Serine-rich peptide linkers
This application is a continuation of application Ser. No. 07/842,149, filed as PCT/US92/01478, Feb. 27, 1992 now abandoned, which is a continuation in part of application Ser. No. 07/662,226, filed Feb. 7, 1991, also abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the fields of peptide linkers, fusion proteins and single-chain antibodies. 
     BACKGROUND OF THE INVENTION 
     Two or more polypeptides may be connected to form a fusion protein. This is accomplished most readily by fusing the parent genes that encode the proteins of interest. Production of fusion proteins that recover the functional activities of the parent proteins may be facilitated by connecting genes with a bridging DNA segment encoding a peptide linker that is spliced between the polypeptides connected in tandem. The present invention addresses a novel class of linkers that confer unexpected and desirable qualities on the fusion protein products. 
     An example of one variety of such fusion proteins is an antibody binding site protein also known as a single-chain Fv (sFv) which incorporates the complete antibody binding site in a single polypeptide chain. Antibody binding site proteins can be produced by connecting the heavy chain variable region (V H ) of an antibody to the light chain variable region (V L ) by means of a peptide linker. See, PCT International Publication No. WO 88/09344 the teachings of which are hereby incorporated herein by reference. Suck sFv proteins have been produced to date that faithfully reproduce the binding affinities and specificities of the parent monoclonal antibody. However, there have been some drawbacks associated with them, namely, that some sFv fusion proteins have tended to exhibit low solubility in physiologically acceptable media. For example, the anti-digoxin 26-10 sFv protein, which binds to the cardiac glycoside digoxin, can be refolded in 0.01M NaOAc buffer, pH 5.5, to which urea is added to a final concentration of 0.25M to produce approximately 22% active anti-digoxin sFv protein. The anti-digoxin sFv is inactive as a pure protein in phosphate buffered saline (PBS) which is a standard buffer that approximates the ionic strength and neutral pH conditions of human serum. In order to retain digoxin binding activity in PBS the 26-10 sFv must be stored in 0.01M sodium acetate, pH 5.5, 0.25M urea diluted to nanomolar concentrations in PBS containing 1% horse serum or 0.1% gelatin, a concentration which is too low for most therapeutic or pharmaceutical use. 
     Therefore, it is an object of the invention to design and prepare fusion proteins which are 1) soluble at high concentrations in physiological media, and 2) resistant to proteolytic degradation. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a peptide linker comprising a large proportion of serine residues which, when used to connect two polypeptide domains, produces a fusion protein which has increased solubility in aqueous media and improved resistance to proteolysis. In one aspect, the invention provides a family of biosynthetic proteins comprising first and second protein domains which are biologically active individually or act together to effect biological activity, wherein the domains are connected by a peptide linker comprising the sequence (X, X, X, X, Gly) y  wherein y typically is 2 or greater, up to two Xs in each unit are Thr, and the remaining Xs in each unit are Ser. Preferably, the linker takes the form (Ser, Ser, Ser, Ser, Gly) y  where Y is greater than 1. The linker preferably comprises at least 75 percent serine residues. 
     The linker can be used to prepare single chain binding site proteins wherein one of the protein domains attached to the linker comprises or mimicks the structure of an antibody heavy chain variable region and the other domain comprises or mimicks the structure of an antibody light chain variable domain. A radioactive isotope advantageously may be attached to such structures to produce a family of imaging agents having high specificity for target structure dictated by the particular affinity and specificity of the single chain binding site. Alternatively, the linker may be used to connect a polypeptide ligand and a polypeptide effector. For example, a ligand can be a protein capable of binding to a receptor or adhesion molecule on a cell in vivo, and the effector a protein capable of affecting the metabolism of the cell. Examples of such constructs include those wherein the ligand is itself a single chain immunoglobulin binding site or some other form of binding protein or antibody fragment, and the effector is, for example, a toxin. 
     Preferred linkers for sFv comprise between 8 and 40 amino acids, more preferably 10-15, most preferably 13, wherein at least 40%, and preferably 50% are serine. Glycine is a preferred amino acid for remaining residues; threonine may also be used; and preferably, charged residues are avoided. 
     Fusion proteins containing the serine-rich peptide linker are also the subject of the present invention, as are DNAs encoding the proteins, cells expressing them, and method of making them. 
     The serine-rich peptide linkers of the present invention can be used to connect the subunit polypeptides of a biologically active protein, that is, linking one polypeptide domain with another polypeptide domain, thereby forming a biologically active fusion protein; or to fuse one biologically active polypeptide to another biologically active peptide, thereby forming a bifunctional fusion protein expressing both biological activities. A particularly effective linker for forming this protein contains the following amino acid sequence (sequence ID No. 1): 
     
         -Ser-Gly-Ser-Ser-Ser-Ser-Gly-Ser-Ser-Ser-Ser-Gly-Ser-. 
    
     The serine-rich linkers of the present invention produce proteins which are biologically active and which remain in solution at a physiologically acceptable pH and ionic strength at much higher concentrations than would have been predicted from experience. The serine-rich peptide linkers of the present invention often can provide significant improvements in refolding properties of the fusion protein expressed in procaryotes. The present serine-rich linkers are resistant to proteolysis, thus fusion proteins which are relatively stable in vivo can be made using the present linker and method. In particular, use of the linkers of the present invention to fuse domains mimicking V H  and V L  from monoclonal antibody results in single chain binding site proteins which dissolve in physiological media, retain their activity at high concentrations, and resist lysis by endogenous proteases. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The serine-rich peptide linkers of the present invention are used to link through peptide bonded structure two or more polypeptide domains. The polypeptide domains individually may be biologically active proteins or active polypeptide segments, for example, in which case a multifunctional protein is produced. Alternatively, the two domains may interact cooperatively to effect the biological function. The resulting protein containing the linker(s) is referred to herein as a fusion protein. 
     The preferred length of a serine-rich peptide of the present invention depends upon the nature of the protein domains to be connected. The linker must be of sufficient length to allow proper folding of the resulting fusion protein. The length required can be estimated as follows: 
     1. Single-Chain Fv (sFv). For a single chain antibody binding site comprising mimicks of the light and heavy chain variable regions of an antibody protein (hereinafter, sFv), the linker preferably should be able to span the 3.5 nanometer (nm) distance between its points of covalent attachment between the C-terminus of one and the N-terminus of the other V domain without distortion of the native Fv conformation. Given the 0.38 nm distance between adjacent peptide bonds, a preferred linker should be at least about 10 residues in length. Most preferably, a 13-15 amino acid residue linker is used in order to avoid conformational strain from an overly short connection, while avoiding steric interference with the combining site from an excessively long peptide. 
     2. Connecting domains in a dimeric or multimeric protein for which a 3-dimensional conformation is known. Given a 3-dimensional structure of the protein of interest, the minimum surface distance between the chain termini to be bridged, d (in nanometers), should be determined, and then the approximate number of residues in the linker, n, is calculated by dividing d by 0.38 nm (the peptide unit length). A preferred length should be defined ultimately by empirically testing linkers of different sizes, but the calculated value provides a good first approximation. 
     3. Connecting domains in a dimeric or multimeric protein for which no 3-dimensional conformation is known. In the absence of information regarding the protein&#39;s 3-dimensional structure, the appropriate linker length can be determined operationally by testing a series of linkers (e.g., 5, 10, 15, 20, or 40 amino acid residues) in order to find the range of usable linker sizes. Fine adjustment to the linker length then can be made by comparing a series of single-chain proteins (e.g., if the usable n values were initially 15 and 20, one might test 14, 15, 16, 17, 18, 19, 20, and 21) to see which fusion protein has the highest specific activity. 
     4. Connection of independent domains (i.e., independently functional proteins or polypeptides) or elements of secondary structure (alpha or beta strands). For optimal utility, this application requires empirically testing serine-rich linkers of differing lengths to determine what works well. In general, a preferred linker length will be the smallest compatible with full recovery of the native functions and structures of interest. Linkers wherein 1≦y≦4 work well in many instances. 
