To obtain information about the functional importance of amino acids required for effective erythropoietin (EPO) mimetic action, the conserved residues of a peptide mimetic of EPO, recently discovered by phage display, were subjected to an alanine replacement strategy. Further, to identify a minimal mimetic peptide sequence, a series of truncation peptides has been generated. One EPO mimetic peptide sequence, EMP1, was targeted and more than 25 derivatives of this sequence were evaluated for their ability to compete with [125I]EPO for receptor binding and for their ability to support the proliferation of two EPO-responsive cell lines. Two hydrophobic amino acids, Tyr4 and Trp13, appear essential for mimetic action, and aromatic residues appear to be important at these sites. These findings are consistent with the previously reported X-ray crystal structure of EMP1 complexed with the extracellular domain of the EPO receptor (EPO binding protein; EBP). In our efforts to define the structural elements required for EPO mimetic action, a 13 amino acid peptide was identified which possesses mimetic properties and contains a minimal agonist epitope. The ability of this peptide to effectively serve as a mimetic capable of the induction of EPO-responsive cell proliferation appears to reside within a single residue, equivalent to position Tyr4 of EMP1, when present in a sequence that includes the cyclic core peptide structure. Although these peptides are less potent than EPO, they should serve as an excellent starting point for the design of compounds with EPO mimetic activity.

The erythropoietin receptor (EPOR) is a member of a family of cytokine and growth factor receptors that share conserved features in their extracellular and cytoplasmic domains. We have used site-specific mutagenesis within the extracellular domain of the EPOR to search for amino acid residues involved in erythropoietin (EPO) binding. Mutant proteins were expressed in bacteria as soluble EPO binding proteins (EBP) and characterized for EPO binding activity in a number of different assays. Substitution of phenylalanine at position 93 (Phe93) with alanine (F93A mutation) resulted in a drastic reduction in EPO binding in the EBP. More conservative tyrosine or tryptophan substitutions at Phe93 resulted in much less dramatic effects on EPO binding. Biophysical studies indicated that the F93A mutation does not result in gross structural alterations in the EBP. Furthermore, the F93A mutation in full-length EPOR expressed in COS cells abolished detectable EPO binding. This was not a result of processing or transport defects, since mutant receptor was present on the surface of the cells. Mutations in the region immediately around Phe93 and in residues homologous to other reported ligand binding determinants of the cytokine receptor family had small to moderate effects on EPO binding. These data indicate that Phe93 is a critical EPO binding determinant of the EPOR. Furthermore, since Phe93 aligns with critical ligand binding determinants in other receptors of the cytokine receptor family, these data suggest that receptors of this family may use common structural motifs to bind their cognate ligands.

The potency of previously isolated peptides that are modest agonists of the EPO receptor was dramatically increased by PEG-induced dimerization. The EPO receptor is thought to be dimerized during activation, so our results are consistent with the proposed 2:2 receptor : peptide stoichiometry. The conversion of an inactive peptide into an agonist further supports the idea that dimerization can mediate receptor activation.

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