A combinatorial method for constructing libraries of long peptides displayed by filamentous phage.

We describe the construction and screening of a random peptide library displayed by filamentous phage. The peptides are expressed in multiple copies on the filamentous phage M13 as amino-terminal fusions with the major coat protein, the product of gene VIII. These libraries are efficiently screened for reactive peptides, using a combination of panning in solution followed by a plaque lift assay. Advantages of this system are that both high- and low-affinity phage clones are simultaneously identified and the analysis of non-reactive phage is minimized. The vector system utilized to construct this library enables it to be used for the construction of peptide libraries employing a combinatorial cloning strategy. This feature makes it especially suitable for construction of peptide libraries using codon-based oligonucleotide synthesis. The vectors also allow rapid optimization and modification of lead peptides by codon-based mutagenesis. A 20-amino acid long random peptide library of 1 x 10(9) members was constructed and screened for peptides that bound to (i) a monoclonal antibody recognizing the amino-terminus of beta-endorphin; (ii) a monoclonal antibody recognizing a peptide epitope derived from the v-ros oncogene product; and (iii) the constant region of murine IgG2b. The approach described here provides a means for the construction of customized libraries that can be screened with a variety of target molecules.