     After the ideal length of the peptide linker is determined, the percentage of serine residues present in the linker can be optimized. As was stated above, preferably at least 75% of a peptide linker of the present invention is serine residues. The currently preferred linker is (SerSerSerSerGly) y  [residues 3-7 of sequence ID No. 1] where y comprises an integer from 1 to 5. Additional residues may extend C-terminal or N-terminal of the linker; preferably such additional residues comprising Ser, Thr, or Gly. Up to two of each of the serine residues on each segment may be replaced by Thr, but this has the tendency to decrease the water solubility of the fusion constructs. For constructs wherein the two linked domains cooperate to effect a single biological function, such as an sFv, it is preferred to avoid use of charged residues. Generally, in linkers of more than 10 residues long, any naturally occurring amino acid may be used once, possibly twice, without unduly degrading the properties of the linker. 
     The serine-rich peptide linker can be used to connect a protein or polypeptide domain with a biologically active peptide, or one biologically active peptide to another to produce a fusion protein having increased solubility, improved folding properties and greater resistance to proteolysis in comparison to fusion proteins using non-serine rich linkers. The linker can be used to make a functional fusion protein from two unrelated proteins that retain the activities of both proteins. For example, a polypeptide toxin can be fused by means of a linker to an antibody, antibody fragment, sFv or peptide ligand capable of binding to a specific receptor to form a fusion protein which binds to the receptor on the cell and kills the cell. 
     Fusion protein according to the present invention can be produced by amino acid synthesis, if the amino acid sequence is known, or preferably by art-recognized cloning techniques. For example, an oligonucleotide encoding the serine-rich linker is ligated between the genes encoding the domains of interest to form one fused gene encoding the entire single-chain protein. The 5&#39; end of the linker oligonucleotide is fused to the 3&#39; end of the first gene, and the 3&#39; end of the linker is fused to the 5&#39; end of the second gene. Any number of genes can be connected in tandem array to encode multi-functional fusion proteins using the serine-rich polypeptide linker of the present invention. The entire fused gene can be transfected into a host cell by means of an appropriate expression vector. 
     In a preferred embodiment of the present invention, amino acid sequences mimicking the light (VL) and heavy (V H ) chain variable regions of an antibody are linked to form a single chain antibody binding site (sFv) which preferably is free of immunoglobulin constant region. Single chain antibody binding sites are described in detail, for example, in U.S. Pat. No. 5,019,513, the disclosure of which is incorporated herein by reference. A particularly effective serine-rich linker for an sFv protein is a linker having the following amino acid sequence: ##STR1## That is, in this embodiment y=2; Ser, Gly precedes the modular sequences, and Ser follows them. The serine-rich linker joins the V H  with the V L  (or vice versa) to produce a novel sFv fusion protein having substantially increased solubility, and resistance to lysis by endogenous proteases. 
     A preferred genus of linkers comprises a sequence having the formula: ##STR2## 
     Where up to two Xs in each unit can be Thr, the remaining Xs are Ser, and y in between 1 and 5. 
     A method for producing a sFv is described in PCT Application No. US88/01737, the teachings of which are incorporated herein by reference. In general, the gene encoding the variable region from the heavy chain (V H ) of an antibody is connected at the DNA level to the variable region of the light chain (V L ) by an appropriate oligonucleotide. Upon translation, the resultant hybrid gene forms a single p61ypeptide chain comprising the two variable domains bridged by a linker peptide. 
     The sFv fusion protein comprises a single polypeptide chain with the sequence V H  -&lt;linker&gt;-V L  or V L  -&lt;linker&gt;-V H , as opposed to the classical Fv heterodimer of V H  and V L . About 3/4 of each variable region polypeptide sequence is partitioned into four framework regions (FRs) that form a scaffold or support structure for the antigen binding site, which is constituted by the remaining residues defining three complementary determining regions (CDRs) which form loops connecting the FRs. The sFv is thus preferably composed of 8 FRs, 6 CDRs, and a linker segment, where the V H  sequence can be abbreviated as: 
     
         FR1-H1-FR2-H2-FR3-H3-FR4; 
    
     and the V L  sequence as 
     
         FR1-L1-FR2-L2-FR3-L3-FR4. 
    
     The predominant secondary structure in immunoglobulin V regions is the twisted β-sheet. A current interpretation of Fv architecture views the FRs as forming two concentric β-barrels, with the CDR loops connecting antiparallel β-strands of the inner barrel. The CDRs of a given murine monoclonal antibody may be grafted onto the FRs of human Fv regions in a process termed &#34;humanization&#34; or CDR replacement. Humanized antibodies promise minimal immunogenicity when sFv fusion proteins are administered to patients. Humanized single chain biosynthetic antibody binding sites, and how to make and use them, are described in detail in U.S. Pat. No. 5,019,513, as are methods of producing various other FR/CDR chimerics. 
     The general features of a viable peptide linker for an sFv fusion protein are governed by the architecture and chemistry of Fv regions. It is known that the sFv may be assembled in either domain order, V H  -linker-V L  or V L  -linker-V H , where the linker bridges the gap between the carboxyl (C) and amino (N) termini of the respective domains. For purposes of sFv design, the C-terminus of the amino-terminal V H  or V L  domain is considered to be the last residue of that sequence which is compactly folded, corresponding approximately to the end of the canonical V region sequence. The amino-terminal V domain is thus defined to be free of switch region residues that link the variable and constant domains of a given H or L chain, which makes the linker sequence an architectural element in sFv structure that corresponds to bridging residues, regardless of their origin. In several examples, fused sFv constructs have incorporated residues from the switch region, even extending into the first constant domain. 
     In principle, sFv proteins may be constructed to incorporate the Fv region of any monoclonal antibody regardless of its class or antigen specificity. Departures from parent V region sequences may involve changes in CDRs to modify antigen affinity or specificity, or to redefine complementarity, as well as wholesale alteration of framework regions to effect humanization of the sFv or for other purposes. In any event, an effective assay, e.g., a binding assay, must be available for the parent antibody and its sFv analogue. Design of such an assay is well within the skill of the art. Fusion proteins such as sFv immunotoxins intrinsically provide an assay by their toxicity to target cells in culture. 
     The construction of a single-chain Fv typically is accomplished in two or three phases: (1) isolation of cDNA for the variable regions; (2) modification of the isolated V H  and V L  domains to permit their joining to form a single chain via a linker; (3) expression of the single-chain Fv protein. The assembled sFv gene may then be progressively altered to modify sFv properties. Escherichia coli (E. coli) has generally been the source of most sFv proteins although other expression systems can be used to generate sFv proteins. 
     The V H  and V L  genes for a given monoclonal antibody are most conveniently derived from the cDNA of its parent hybridoma cell line. Cloning of V H  and V L  from hybridoma cDNA has been facilitated by library construction kits using lambda vectors such as Lambda ZAP R  (Stratagene). If the nucleotide and/or amino acid sequences of the V domains are known, then the gene or the protein can be made synthetically. Alternatively, a semisynthetic approach can be taken by appropriately modifying other available cDNA clones or sFv genes by site-directed mutagenesis. Many alternative DNA probes have been used for V gene cloning from hybridoma cDNA libraries. Probes for constant regions have general utility provided that they match the class of the relevant heavy or light chain constant domain. Unrearranged genomic clones containing the J-segments have even broader utility, but the extent of sequence homology and hybridization stringency may be unknown. Mixed pools of synthetic oligonucleotides based on the J-regions of known amino acid sequence have been used. If the parental myeloma fusion partner was transcribing an endogenous immunoglobulin gene, the authentic clones for the V genes of interest should be distinguished from the genes of endogenous origin by examining their DNA sequences in a Genbank homology search. 
