According to the completion of the genome project for both human and various target animals and plants and the development of bioinformatics, mRNA has been proved to act as a messenger transmitting genetic information of DNA to a protein and at the same time to regulate the gene expression.
Since the beginning of year 2000, micro-RNA (miRNA) or its precursor pre-miRNA has been proved to regulate 10-20% of gene functions. In prokaryotes, some parts of mRNA are directly bound with a metabolite, suggesting that it has ribo-switch that regulates the functions of metabolite related protein. It has been also confirmed that the secondary structure of untranslating region of mRNA of higher animals regulates mRNA stability and translation efficiency.
The numbers of such RNA that has regulatory function are considerable. The structure of the RNA is composed of a series of hairpin structures in which stems and loops (basic motif) are arranged serially. It is also presumed that pharmacophore of natural miRNA or biologically significant mRNA might be the specific stem-loop (hairpin) structure, which is less than 30 nt, considering the size of binding region of ribo-switch to a compound.
Although every mRNA has been proved to have secondary structure, the confirmed mRNA hairpin structures are very few, which are only exemplified by Rev Response Element (RRE) of HIV-1, trans-activation response element (TAR) of HIV-1, Thymidylate Synthase mRNA of various tumor cells and Ion Responsive Element (IRE) involved in homeostasis of iron ion and dementia, which is attributed to the lack of biological methods, the lack of information on RNA-binding protein and insufficient information on hairpin structure, etc. However, RNA targets having the hairpin structure are highly expected to be major biological targets and so great effort has to be made to find out ligands against such pouring RNA targets.
Polyamines having several amine groups have been produced by imitating the conventional RNA pro-binding aminoglycoside compound, which have also been confirmed to be bound with RNA targets very well (Lawton et al., J. Am. Chem. Soc., 126: 12762-12763, 2004). Successively, morphology of a protein was observed according to the methylation of an amino acid containing amine group existing in natural RNA binding protein (Das and Frankel, Biopolymers, 70: 80-85, 2003). From the investigation of natural RNA binding proteins and binding peptides was confirmed that lysine or arginine which contains a large number of amine groups was included in the peptides and such proteins or peptides were already methylated considerably (Tan and Fankel, Proc Natl Acad Sci USA. 92, 5282-5286, 1995). It was additionally confirmed that RNA binding capacity was increased as methylation of arginine of RNA binding protein proceeded (Liu and Dreyfuss, Mol Cell Biol. 15, 2800-2808, 1995).
There have been a great numbers of reports on RNA binding capacity of a peptide containing amine group or RNA binding capacity depending on the methylation of amine group and natural RNA binding peptides and methylation of them. However, there was no report yet on synthesized RNA binding peptide or specific RNA binding capacity of a methylated peptide. Therefore, to obtain a peptide specifically binding to RNA, the present inventors prepared a peptide composed of 15 amino acids containing 7 alpha-helical lysines. In the meantime, to ensure the diversity of such peptides, a library was constructed by using the combination of methylated lysines. Then, the present inventors completed this invention by selecting peptides showing the strongest binding capacity to RRE-RNA of HIV-1 from those synthesized from the library. The peptide of the present invention thus has not only strong but also specific RRE RNA binding capacity, so that it can be used as a therapeutic agent for AIDS.