the present invention will now be described in more detail . the present invention is processed by steps of acquisition and cloning of genes of a target fusion protein , construction of expression vectors of a target gene , transfection of animal cells and eatp expression , and purification of the expressed eatp and activity measurement . complementary dna ( cdna ) of epo can be acquired by employing a conventional reverse transcription - polymerase chain reaction ( rt - pcr ) technique using a rt - pct premix kit available from bioneer corp ., korea , in which primers ep1 and ec2 complementary to both terminals of epo cdna previously prepared from a cdna library of the human embryonic liver ( available from invitrogen corp .) are used . epo cdna is cloned into a cloning vector pgem - t ( promega corp . ), which is termed pgemt - epo , and its base sequence is identified for use as template in subsequent operations . ctp variant genes of a hcg β subunit used in the present invention are obtained by artificial synthesis and self - priming pcr . the synthesized gene fragments are ea1 , a2 , a3 and a4 : ( seq id no : 5 ) ea1 : aggggaggcctgcaggacaggggactcctcttccg ( seq id no : 6 ) a2 : ggaagg gc ggggggaggggcctt g g c ggaagagga ( seq id no : 7 ) a3 : cc gc ccttccaagccca g cccgactcccggggccg ( seq id no : 8 ) a4 : ttattgtgggaggatcggggtgtc g g c gggccccg each 1 μl of four genes is taken ( 50 pmole / μl ) to be subjected to pcr using a high fidelity taq system ( boehringer manheim corp .). gene fragments ( modified ctp genes ) of approximately 100 bps in size are identified in a 1 % agarose gel . these genes encode a peptide obtained by replacing 4 ser residues at positions 121 , 127 , 132 and 138 among 28 carboxy terminal amino acids at positions 118 - 145 of a hcg β subunit , with ala residues ( see fig1 ). pcr is performed using a pgemt - epo as template and ep1 and ec2 as primers , yielding only epo genes . then , pcr is further performed using both the epo genes and the modified ctp genes as templates and using ep11 and ep22 primers by means of the high fidelity taq system , thereby acquiring a desired fusion protein with gene fragments of approximately 630 bps ( to be termed eatp genes ). these genes are cloned into pgem - t cloning vectors and then base sequences are identified ( to be termed pgemt - eatp ) ( see fig3 ). pcdna3 . 1 vector ( invitrogen corp .) is used as an expression vector . both termini of the eatp gene in pgemt - eatp have hind iii and bamh i restriction enzyme sites derived from the primers ep11 and ep22 . pcdna3 . 1 and the obtained pgemt - eatp are treated with hind iii and bamh i . the linearized pcdna3 . 1 and eatp gene are obtained from an agarose gel using a qiagen elution kit , followed by ligation , thereby transforming e . coli nm522 . plasmids are isolated from colonies resulting after incubation overnight in an lb - ampicillin solid medium , and are treated with the restriction enzymes hind iii and bamh i . then , only colonies inserted eatp are selected by 1 % agarose gel electrophoresis . the resultant plasmids are termed pcdna3 . 1 - eatp ( see fig3 ). cho cells ( dg44 ) are grown in a 60 mm dish to prepare 40 - 80 % confluent cells ( 1 - 4 × 10 5 cells / 60 mm dish ). 3 μl of a superfection reagent ( boehringer manheim corp .) and 97 μl of media ( α - mem with media , serum free and non - antibiotic ) are mixed sufficiently , and approximately 2 μg of a plasmid pcdna3 . 1 - eatp dna ( more than 0 . 1 μg / μl ) and 0 . 2 μg of a dihydrofolate reductase ( dhfr ) gene containing vector pltrdhfr26 ( atcc37295 ) are added to the resultant mixture and reacted at room temperature for 5 - 10 minutes to then be added to the cells . after one day , the media are replaced with α - mem without media ( containing 500 μg / ml g418 ) with 10 % fbs . the cells are replenished with media with 500 μg / ml g418 and cultured for 7 - 10 days . then , cells without g418 - resistant genes and cells of negative control group all die . after cells selected from the g418 media are sufficiently cultured , an eatp protein expressed from the media is confirmed using an epo elisa kit ( boehringer manheim corp .). using an anti - epo monoclonal antibody ( r & amp ; d inc . ), affinity resins for purification are prepared as follows . 