     The cloning steps described above may be simplified by the use of polymerase chain reaction (PCR) technology. For example, immunoglobulin cDNA can be transcribed from the monoclonal cell line by reverse transcriptase prior to amplification by Tag polymerase using specially designed primers. Primers used for isolation of V genes may also contain appropriate restriction sequences to speed sFv and fusion protein assembly. Extensions of the appropriate primers preferably also should encode parts of the desired linker sequence such that the PCR amplification products of V H  and V L  genes can be mixed to form the single-chain Fv gene directly. The application of PCR directly to human peripheral blood lymphocytes offers the opportunity to clone human V regions directly in bacteria. See., Davis et al. Biotechnology, 9, (2):165-169 (1991). 
     Refinement of antibody binding sites is possible by using filamentous bacteriophage that allow the expression of peptides or polypeptides on their surface. These methods have permitted the construction of phage antibodies that express functional sFv on their surface as well as epitope libraries that can be searched for peptides that bind to particular combining sites. With appropriate affinity isolation steps, this sFv-phage methodology offers the opportunity to generate mutants of a given sFv with desired changes in specificity and affinity as well as to provide for a refinement process in successive cycles of modification. See McCafferty et al., Nature, 348:552 (1990), Parmely et al. Gene, 38:305 (1988), Scott et al. Science, 249:386 (1990), Devlin et al. Science, 249:404 (1990)., and Cwirla et al., Proc. Nat. Acad. Sci. U.S.A., 87:6378 (1990). 
     The placement of restriction sites in an sFv gene can be standardized to facilitate the exchange of individual V H , V L  linker elements, or leaders (See U.S. Pat. No. 5,019,513, supra). The selection of particular restriction sites can be governed by the choice of stereotypical sequences that may be fused to different sFv genes. In mammalian and bacterial secretion, secretion signal peptides are cleaved from the N-termini of secreted proteins by signal peptidases. The production of sFv proteins by intracellular accumulation in inclusion bodies also may be exploited. In such cases a restriction site for gene fusion and corresponding peptide cleavage site are placed at the N-terminus of either V H  or V L . Frequently a cleavage site susceptible to mild acid for release of the fusion leader is chosen. 
     In a general scheme, a SacI site serves as an adapter at the C-Terminal end of V H . A large number of V H  regions end in the sequence -Val-Ser-Ser-, which is compatible with the codons for a SacI site (G AGC TCT), to which the linker may be attached. The linker of the present invention can be arranged such that a -Gly-Ser- is positioned at the C-terminal end of the linker encoded by GGA-TCC to generate a BamHI site, which is useful provided that the same site is not chosen for the beginning of V H . 
     Alternatively, an XhoI site (CTCGAG) can be placed at the C-terminal end of the linker by including another serine to make a -Gly-Ser-Ser- sequence that can be encoded by GGC-TCG-AGN-, which contains the XhoI site. For sFv genes encoding V H  -Linker-V L , typically a PstI site is positioned at the 3&#39; end of the V L  following the new stop condon, which forms a standard site for ligation to expression vectors. If any of these restriction sites occur elsewhere in the cDNA, they can be removed by silent base changes using site directed mutagenesis. Similar designs can be used to develop a standard architecture for V L  -V H  constructions. 
     Expression of fusion proteins in E. coli as insoluble inclusion bodies provides a reliable method for producing sFv proteins. This method allows for rapid evaluation of the level of expression and activity of the sFv fusion protein while eliminating variables associated with direct expression or secretion. Some fusion partners tend not to interfere with antigen binding which may simplify screening for sFv fusion protein during purification. Fusion protein derived from inclusion bodies must be purified and refolded in vitro to recover antigen binding activity. Mild acid hydrolysis can be used to cleave a labile Asp-Pro peptide bond between the leader and sFv yielding proline at the sFv amino terminus. In other situations, leader cleavage can rely on chemical or enzymatic hydrolysis at specifically engineered sites, such as CNBr cleavage of a unique methionine, hydroxylamine cleavage of the peptide bond between Asn-Gly, and enzymatic digestion at specific cleavage sites such as those recognized by factor Xa, enterokinase or V8 protease. 
     Direct expression of intracellular sFv proteins which yields the desired sFv without a leader attached is possible for single-chain Fv analogues and sFv fusion proteins. Again, the isolation of inclusion bodies must be followed by refolding and purification. This approach avoids the steps needed for leader removal but direct expression can be complicated by intracellular proteolysis of the cloned protein. 
     The denaturation transitions of Fab fragments from polyclonal antibodies are known to cover a broad range of denaturant. The denaturation of monoclonal antibody Fab fragments or component domains exhibit relatively sharp denaturation transitions over a limited range of denaturant. Thus, sFv proteins can be expected to differ similarly covering a broad range of stabilities and denaturation properties which appear to be paralleled by their preferences for distinct refolding procedures. Useful refolding protocols include dilution refolding, redox refolding and disulfide restricted refolding. In general, all these procedures benefit from the enhanced solubility conferred by the serine-rich linker of the present invention. 
     Dilution refolding relies on the observation that fully reduced and denatured antibody fragments can refold upon removal of denaturant and reducing agent with recovery of specific binding activity. Redox refolding utilizes a glutathione redox couple to catalyze disulfide interchange as the protein refolds into its native state. For an sFv protein having a prior art linker such as (GlyGlyGLyGlySer) 3 , the protein is diluted from a fully reduced state in 6M urea into 3M urea+25 mM Tris-HCL+10 mM EDTA, pH 8, to yield a final concentration of approximately 0.1 mg/ml. In a representative protein, the sFv unfolding transition begins around 3M urea and consequently the refolding buffer represents near-native solvent conditions. Under these conditions, the protein can presumably reform approximations to the V domain structures wherein rapid disulfide interchange can occur until a stable equilibrium is attained. After incubation at room temperature for 16 hours, the material is dialyzed first against urea buffer lacking glutathione and then against 0.01M sodium acetate+0.25M urea, pH 5.5. 
     In contrast to the sFv protein having the prior art linker described above, with the same sFv protein, but having a serine-rich linker of the present invention, the 3M urea-glutathione refolding solution can be dialyzed directly into 0.05M potassium phosphate, pH 7, 0.15 NaCl (PBS). 
     Disulfide restricted refolding offers still another route to obtaining active sFv which involves initial formation of intrachain disulfides in the fully denatured sFv. This capitalizes on the favored reversibility of antibody refolding when disulfides are kept intact. Disulfide crosslinks should restrict the initial refolding pathways available to the molecule as well as other residues adjacent to cysteinyl residues that are close in the native state. For chains with the correct disulfide paring the recovery of a native structure should be favored while those chains with incorrect disulfide pairs must necessarily produce non-native species upon removal of denaturant. Although this refolding method may give a lower yield than other procedures, it may be able to tolerate higher protein concentrations during refolding. 
     Proteins secreted into the periplasmic space or into the culture medium appear to refold properly with formation of the correct disulfide bonds. In the majority of cases the signal peptide sequence is removed by a bacterial signal peptidase to generate a product with its natural amino terminus. Even though most secretion systems currently give considerably lower yields than intracellular expression, the rapidity of obtaining correctly folded and active sFv proteins can be of decisive value for protein engineering. The ompA or pelB signal sequence can be used to direct secretion of the sFv. 
     If some sFv analogues or fusion proteins exhibit lower binding affinities than the parent antibody, further purification of the sFv protein or additional refinement of antigen binding assays may be needed. On the other hand, such sFv behavior may require modification of protein design. Changes at the amino-termini of V domains may on occasion perturb a particular combining site. Thus, if an sFv were to exhibit a lower affinity for antigen than the parent Fab fragment, one could test for a possible N-terminal perturbation effect. For instance, given a V L  -V H  that was suspect, the V H  -V L  construction could be made and tested. If the initially observed perturbation were changed or eliminated in the alternate sFv species, then the effect could be traced to the initial sFv design. 
    
    
     The invention will be understood further from the following nonlimiting examples. 
