Binding epitope of somatostatin defined by phage-displayed peptide libraries.

We have developed a versatile phagemid system to display peptides on the surface of M13 bacteriophage at a copy number which approaches monovalency. In this system, a phagemid encodes a peptide fused to the ammo-terminus of the second domain (dII) of the minor coat protein pIII under control of the inducible lac promoter. The fusion protein is displayed in combination with several copies of wild-type pIII on the surface of phage. Two diverse random octapeptide libraries, one linear and one which contained flanking cysteines capable of forming disulflde bridges, were were generated using an in vitro mutagenesis approach and affinity selected on an anti-somatostatin mAb. Peptides with high affinity for the mAb were enriched only from the cyclic library and the tetrapeptide, FWKT, was identified by consensus as the binding epitope. The selected peptides exhibited not only the primary amino acid sequence but also shared structural features with somatostatin. One peptide, CRFWKTWC, also exhibited nanomolar affinities for the five known somatostatin receptor subtypes. This system can easily be adapted to display individual peptides or a wide range of custom peptide libraries.