A random phage peptide library was constructed for the filamentous bac
teriophage fuse5, The library was made by inserting a degenerate oligo
nucleotide which encodes 15 variable amino acids into the NH2-terminal
region of the phage gene III protein, This library, containing 1x10(9
) different phages, was screened with a human immunoglobulin fusion pr
otein containing the extracellular region of human thrombopoietin rece
ptor, Several phages were isolated following four cycles of enrichment
and amplification, These phages specifically bound to the fusion prot
ein, One phage peptide acted as an agonist of the thrombopoietin recep
tor, since it stimulated the proliferation of thrombopoietin-dependent
cells and the differentiation of mouse bone marrow cells to megakaryo
cytes. The amino acid sequence of this peptide is not present in the p
rimary amino acid sequence of thrombopoietin, This discovery may lead
to the design of a small-molecular mimic of thrombopoietin.