     EXAMPLES 
     Example 1 
     Preparation and Evaluation of an Anti-digoxin 26-10 sFv Having a Serine-rich Linker 
     An anti-digoxin 26-10 sFv containing a serine-rich peptide linker (Sequence No. 1, identified below) of the present invention was prepared as follows: ##STR3## 
     A set of synthetic oligonucleotides was prepared using phosphoramidite chemistry on a Cruachem DNA synthesizer, model PS250. The nucleotide sequence in the appropriate reading frame encodes the polypeptide from 1-12 while residue 13 is incorporated as part of the Bam Hl site that forms upon fusion to the downstream Ban Hl fragment that encodes V L  ; and the first serine residue in the linker was attached to a serine at the end of the 26-10 V H  region of the antibody. This is shown more clearly in Sequence ID Nos. 4 and 5. 
     The synthetic oligonucleotide sequence which was used in the cassette mutagenesis was as follows: ##STR4## 
     The complementary oligomers, when annealed to each other, present a cohesive end of a SacI site upstream and a BamHI site downstream. 
     The nucleotide sequence was designed to contain useful 6-base restriction sites which will allow combination with other single chain molecules and additional modifications of the leader. The above-described synthetic oligonucleotides were assembled with the V H  and V L  regions of the anti-digoxin 26-10 gene as follows: 
     A pUC plasmid containing the 26-10 sFv gene (disclosed in PCT International Publication No. WO 88/09344) containing a (Gly-Gly-Gly-Gly-Ser) n  linker between a SacI site at the end of the V H  region and a unique BamHI site which had been inserted at the beginning of V L  region was opened at SacI and BamHI to release the sequence encoding for the prior art linker and to accept the oligonucleotides defined by Sequence No. 2. The resulting plasmid was called pH899. 
     The new 26-10 sFv gene of pH899 was inserted into an expression vector, pH895, for fusion with a modified fragment B (MFB) of staphlococcal protein A. (See Sequence ID No. 4.) The modified FB leader has glutamyl resides at positions FB-36 and FB-37 instead of 2 aspartyl residues, which reduces unwanted ancillary cleavage during acid treatment. The modified pH895 is essentially equivalent to pC105 (except for the slightly modified leader) as previously described in Biochemistry, 29(35):8024-8030 (1990). The assembly was done by replacing the old sFv fragment with the new sFv between XbaI (in V H ) and PstI (at the end of sFv) in the expression plasmid pH895, opened at unique XbaI and PstI sites. The resulting new expression vector was named pH908. An expression vector utilizing an MLE-MFB leader was constructed as follows. 
     The mFB-sFv gene was retrieved by treating pH908 with EcoRI and PstI and inserted into a trp expression vector containing the modified trp LE leader peptide (MLE) producing plasmid pH912. This vector resembled essentially the pD312 plasmid as described in PNAS, 85: 5879-5883 (1988) but having removed from it the EcoRI site situated between the Tet-R gene and the SspI site. Plasmid pH912 contained the MLE-mFB-sFv gene shown in sequence 4. The MLE starts at the N-terminus of the protein and ends at the glutamic acid residue, amino acid residue 59. The mFB leader sequence starts at the methionine residue, amino acid residue 61, and ends at the aspartic acid residue, amino acid residue 121. Phenylanine residue 60 is technically part of the Eco RI restriction site sequence at the junction of the MLE and mFB. 
     Expression of sFv transfected into E. coli (strain JM101) by the plasmid pH912 was under control of the trp promoter. E. coli was transformed by pH912 under selection by tetracycline. Expression was induced in M9 minimal medium by addition,of indole acrylic acid (10 μg/ml) at a cell density with A 600  =1 resulting in high level expression and formation of inclusion bodies which were harvested from cell paste. 
     After expression in E. coli of the sFv protein containing the novel linker of the present invention, the resultant cells were suspended in 25 mM Tris-HCl, pH 8, and 10mmM EDTA treated with 0.1% lysozyme overnight, sonicated at a high setting for three 5 minute periods in the cold, and spun in a preparative centrifuge at 11,200×g for 30 minutes. For large scale preparation of inclusion bodies, the cells are concentrated by ultrafiltration and then lysed with a laboratory homogenizer such as with model 15MR, APV homogenizer manufactured by Gaulin, Inc. The inclusion bodies are then collected by centrifugation. The resultant pellet was then washed with a buffer containing 3M urea, 25 mM Tris-HCl, pH8, and 10 mEDTA. 
     The purification of the 26-10 sFv containing the linker of the present invention from the MLE-mFB-sFv fusion protein was then accomplished according to the following procedure: 
     1) Solubilization of Fusion Protein in Guanidine Hydrochloride 
     The MLE-mFB-sFv inclusion bodies were weighed and were then dissolved in a 6.7M GuHCl (guanidine hydrochloride) which had been dissolved in 10% acetic acid. An amount of GuHCl equal to the weight of the recovered inclusion bodies was then added to the solution and dissolved to compensate for the water present in the inclusion body pellet. 
     2) Acid Cleavage of the Unique Asp-PrO Bond at the Junction of the Leader and 26-10 sFv 
     The Asp-Pro bond (amino acid residues 121 and 122 of Sequence Nos. 4 and 5) was cleaved in the following manner. Glacial acetic acid was added to the solution of step 1 to 10% of the total volume of the solution. The pH of the solution was then adjusted to 2.5 with concentrated HCl. This solution was then incubated at 37° C. for 96 hours. The reaction was stopped by adding 9 volumes of cold ethanol, stored at -20° C. for several hours, followed by centrifugation to yield a pellet of precipitated 26-10 sFv and uncleaved fusion protein. The heavy chain variable region of the sFv molecule extended from amino acid residue 123 to 241; the linker included amino acid residues 242 to 254; and the variable light region extended from amino acid residue 255 to 367 of Sequence Nos. 4 and 5. Note also that Sequence No. 6 and 7 shows a similar sFv starting with methionine at residues 1 followed by V H  (residues 2-120), linker (121-133), and V L  (134-246). This gene product was expressed directly by the T7 expression system with formation of inclusion bodies. 
     3) Re-dissolution of Cleavage Products 
     The precipitated sFv cleavage mixture from step 2 was weighed and dissolved in a solution of 6M GuHl+25 mM Tris HCl+10 mM EDTA having a pH of 8.6. Solid GuHCl in an amount equal to the weight of the sFv cleavage mixture from step two was then added and dissolved in the solution. The pH of the solution was then adjusted to 8.6 and dithiothreitol was added to the solution such that the resultant solution contained 10 mM dithiothreitol. The solution was then incubated at room temperature for 5 hours. 
     4) Renaturation of 26-10 sFv 
     The solution obtained from step 3 was then diluted 70-fold to a concentration of about 0.2mg of protein/ml with a buffer solution containing 3M urea, 25 mM Tris-HCl, pH 8, 10 mM EDTA 1 mM oxidized gluthathione, 0.1 mM reduced gluthathione, and incubated at room temperature for 16 hours. The resultant protein solution was then dialyzed in the cold against PBSA to complete the refolding of the sFv protein. 
     (5) Affinity Purification of the Active Anti-digoxin 26-10 sFv 
     The refolded protein from step 4 was loaded onto a column containing ouabain-amine-Sepharose 4B, and the column was washed successively with PBSA, followed by two column volumes of 1M NaCl in PBSA and then again with PBSA to remove salt. Finally, the active protein was displaced from the resin by 20 mM ouabain in PBSA. Absorbance measurements at 280 nm indicated which fractions contained active protein. However, the spectra of the protein and ouabain overlap. Consequently, ouabain was removed by exhaustive dialysis against PBSA in order to accurately quantitate the protein yield. 
     6) Removal of Uncleaved Fusion Protein and the MLE-mFB Leader 
     Finally, the solution from step 5 containing the active refolded protein (sFv and MLE-mFB-sFv) was chromatographed on an IgG-Sepharose column in PBSA buffer. The uncleaved MLE-mFB-sFv protein bound to the immobilized immunoglobulin and the column effluent contained essentially pure sFv. 
