Selection of phage displayed peptides from a random 10-mer library recognising a peptide target.
Peptide display libraries are powerful tools in the search for detailed information about protein-protein interactions. Usual targets for isolation of phage displayed peptide ligands include antibodies, various receptors, other full size proteins or larger fragments thereof. Smaller protein fragments such as synthetic peptides have not been reported as targets for screening of peptide display libraries. To investigate whether a protein target used for screening of a peptide display library could be scaled down to peptide size. As the peptide target we wanted to use a sequence derived from the cytosolic tail of MHC class II associated invariant chain containing a leucine class endosomal sorting signal, known to be recognised as an autonomous functional unit during targeting of class II complexes to antigen processing compartments. A screening procedure where a synthetic 15-mer invariant chain peptide was coupled to a methacrylate matrix of high binding capacity was developed, and three rounds of selection were performed from a random 10-mer fUSE5 display library. The peptide display library was successfully enriched for phage clones with affinity for the invariant chain peptide. Furthermore, the binding phage clones were able to distinguish between a functional and a mutated form of the target. These clones therefore displayed possible peptide mimetics of signal recognition sites in the cellular sorting machinery. The size of a protein target may be scaled down to peptide size and be recognised by a 10-mer peptide displayed on filamentous phage. This approach may particularly be useful when the peptide target contains a functional unit for recognition.