0 . 3 g of cnbr - activated sepharose 4b is swollen in 1 mm hcl for 20 minutes and loaded onto a column , followed by washing with 1 mm hcl . then , the resultant resin is further washed in 4 ml coupling buffer solution ( 0 . 1 m nahco 3 and 0 . 5 m nacl , ph 8 . 3 ) transferred to a tube and immediately mixed with anti - epo monoclonal antibody in the coupling buffer solution ( 500 μg / vial ), and then reacted at room temperature for 2 hours . at this time , the tube is sufficiently shaken . then , the resultant product is replaced with a blocking buffer ( 0 . 2 m glycine , ph 8 . 0 ) and reacted at room temperature for 2 hours with agitation . the resultant resin is washed sequentially with a 6 . 5 ml coupling buffer solution , a 6 . 5 ml acetate buffer solution ( 0 . 1 m acetic acid , 0 . 5 m nacl , ph 4 ) and a 6 . 5 ml coupling buffer solution . the prepared resin is packed into a column and then subjected to purification as follows . cells are grown in a serum - free medium for one day and then only the medium is concentrated approximately 5 times using a ultrafiltration filter , for example , centriprep ( having a nominal molecular weight cutoff of 10 , 000 ) ( millipore corp .). then , the concentrated solutions are loaded onto a column equilibrated with phosphate buffered saline ( pbs ) at a flow rate about 20 ml / hr and washed again with pbs . the target proteins are eluted in an elution buffer solution ( 0 . 1 m glycine , ph 2 . 8 ) and then immediately titrated with 1 m tris solution for adjustment to ph 7 . 5 . the purity of the purified eatp is 97 % or higher as verified by sds - page and silver staining ( see fig4 ). biological activities of the expressed and appropriately purified epo and eatp are measured by a bioassay using spleen cells of a mouse treated with phenylhydrazine . the result shows that the activity of eatp is higher than that of epo , suggesting that the presence of added carboxy terminals in eatp does not inhibit the activity of epo . in order to confirm whether the prepared candidate materials actually have a longer in vivo half - life , pharmacokinetic tests are performed on mice . here , the candidate materials are intravenously administered to four mice at dosages of 20 units for each mouse . to evaluate the concentration profile in blood , blood is gathered from the mice and the concentration in the gathered blood is measured using an eia kit ( boehringer manheim corp .). the pharmacokinetic test performed on mice shows that candidate material eatp has a much longer half - life than the control material epo ( see fig5 ). the present invention is further illustrated in the following examples , which should not be misconstrued as limiting the scope of the invention . cdna of epo was acquired by employing a conventional rt - pcr technique using a rt - pct premix kit ( bioneer corp ., korea ), in which primers ep1 and ec2 complementary to both terminals of epo cdna previously prepared from a cdna library of the human embryonic liver ( invitrogen corp .) were used . 30 cycles of pcr reactions were performed in the conditions of 35 seconds at 55 ° c . ( annealing ), 40 seconds at 72 ° c ., and 20 seconds at 94 ° c ., yielding epo cdna . the obtained epo cdna was cloned into a cloning vector pgem - t ( promega corp .). in other words , the product of pcr was eluted from 1 % agarose , ligated to pgem - t , followed by transformation of e . coli nm522 . after overnight incubation in an x - gal / iptg smeared lb - ampicillin solid medium , plasmid dna was isolated from white colonies and reacted with restriction enzymes sac i and sac ii to select colonies having epo cdna inserts therein . the obtained vector