     In conclusion, the incorporation of a serine-rich peptide linker of 13 residues [Ser-Gly-(Ser-Ser-Ser-Ser-Gly) 2  -Ser- ]  in the 26-10 sFv yielded significant improvements over the 26-10 sFv with a glycine-rich linker of 15 residues, [-(Gly-Gly-Gly-Gly-Ser) 3  ]. 
     The serine-rich peptide linker of the present invention results in a number of improvements over the previous peptide linkers including: 
     1. Refolding and storage conditions are consistent with normal serum conditions, thereby making applications to pharmacology and toxicology accessible. The 26-10 sFv can be renatured in PBS (0.05M potassium phosphate, 0.15M NaCl, pH 7.0); 0.03% azide is added as a bacteriostatic agent for laboratory purposes but would be excluded in any animal or clinical applications. The old linker, 26-10 sFv had to be renatured into 0.01M sodium acetate, pH 5.5, with 0.25M urea added to enhance the level of active protein. 
     2. Solubility was vastly improved from a limit of about 5OD 280  units per ml (about 3 mg/ml) to 52 OD 280  units per ml (about 33 mg/ml), and possibly greater in buffers other than PBSA. The highly concentrated protein solution was measured directly with a 0.2 mm path length cell. The protein concentration was estimated by multiplying by 50 the absorptions at 280 nm, subtracting twice the scattering absorbance at 333 nm, which yields a corrected A280 of about 52 units per ml. 
     3. Fidelity of the antigen binding site was retained by the new serine-rich linker 26-10 sFv, which is consistent with an uncharged linker peptide that has minimal interactions with the V domains. 
     4. Enhanced stability at normal serum pH and ionic strength. In PSBSA, 26-10 sFv with the (GGGGS) 3  linker loses binding activity irreversibly whereas the 26-10 sFv containing the new serine-rich linker is completely stable in PBSA. 
     5. Enhanced resistance to proteolysis. The presence of the serine-rich linker improves resistance to endogenous proteases in vivo, which results in a longer plasma/half-life of the fusion protein. 
     Example 2 
     Preparation of a Fusion Protein Having a Serine Rich Linker 
     A fusion protein was prepared containing a serine rich linker linking two unrelated proteins. A fusion gene was constructed as described in Example 1 above, except that in lieu of the V L  and V H  genes described in Example 1, genes encoding the following proteins were fused: the dominant dhfr gene (Sequence No. 8, residues 1-576) and the neogene (Sequence No. 8, residues 621-1416) were fused with a linker having the sequence: ##STR5## The four residues SVTV (numbers 189-192 of Seq. ID No. 8) can be regarded as part of the linker. These were left over from the sFv from which the linker sequences used in this example was derived. The resulting protein was a functional fusion protein encoding domains from two unrelated proteins which retained the activity of both. Thus, this DNA included on a plasmid inparts to successfully transfected cells resistance to both methotrexate, due to the action of the DHFR enzyme, and to neomycin, due to the action of the neo expression product. 
     Equivalents 
     One skilled in the art will recognize many equivalents to the specific embodiments described herein. Such equivalents are intended to be encompassed by the following claims. 
     
         __________________________________________________________________________
SEQUENCE LISTING                                                          
(1) GENERAL INFORMATION:                                                  
(iii) NUMBER OF SEQUENCES: 9                                              
(2) INFORMATION FOR SEQ ID NO:1:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 13 amino acids                                                
(B) TYPE: amino acid                                                      
(C) STRANDEDNESS: single                                                  
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: peptide                                               
(ix) FEATURE:                                                             
 (A) NAME/KEY: Peptide                                                    
(B) LOCATION: 1..13                                                       
(D) OTHER INFORMATION: /note=&#34;(SER)4-GLY LINKER. THE                      
REPEATING SEQUENCE &#34;(SER)4-GLY&#34;(E.G., RES. 3-7)                           
MAY BE REPEATED MULTIPLE TIMES (SEE SPECIFICATION.)                       
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                   
SerGlySerSerSerSerGlySerSerSerSerGlySer                                   
1 510                                                                     
(2) INFORMATION FOR SEQ ID NO:2:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 36 base pairs                                                 
(B) TYPE: nucleic acid                                                    
(C) STRANDEDNESS: single                                                  
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: syn DNA                                               
(ix) FEATURE:                                                             
(A) NAME/KEY: misc.sub.-- feature                                         
(B) LOCATION: 1..36                                                       
 (D) OTHER INFORMATION: /note=&#34;LINKER SEQUENCE (TOP                       
STRAND)&#34;                                                                  
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                   
CCTCCGGATCTTCATCTAGCGGTTCCAGCTCGAGTG36                                    
(2) INFORMATION FOR SEQ ID NO:3:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 13 amino acids                                                
(B) TYPE: amino acid                                                      
(C) STRANDEDNESS: single                                                  
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: peptide                                               
(ix) FEATURE:                                                             
(A) NAME/KEY: Peptide                                                     
(B) LOCATION: 1..13                                                       
(D) OTHER INFORMATION: /note=&#34;(XAA)4-GLY LINKER, WHERE                    
RES.3-7 ARE THE REPEATING UNIT AND UP TO 2 OF THE XAA&#39;S                   
IN REPEAT UNIT CAN BE THR, THE REMAINDER SER.                             
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                   
 XaaGlyXaaXaaXaaXaaGlyXaaXaaXaaXaaGlyXaa                                  
1510                                                                      
(2) INFORMATION FOR SEQ ID NO:4:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 1110 base pairs                                               
(B) TYPE: nucleic acid                                                    
(C) STRANDEDNESS: single                                                  
 (D) TOPOLOGY: linear                                                     
(ii) MOLECULE TYPE: syn DNA                                               
(ix) FEATURE:                                                             
(A) NAME/KEY: CDS                                                         
(B) LOCATION: 1..1101                                                     
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                   
ATGAAAGCAATTTTCGTACTGAAAGGTTCACTGGACAGAGATCTGGAC48                        
MetLysAlaIlePheValLeuLysGl ySerLeuAspArgAspLeuAsp                         
151015                                                                    
TCTCGTCTGGATCTGGACGTTCGTACCGACCACAAAGACCTGTCTGAT96                        
SerArgLeuAspLeuAspValAr gThrAspHisLysAspLeuSerAsp                         
202530                                                                    
CACCTGGTTCTGGTCGACCTGGCTCGTAACGACCTGGCTCGTATCGTT144                       
HisLeuValLeuValAspLeuAl aArgAsnAspLeuAlaArgIleVal                         
354045                                                                    
ACTCCCGGGTCTCGTTACGTTGCGGATCTGGAATTCATGGCTGACAAC192                       
ThrProGlySerArgTyrValAlaAs pLeuGluPheMetAlaAspAsn                         
505560                                                                    
AAATTCAACAAGGAACAGCAGAACGCGTTCTACGAGATCTTGCACCTG240                       
LysPheAsnLysGluGlnGlnAsnAlaPheTy rGluIleLeuHisLeu                         
65707580                                                                  
CCGAACCTGAACGAAGAGCAGCGTAACGGCTTCATCCAAAGCCTGAAA288                       
ProAsnLeuAsnGluGluGlnArgAs nGlyPheIleGlnSerLeuLys                         
859095                                                                    
GAAGAGCCGTCTCAGTCTGCGAATCTGCTAGCGGATGCCAAGAAACTG336                       
GluGluProSerGlnSerAlaAs nLeuLeuAlaAspAlaLysLysLeu                         
100105110                                                                 
