Abstract

From a filamentous phage library displaying random hexapeptides, we selected clones displaying peptides that bind S-protein, a 104-amino-acid (aa) fragment of bovine pancreatic ribonuclease (RNase). The selected peptides show a sequence motif, (F/Y)NF(E/V)(I/V)(L/V), that bears little resemblance to S-peptide, a 20-aa fragment of RNase that is S-protein's natural ligand. One of the displayed peptides, YNFEVL, was synthesized chemically and shown by isothermal titration calorimetry to bind S-protein with a dissociation equilibrium constant of 5.5 βM at 25°C, an affinity comparable to that of previously studied S-peptide variants. The YNFEVL peptide is an antagonist of S-peptide, in that it blocks the ability of S-peptide to restore enzyme activity to S-protein. The S-protein/S-peptide system preserves the essential features of a pharmacologically significant receptor/hormone couple, and the S-peptide antagonist can therefore be regarded as a new RNase-specific ‘drug’. This work illustrates the potential value of phage display libraries for discovering novel classes of pharmaceuticals.

Keywords: Drug discovery; random hexapeptide library; filamentous bacteriophage; peptide epitope; hormone receptor; titration calorimetry

Abbreviations: A, absorbance (1 cm); aa, amino acid(s); BSA, bovine serum albumin; cpm, counts per min; ΔH_b, enthalpy of binding interaction; K_d, dissociation equilibrium constant; pill, coat protein encoded by filamentous phage gene III; RNase, bovine pancreatic ribonuclease; S-peptide and S-protein, fragments produced by limited cleavage of RNase with subtilisin; S15, first 15 aa of S-peptide; TBS, 0.15 M NaCl/50mM Tris·HCl pH 7.5; TBS/Tween, TBS with 0.5% (v/v) Tween-20