pp125^FAK-Dependent Tyrosine Phosphorylation of Paxillin Creates a High-Affinity Binding Site for Crk

Abstract

Paxillin, a focal-adhesion-associated protein, becomes phosphorylated in response to a number of stimuli which also induce the tyrosine phosphorylation of the focal-adhesion-associated protein tyrosine kinase pp125^FAK. On the basis of their colocalization and coordinate phosphorylation, paxillin is a candidate for a substrate of pp125^FAK. We describe here conditions under which the phosphorylation of paxillin on tyrosine is pp125^FAK dependent, supporting the hypothesis that paxillin phosphorylation is regulated by pp125^FAK. pp125^FAK must localize to focal adhesions and become autophosphorylated to induce paxillin phosphorylation. Phosphorylation of paxillin on tyrosine creates binding sites for the SH2 domains of Crk, Csk, and Src. We identify two sites of phosphorylation as tyrosine residues 31 and 118, each of which conforms to the Crk SH2 domain binding motif, (P)YXXP. These observations suggest that paxillin serves as an adapter protein, similar to insulin receptor substrate 1, and that pp125^FAK may regulate the formation of signaling complexes by directing the phosphorylation of paxillin on tyrosine.