AACGATGCGCAGGCACCGAAATCGGATCCCGAAGTTCAACTGCAACAG384                       
AsnAspAlaGlnAlaProLysSe rAspProGluValGlnLeuGlnGln                         
115120125                                                                 
TCTGGTCCTGAATTGGTTAAACCTGGCGCCTCTGTGCGCATGTCCTGC432                       
SerGlyProGluLeuValLysProGl yAlaSerValArgMetSerCys                         
130135140                                                                 
AAATCCTCTGGGTACATTTTCACCGACTTCTACATGAATTGGGTTCGC480                       
LysSerSerGlyTyrIlePheThrAspPheTy rMetAsnTrpValArg                         
145150155160                                                              
CAGTCTCATGGTAAGTCTCTAGACTACATCGGGTACATTTCCCCATAC528                       
GlnSerHisGlyLysSerLeuAspTy rIleGlyTyrIleSerProTyr                         
165170175                                                                 
TCTGGGGTTACCGGCTACAACCAGAAGTTTAAAGGTAAGGCGACCCTT576                       
SerGlyValThrGlyTyrAsnGl nLysPheLysGlyLysAlaThrLeu                         
180185190                                                                 
ACTGTCGACAAATCTTCCTCAACTGCTTACATGGAGCTGCGTTCTTTG624                       
ThrValAspLysSerSerSerTh rAlaTyrMetGluLeuArgSerLeu                         
195200205                                                                 
ACCTCTGAGGACTCCGCGGTATACTATTGCGCGGGCTCCTCTGGTAAC672                       
ThrSerGluAspSerAlaValTyrTy rCysAlaGlySerSerGlyAsn                         
210215220                                                                 
AAATGGGCCATGGATTATTGGGGTCATGGTGCTAGCGTTACTGTGAGC720                       
LysTrpAlaMetAspTyrTrpGlyHisGlyAl aSerValThrValSer                         
225230235240                                                              
TCCTCCGGATCTTCATCTAGCGGTTCCAGCTCGAGTGGATCCGACGTC768                       
SerSerGlySerSerSerSerGlySe rSerSerSerGlySerAspVal                         
245250255                                                                 
GTAATGACCCAGACTCCGCTGTCTCTGCCGGTTTCTCTGGGTGACCAG816                       
ValMetThrGlnThrProLeuSe rLeuProValSerLeuGlyAspGln                         
260265270                                                                 
GCTTCTATTTCTTGCCGCTCTTCCCAGTCTCTGGTCCATTCTAATGGT864                       
AlaSerIleSerCysArgSerSe rGlnSerLeuValHisSerAsnGly                         
275280285                                                                 
AACACTTACCTGAACTGGTACCTGCAAAAGGCTGGTCAGTCTCCGAAG912                       
AsnThrTyrLeuAsnTrpTyrLeuGl nLysAlaGlyGlnSerProLys                         
290295300                                                                 
CTTCTGATCTACAAAGTCTCTAACCGCTTCTCTGGTGTCCCGGATCGT960                       
LeuLeuIleTyrLysValSerAsnArgPheSe rGlyValProAspArg                         
305310315320                                                              
TTCTCTGGTTCTGGTTCTGGTACTGACTTCACCCTGAAGATCTCTCGT1008                      
PheSerGlySerGlySerGlyThrAs pPheThrLeuLysIleSerArg                         
325330335                                                                 
GTCGAGGCCGAAGACCTGGGTATCTACTTCTGCTCTCAGACTACTCAT1056                      
ValGluAlaGluAspLeuGlyIl eTyrPheCysSerGlnThrThrHis                         
340345350                                                                 
GTACCGCCGACTTTTGGTGGTGGCACCAAGCTCGAGATTAAACGT1101                         
ValProProThrPheGlyGlyGl yThrLysLeuGluIleLysArg                            
355360365                                                                 
TAACTGCAG1110                                                             
(2) INFORMATION FOR SEQ ID NO:5:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
( A) LENGTH: 367 amino acids                                              
(B) TYPE: amino acid                                                      
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: protein                                               
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                   
MetLysAlaIlePheValLeuLysGlySerLeuAspArgAspLeuAsp                          
1510 15                                                                   
SerArgLeuAspLeuAspValArgThrAspHisLysAspLeuSerAsp                          
202530                                                                    
HisLeuValLeuValAspLeuAlaArgAsnAspLeuAlaArgIleVal                          
 354045                                                                   
ThrProGlySerArgTyrValAlaAspLeuGluPheMetAlaAspAsn                          
505560                                                                    
LysPheAsnLysGluGlnGlnAsn AlaPheTyrGluIleLeuHisLeu                         
65707580                                                                  
ProAsnLeuAsnGluGluGlnArgAsnGlyPheIleGlnSerLeuLys                          
85 9095                                                                   
GluGluProSerGlnSerAlaAsnLeuLeuAlaAspAlaLysLysLeu                          
100105110                                                                 
AsnAspAlaGlnAlaProLysSerAspProGluV alGlnLeuGlnGln                         
115120125                                                                 
SerGlyProGluLeuValLysProGlyAlaSerValArgMetSerCys                          
130135140                                                                 
LysSer SerGlyTyrIlePheThrAspPheTyrMetAsnTrpValArg                         
145150155160                                                              
GlnSerHisGlyLysSerLeuAspTyrIleGlyTyrIleSerProTyr                          
 165170175                                                                
SerGlyValThrGlyTyrAsnGlnLysPheLysGlyLysAlaThrLeu                          
180185190                                                                 
ThrValAspLysSer SerSerThrAlaTyrMetGluLeuArgSerLeu                         
195200205                                                                 
ThrSerGluAspSerAlaValTyrTyrCysAlaGlySerSerGlyAsn                          
210215 220                                                                
LysTrpAlaMetAspTyrTrpGlyHisGlyAlaSerValThrValSer                          
225230235240                                                              
SerSerGlySerSerSerSerGlySerSerSerSerG lySerAspVal                         
245250255                                                                 
ValMetThrGlnThrProLeuSerLeuProValSerLeuGlyAspGln                          
26026527 0                                                                
AlaSerIleSerCysArgSerSerGlnSerLeuValHisSerAsnGly                          
275280285                                                                 
AsnThrTyrLeuAsnTrpTyrLeuGlnLysAlaGlyGlnSerProLys                          
290 295300                                                                
LeuLeuIleTyrLysValSerAsnArgPheSerGlyValProAspArg                          
305310315320                                                              
PheSerGlySerGlySer GlyThrAspPheThrLeuLysIleSerArg                         
325330335                                                                 
ValGluAlaGluAspLeuGlyIleTyrPheCysSerGlnThrThrHis                          
340 345350                                                                
ValProProThrPheGlyGlyGlyThrLysLeuGluIleLysArg                             
355360365                                                                 
(2) INFORMATION FOR SEQ ID NO:6:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 747 base pairs                                                
 (B) TYPE: nucleic acid                                                   
(C) STRANDEDNESS: single                                                  
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: syn DNA                                               
(ix) FEATURE:                                                             
(A) NAME/KEY: CDS                                                         
(B) LOCATION: 1..747                                                      
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                   
ATGGAAGTTCAACTGCAACAGTCTGGTCCTGAATTGGTTAAACCTGGC 48                       
MetGluValGlnLeuGlnGlnSerGlyProGluLeuValLysProGly                          
151015                                                                    
GCCTCTGTGCGCATGTCCTGCAAATCCTCTGGGTACATTTTCACCGAC 96                       
AlaSerValArgMetSerCysLysSerSerGlyTyrIlePheThrAsp                          
202530                                                                    
TTCTACATGAATTGGGTTCGCCAGTCTCATGGTAAGTCTCTAGACTAC 144                      
PheTyrMetAsnTrpValArgGlnSerHisGlyLysSerLeuAspTyr                          
354045                                                                    
ATCGGGTACATTTCCCCATACTCTGGGGTTACCGGCTACAACCAGAAG 192                      
IleGlyTyrIleSerProTyrSerGlyValThrGlyTyrAsnGlnLys                          
