PP125(FAK)-DEPENDENT TYROSINE PHOSPHORYLATION OF PAXILLIN CREATES A HIGH-AFFINITY BINDING-SITE FOR CRK

Paxillin, a focal-adhesion-associated protein, becomes phosphorylated in response to a number of stimuli which also induce the tyrosine phos phorylation of the focal-adhesion-associated protein tyrosine kinase p p125(FAK). On the basis of their colocalization and coordinate phospho rylation, paxillin is a candidate for a substrate of pp125(FAK). We de scribe here conditions under which the phosphorylation of paxillin on tyrosine is pp125(FAK) dependent, supporting the hypothesis that paxil lin phosphorylation is regulated by pp125(FAK) pp125(FAK) must localiz e to focal adhesions and become autophosphorylated to induce paxillin phosphorylation. Phosphorylation of paxillin on tyrosine creates bindi ng sites for the SH2 domains of Crk, Csk, and Src. We identify two sit es of phosphorylation as tyrosine residues 31 and 118, each of which c onforms to the Crk SH2 domain binding motif, (P)YXXP. These observatio ns suggest that paxillin serves as an adapter protein, similar to insu lin receptor substrate 1, and that pp125(FAK) may regulate the formati on of signaling complexes by directing the phosphorylation of paxillin on tyrosine.