A RIBONUCLEASE S-PEPTIDE ANTAGONIST DISCOVERED WITH A BACTERIOPHAGE DISPLAY LIBRARY

From a filamentous phage library displaying random hexapeptides, we se lected clones displaying peptides that bind S-protein, a 104-amino-aci d (aa) fragment of bovine pancreatic ribonuclease (RNase). The selecte d peptides show a sequence motif, (F/Y)NF(E/V)(I/V)(L/V), that bears l ittle resemblance to S-peptide, a 20-aa fragment of RNase that is S-pr otein's natural ligand. One of the displayed peptides, YNFEVL, was syn thesized chemically and shown by isothermal titration calorimetry to b ind S-protein with a dissociation equilibrium constant of 5.5 muM at 2 5-degrees-C, an affinity comparable to that of previously studied S-pe ptide variants. The YNFEVL peptide is an antagonist of S-peptide, in t hat it blocks the ability of S-peptide to restore enzyme activity to S -protein. The S-protein/S-peptide system preserves the essential featu res of a pharmacologically significant receptor/hormone couple, and th e S-peptide antagonist can therefore be regarded as a new RNase-specif ic 'drug'. This work illustrates the potential value of phage display libraries for discovering novel classes of pharmaceuticals.