505560                                                                    
TTTAAAGGTAAGGCGACCCTTACTGTCGACAAATCTTCCTCAACTGCT240                       
Ph eLysGlyLysAlaThrLeuThrValAspLysSerSerSerThrAla                         
65707580                                                                  
TACATGGAGCTGCGTTCTTTGACCTCTGAGGACTCCGCGGTATACTAT 288                      
TyrMetGluLeuArgSerLeuThrSerGluAspSerAlaValTyrTyr                          
859095                                                                    
TGCGCGGGCTCCTCTGGTAACAAATGGGCGATGGATTATTGGGGTCAT 336                      
CysAlaGlySerSerGlyAsnLysTrpAlaMetAspTyrTrpGlyHis                          
100105110                                                                 
GGTGCTAGCGTTACTGTGAGCTCCTCCGGATCTTCATCTAGCGGTTCC 384                      
GlyAlaSerValThrValSerSerSerGlySerSerSerSerGlySer                          
115120125                                                                 
AGCTCGAGTGGATCCGACGTCGTAATGACCCAGACTCCGCTGTCTCTG 432                      
SerSerSerGlySerAspValValMetThrGlnThrProLeuSerLeu                          
130135140                                                                 
CCGGTTTCTCTGGGTGACCAGGCTTCTATTTCTTGCCGCTCTTCCCAG480                       
Pr oValSerLeuGlyAspGlnAlaSerIleSerCysArgSerSerGln                         
145150155160                                                              
TCTCTGGTCCATTCTAATGGTAACACTTACCTGAACTGGTACCTGCAA 528                      
SerLeuValHisSerAsnGlyAsnThrTyrLeuAsnTrpTyrLeuGln                          
165170175                                                                 
AAGGCTGGTCAGTCTCCGAAGCTTCTGATCTACAAAGTCTCTAACCGC 576                      
LysAlaGlyGlnSerProLysLeuLeuIleTyrLysValSerAsnArg                          
180185190                                                                 
TTCTCTGGTGTCCCGGATCGTTTCTCTGGTTCTGGTTCTGGTACTGAC 624                      
PheSerGlyValProAspArgPheSerGlySerGlySerGlyThrAsp                          
195200205                                                                 
TTCACCCTGAAGATCTCTCGTGTCGAGGCCGAAGACCTGGGTATCTAC 672                      
PheThrLeuLysIleSerArgValGluAlaGluAspLeuGlyIleTyr                          
210215220                                                                 
TTCTGCTCTCAGACTACTCATGTACCGCCGACTTTTGGTGGTGGCACC720                       
Ph eCysSerGlnThrThrHisValProProThrPheGlyGlyGlyThr                         
225230235240                                                              
AAGCTCGAGATTAAACGTTAACTGCAG 747                                           
LysLeuGluIleLysArg                                                        
245                                                                       
(2) INFORMATION FOR SEQ ID NO:7:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 246 amino acids                                               
(B) TYPE: amino acid                                                      
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: protein                                               
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                   
MetGluValGlnLeuGlnGlnS erGlyProGluLeuValLysProGly                         
151015                                                                    
AlaSerValArgMetSerCysLysSerSerGlyTyrIlePheThrAsp                          
202 530                                                                   
PheTyrMetAsnTrpValArgGlnSerHisGlyLysSerLeuAspTyr                          
354045                                                                    
IleGlyTyrIleSerProTyrSerGlyValThrGlyTyrAs nGlnLys                         
505560                                                                    
PheLysGlyLysAlaThrLeuThrValAspLysSerSerSerThrAla                          
65707580                                                                  
Tyr MetGluLeuArgSerLeuThrSerGluAspSerAlaValTyrTyr                         
859095                                                                    
CysAlaGlySerSerGlyAsnLysTrpAlaMetAspTyrTrpGlyHis                          
 100105110                                                                
GlyAlaSerValThrValSerSerSerGlySerSerSerSerGlySer                          
115120125                                                                 
SerSerSerGlySerAspValV alMetThrGlnThrProLeuSerLeu                         
130135140                                                                 
ProValSerLeuGlyAspGlnAlaSerIleSerCysArgSerSerGln                          
145150155 160                                                             
SerLeuValHisSerAsnGlyAsnThrTyrLeuAsnTrpTyrLeuGln                          
165170175                                                                 
LysAlaGlyGlnSerProLysLeuLeuIleTyrLysVa lSerAsnArg                         
180185190                                                                 
PheSerGlyValProAspArgPheSerGlySerGlySerGlyThrAsp                          
195200205                                                                 
Phe ThrLeuLysIleSerArgValGluAlaGluAspLeuGlyIleTyr                         
210215220                                                                 
PheCysSerGlnThrThrHisValProProThrPheGlyGlyGlyThr                          
2252 30235240                                                             
LysLeuGluIleLysArg                                                        
245                                                                       
(2) INFORMATION FOR SEQ ID NO:8:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 1415 base pairs                                               
(B) TYPE: nucleic acid                                                    
(C) STRANDEDNESS: single                                                  
(D) TOPOLOGY: linear                                                      
 (ii) MOLECULE TYPE: syn DNA                                              
(ix) FEATURE:                                                             
(A) NAME/KEY: CDS                                                         
(B) LOCATION: 1..1415                                                     
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                   
ATGGTTCGACCATTGAACTGCATCGTCGCCGTGTCCCAAAATATGGGG48                        
MetValArgProLeuAsnCysIleValAlaValSe rGlnAsnMetGly                         
151015                                                                    
ATTGGCAAGAACGGAGACCGACCCTGGCCTCCGCTCAGGAACGAGTTC96                        
IleGlyLysAsnGlyAspArgProTrpProPr oLeuArgAsnGluPhe                         
202530                                                                    
AAGTACTTCCAAAGAATGACCACAACCTCTTCAGTGGAAGGTAAACAG144                       
LysTyrPheGlnArgMetThrThrThrSerSe rValGluGlyLysGln                         
354045                                                                    
AATCTGGTGATTATGGGTAGGAAAACCTGGTTCTCCATTCCTGAGAAG192                       
AsnLeuValIleMetGlyArgLysThrTrpPheSe rIleProGluLys                         
505560                                                                    
AATCGACCTTTAAAGGACAGAATTAATATAGTTCTCAGTAGAGAACTC240                       
AsnArgProLeuLysAspArgIleAsnIleValLeuSerAr gGluLeu                         
65707580                                                                  
AAAGAACCACCACGAGGAGCTCATTTTCTTGCCAAAAGTTTGGATGAT288                       
LysGluProProArgGlyAlaHisPheLeuAlaLy sSerLeuAspAsp                         
859095                                                                    
GCCTTAAGACTTATTGAACAACCGGAATTGGCAAGTAAAGTAGACATG336                       
AlaLeuArgLeuIleGluGlnProGluLeuAl aSerLysValAspMet                         
100105110                                                                 
GTTTGGATAGTCGGAGGCAGTTCTGTTTACCAGGAAGCCATGAATCAA384                       
ValTrpIleValGlyGlySerSerValTyrGl nGluAlaMetAsnGln                         
115120125                                                                 
CCAGGCCACCTCAGACTCTTTGTGACAAGGATCATGCAGGAATTTGAA432                       
ProGlyHisLeuArgLeuPheValThrArgIleMe tGlnGluPheGlu                         
130135140                                                                 
AGTGACACGTTTTTCCCAGAAATTGATTTGGGGAAATATAAACTTCTC480                       
SerAspThrPhePheProGluIleAspLeuGlyLysTyrLy sLeuLeu                         
145150155160                                                              
CCAGAATACCCAGGCGTCCTCTCTGAGGTCCAGGAGGAAAAAGGCATC528                       
ProGluTyrProGlyValLeuSerGluValGlnGl uGluLysGlyIle                         
165170175                                                                 
AAGTATAAGTTTGAAGTCTACGAGAAGAAAGACGCTAGCGTTACTGTG576                       
LysTyrLysPheGluValTyrGluLysLysAs pAlaSerValThrVal                         
180185190                                                                 
AGCTCCTCCGGATCTTCATCTAGCGGTTCCAGCTCGAGTGGATCTATG624                       