was termed pgemt - epo and its base sequence was identified for use as a template in subsequent processes . modified ctp genes of a hcg β subunit were obtained by artificial synthesis and self - priming pcr . the synthesized gene fragments were ea1 , a2 , a3 and a4 . each 1 μl of four genes was taken ( 50 pmole / μl ) to be subjected to 15 cycles of pcr using a high fidelity taq system ( boehringer manheim corp .) under conditions of 40 seconds at 55 ° c . ( annealing ), 40 seconds at 72 ° c . and 20 seconds at 94 ° c . gene fragments of approximately 100 bps in size were identified in a 1 % agarose gel ( modified ctp genes ). these genes encode a peptide obtained by replacing 4 ser residues at positions 121 , 127 , 132 and 138 among 28 carboxy terminal amino acids of a hcg β subunit , with ala residues ( fig1 ). pcr was performed using a pgemt - epo template and ep1 and ec2 primers , yielding only epo genes . then , 30 cycles of pcr were further performed using both the epo genes and the modified ctp genes obtained as templates and using ep11 and ep22 primers by means of the high fidelity taq system under conditions of 42 seconds at 57 ° c . ( annealing ), 60 seconds at 72 ° c ., and 20 seconds at 94 ° c . thus , approximately 630 bps of fused gene fragments were obtained ( to be termed eatp genes ). these genes were cloned into pgem - t using above mentioned method ( to be termed pgemt - eatp ), and its sequences were identified . pcdna3 . 1 vector ( invitrogen ) was used as expression vector . both terminus of the eatp gene in pgemt - eatp have hind iii and bamh i restriction sites derived from the primers ep11 and ep22 . pcdna3 . 1 and the obtained pgemt - eatp were treated with the restriction enzymes hind iii and bamh i . the linearized pcdna3 . 1 and eatp gene were obtained from an agarose gel using a qiagen elution kit , followed by ligation , thereby transforming e . coli nm522 . plasmids were isolated from colonies resulting after incubating overnight in an lb - ampicillin solid medium , and were treated with the restriction enzymes hind iii and bamh i . then , only colonies inserted eatp were selected by 1 % agarose gel electrophoresis . the resultant plasmids were termed pcdna3 . 1 - eatp ( see fig3 ). cho cells ( dg44 ) were grown in a 60 mm dish to prepare 40 - 80 % confluent cells ( 1 - 4 × 10 5 cells / 60 mm dish ). 3 μl of a superfection reagent ( boehringer manheim corp .) and 97 μl of media ( α - mem with media , serum - free and non - antibiotic ) were mixed sufficiently , and approximately 2 μg of a plasmid pcdna3 . 1 - eatp dna ( more than 0 . 1 μg / μl ) and 0 . 2 g of a dihydrofolate reductase ( dhfr ) gene containing vector pltrdhfr26 ( atcc37295 ) were added to the resultant mixture and reacted at room temperature for 5 - 10 minutes and then added to the cells . after one day elapsed , the media were replaced with α - mem without media ( containing 500 μg / ml g418 ) with 10 % fbs . the cells were replenished with media containing 500 μg / ml g418 , and cultured for 7 - 10 days . then , cells without g418 - resistant genes and cells of negative control group all died . after cells selected from the g418 media were sufficiently cultured , eatp protein expressed from the media was confirmed using an epo elisa kit ( boehringer manheim corp .). using an anti - epo monoclonal antibody ( r & amp ; d inc . ), affinity resins for purification were prepared as follows . 0 . 3 g of cnbr - activated sepharose 4b was swollen in 1 mm hcl for 20 minutes and loaded onto a column , followed by washing with 1 mm hcl . then , the resultant resin was further washed in 4 ml coupling buffer solution ( 0 . 1 m nahco 3 and 0 . 5 m nacl , ph 8 . 3 ), transferred to a tube and immediately mixed with anti - epo monoclonal antibody in the coupling buffer solution ( 500 μg / vial ), and then reacted at room temperature for 2 hours with agitation . at this time , the tube was sufficiently shaken . then , the resultant product was replaced with a blocking buffer ( 0 . 