Use of random peptide phage-displayed libraries for studying protein phosphorylation and phosphotyrosine-dependent protein-protein interactions

Protein-phosphorylation is a post-translation modification that alters the binding or the enzymatic properties of the modified proteins. In particular, phosphorylation of tyrosine residues is a crucial event in protein-protein interactions occurring during intracellular signal transduction. Several approaches have been used to study these interactions. Combinatorial libraries have proven to be a valuable tool to characterize ligand-target interactions and to analyze binding specificity. Phage displayed libraries offer the advantage of coupling the capsid exposed ligand to its coding sequence, inserted in the phage genome. Furthermore, peptides displayed on phage capsid can be subjected to enzymatic modification by incubation with appropriate enzymes. Therefore, using a receptor specific for the modified products it is possible to select clones bearing the peptides with the desired properties. Here we describe a method to generate "dedicated" libraries, displaying phosphotyrosine containing peptides, using cytosolic phosphotyrosine kinases (PTKs). These are specialized enzymes, implicated in several events of transduction of external signals into the cells. Random peptide phage libraries can be phosphorylated to different extent to assess the PTK substrate requirements or to analyze phosphotyrosine-dependent protein-protein interactions. This approach is useful for studies aimed at understanding the rules underlying protein interaction mechanisms and to design specific inhibitors that could compete in these interactions.