SerSerSerGlySerSerSerSerGlySerSe rSerSerGlySerMet                         
195200205                                                                 
ATTGAACAAGATGGATTGCACGCAGGTTCTCCGGCCGCTTGGGTGGAG672                       
IleGluGlnAspGlyLeuHisAlaGlySerProAl aAlaTrpValGlu                         
210215220                                                                 
AGGCTATTCGGCTATGACTGGGCACAACAGACAATCGGCTGCTCTGAT720                       
ArgLeuPheGlyTyrAspTrpAlaGlnGlnThrIleGlyCy sSerAsp                         
225230235240                                                              
GCCGCCGTGTTCCGGCTGTCAGCGCAGGGGCGCCCGGTTCTTTTTGTC768                       
AlaAlaValPheArgLeuSerAlaGlnGlyArgPr oValLeuPheVal                         
245250255                                                                 
AAGACCGACCTGTCCGGTGCCCTGAATGAACTGCAGGACGAGGCAGCG816                       
LysThrAspLeuSerGlyAlaLeuAsnGluLe uGlnAspGluAlaAla                         
260265270                                                                 
CGGCTATCGTGGCTGGCCACGACGGGCGTTCCTTGCGCAGCTGTGCTC864                       
ArgLeuSerTrpLeuAlaThrThrGlyValPr oCysAlaAlaValLeu                         
275280285                                                                 
GACGTTGTCACTGAAGCGGGAAGGGACTGGCTGCTATTGGGCGAAGTG912                       
AspValValThrGluAlaGlyArgAspTrpLeuLe uLeuGlyGluVal                         
290295300                                                                 
CCGGGGCAGGATCTCCTGTCATCTCACCTTGCTCCTGCCGAGAAAGTA960                       
ProGlyGlnAspLeuLeuSerSerHisLeuAlaProAlaGl uLysVal                         
305310315320                                                              
TCCATCATGGCTGATGCAATGCGGCGGCTGCATACGCTTGATCCGGCT1008                      
SerIleMetAlaAspAlaMetArgArgLeuHisTh rLeuAspProAla                         
325330335                                                                 
ACCTGCCCATTCGACCACCAAGCGAAACATCGCATCGAGCGAGCACGT1056                      
ThrCysProPheAspHisGlnAlaLysHisAr gIleGluArgAlaArg                         
340345350                                                                 
ACTCGGATGGAAGCCGGTCTTGTCGATCAGGATGATCTGGACGAAGAG1104                      
ThrArgMetGluAlaGlyLeuValAspGlnAs pAspLeuAspGluGlu                         
355360365                                                                 
CATCAGGGGCTCGCGCCAGCCGAACTGTTCGCCAGGCTCAAGGCGCGC1152                      
HisGlnGlyLeuAlaProAlaGluLeuPheAlaAr gLeuLysAlaArg                         
370375380                                                                 
ATGCCCGACGGCGAGGATCTCGTCGTGACCCATGGCGATGCCTGCTTG1200                      
MetProAspGlyGluAspLeuValValThrHisGlyAspAl aCysLeu                         
385390395400                                                              
CCGAATATCATGGTGGAAAATGGCCGCTTTTCTGGATTCATCGACTGT1248                      
ProAsnIleMetValGluAsnGlyArgPheSerGl yPheIleAspCys                         
405410415                                                                 
GGCCGGCTGGGTGTGGCGGACCGCTATCAGGACATAGCGTTGGCTACC1296                      
GlyArgLeuGlyValAlaAspArgTyrGlnAs pIleAlaLeuAlaThr                         
420425430                                                                 
CGTGATATTGCTGAAGAGCTTGGCGGCGAATGGGCTGACCGCTTCCTC1344                      
ArgAspIleAlaGluGluLeuGlyGlyGluTr pAlaAspArgPheLeu                         
435440445                                                                 
GTGCTTTACGGTATCGCCGCTCCCGATTCGCAGCGCATCGCCTTCTAT1392                      
ValLeuTyrGlyIleAlaAlaProAspSerGlnAr gIleAlaPheTyr                         
450455460                                                                 
CGCCTTCTTGACGAGTTCTTCTG1415                                               
ArgLeuLeuAspGluPhePhe                                                     
465470                                                                    
(2) INFORMATION FOR SEQ ID NO:9:                                          
(i) SEQUENCE CHARACTERISTICS:                                             
(A) LENGTH: 471 amino acids                                               
(B) TYPE: amino acid                                                      
(D) TOPOLOGY: linear                                                      
(ii) MOLECULE TYPE: protein                                               
(xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                   
MetValArgProLeuAsnCysIleValAlaValSerGlnAsnMetGly                          
1 51015                                                                   
IleGlyLysAsnGlyAspArgProTrpProProLeuArgAsnGluPhe                          
202530                                                                    
LysTyrPheGlnArgMetThr ThrThrSerSerValGluGlyLysGln                         
354045                                                                    
AsnLeuValIleMetGlyArgLysThrTrpPheSerIleProGluLys                          
5055 60                                                                   
AsnArgProLeuLysAspArgIleAsnIleValLeuSerArgGluLeu                          
65707580                                                                  
LysGluProProArgGlyAlaHisPheLeuAlaLysSerLeu AspAsp                         
859095                                                                    
AlaLeuArgLeuIleGluGlnProGluLeuAlaSerLysValAspMet                          
100105110                                                                 
Va lTrpIleValGlyGlySerSerValTyrGlnGluAlaMetAsnGln                         
115120125                                                                 
ProGlyHisLeuArgLeuPheValThrArgIleMetGlnGluPheGlu                          
130 135140                                                                
SerAspThrPhePheProGluIleAspLeuGlyLysTyrLysLeuLeu                          
145150155160                                                              
ProGluTyrProGlyValLeuSer GluValGlnGluGluLysGlyIle                         
165170175                                                                 
LysTyrLysPheGluValTyrGluLysLysAspAlaSerValThrVal                          
180185 190                                                                
SerSerSerGlySerSerSerSerGlySerSerSerSerGlySerMet                          
195200205                                                                 
IleGluGlnAspGlyLeuHisAlaGlySerProAlaAlaTrp ValGlu                         
210215220                                                                 
ArgLeuPheGlyTyrAspTrpAlaGlnGlnThrIleGlyCysSerAsp                          
225230235240                                                              
AlaAl aValPheArgLeuSerAlaGlnGlyArgProValLeuPheVal                         
245250255                                                                 
LysThrAspLeuSerGlyAlaLeuAsnGluLeuGlnAspGluAlaAla                          
 260265270                                                                
ArgLeuSerTrpLeuAlaThrThrGlyValProCysAlaAlaValLeu                          
275280285                                                                 
AspValValThrGluAlaGlyArg AspTrpLeuLeuLeuGlyGluVal                         
290295300                                                                 
ProGlyGlnAspLeuLeuSerSerHisLeuAlaProAlaGluLysVal                          
305310315 320                                                             
SerIleMetAlaAspAlaMetArgArgLeuHisThrLeuAspProAla                          
325330335                                                                 
ThrCysProPheAspHisGlnAlaLysHisArgIleGlu ArgAlaArg                         
340345350                                                                 
ThrArgMetGluAlaGlyLeuValAspGlnAspAspLeuAspGluGlu                          
355360365                                                                 
HisGl nGlyLeuAlaProAlaGluLeuPheAlaArgLeuLysAlaArg                         
370375380                                                                 
MetProAspGlyGluAspLeuValValThrHisGlyAspAlaCysLeu                          
385390 395400                                                             
ProAsnIleMetValGluAsnGlyArgPheSerGlyPheIleAspCys                          
405410415                                                                 
GlyArgLeuGlyValAlaAsp ArgTyrGlnAspIleAlaLeuAlaThr                         
420425430                                                                 
ArgAspIleAlaGluGluLeuGlyGlyGluTrpAlaAspArgPheLeu                          
435440 445                                                                
ValLeuTyrGlyIleAlaAlaProAspSerGlnArgIleAlaPheTyr                          
450455460                                                                 
ArgLeuLeuAspGluPhePhe                                                     
465470                                                                    
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