1 m glycine , ph 8 . 0 ) and reacted at room temperature for 2 hours . the resultant product was washed sequentially with a 6 . 5 ml coupling buffer solution , a 6 . 5 ml acetate buffer solution ( 0 . 1 m acetic acid , 0 . 5 m nacl , ph 4 ) and a 6 . 5 ml coupling buffer solution . the prepared resin was packed into a column and then subjected to purification as follows . cells were grown in a serum - free medium for one day and then only the medium was concentrated approximately 5 times using a ultrafiltration filter of centriprep ( having a nominal molecular weight cutoff of 10 , 000 ) ( millipore corp .). then , the concentrated solutions were loaded onto a column equilibrated with pbs at a flow rate about 20 ml / hr and washed again with pbs . the target proteins were eluted in an elution buffer solution ( 0 . 1 m glycine , ph 2 . 8 ) and then immediately titrated with 1 m tris solution for adjustment to ph 7 . 5 . the purity of the purified eatp was 97 % or higher as verified by sds - page and silver staining ( see fig4 ). phenylhydrazine was administered to a mouse once a day for 2 days at the dose of 60 mg / kg . after 3 days , an enlarged spleen was isolated from the mouse and pulverized with a homogenizer to gain spleen cells . the spleen cells were diluted to a concentration of 6 × 10 6 cells / ml and each 100 μl of the diluted sample was transferred to a 96 - well plate . standard epo ( 0 - 500 mu / ml ) and the expressed epo and eatp ( each 100 mu / ml ) were added to the respective wells . then , the plate was stored in a co 2 incubator maintained at 37 ° c . for 22 hours . 50 μl of dimethyl - 3 h - thymidine ( 20 μci / ml ) was added to each well . the resultant plate was further reacted for 2 hours , and then the sample solutions of each well were adsorbed to a glass filter ( nunc 1 - 73164 ). the filter was washed three times with saline and the radioactivity of the filter was measured using a beta ( β ) counter . the measurements showed that the activity of eatp was substantially equal to or slightly higher than that of epo , suggesting that the presence of added carboxy terminals in eatp does not inhibit the activity of epo . in order to confirm whether the prepared candidate materials actually have a longer in vivo half - life , pharmacokinetic tests were performed on mice . here , the fusion protein purified by the method described in example 5 was intravenously administered to four mice at dosages of 20 units for each mouse . to evaluate the concentration profile in blood , blood was gathered from the mice at regular time intervals , that is , every 30 minutes at the beginning and every 2 hours after 2 hours . and the concentration in the gathered blood was determined using an eia kit ( boehringer manheim .). the result of the pharmacokinetic test is shown in fig5 . as shown in fig5 the candidate material eatp had a much longer ( more than 2 . 5 times longer ) half - life than the control material epo . according to the present invention , the in vivo activity of epo can be enhanced by increasing the in vivo half - life while retaining the intrinsic activity of the epo with its own amino acid , i . e ., without increasing the sugar chain content of epo . met gly val his glu cys pro ala trp leu trp leu leu leu ser leu ile cys asp ser arg val leu glu arg tyr leu leu glu ala lys glu asn ile thr val pro asp thr lys val asn phe tyr ala trp lys arg gln pro trp glu pro leu gln leu his val asp lys ala val ser gly thr ala asp thr phe arg lys leu phe arg val tyr ser asn phe leu
provided is a fusion protein comprising , at its carboxy terminal of human erythropoietin , a mutant having one to four amino acid substitutions in the carboxy terminal peptide fragment of a human chorionic gonadotropin β subunit , for increasing an in vivo half - life activity of epo . the in vivo half - life can be greatly elongated while retaining the intrinsic activity of the epo , without increasing the sugar chain content .