INHIBITION OF CELL SPREADING BY EXPRESSION OF THE C-TERMINAL DOMAIN OF FOCAL ADHESION KINASE (FAK) IS RESCUED BY COEXPRESSION OF SRC OR CATALYTICALLY INACTIVE FAK - A ROLE FOR PAXILLIN TYROSINE PHOSPHORYLATION

pp125(FAK) is tyrosine kinase that appears to regulate the assembly of focal adhesions and thereby promotes cell spreading on the extracellu lar matrix. In some cells, the C terminus of pp125(FAK) is expressed a s a separate protein, pp41/43(FRNK). We have previously shown that ove rexpression of pp41/43(FRNK) inhibits tyrosine phosphorylation of pp12 5(FAK) and paxillin and, in addition, delays cell spreading and focal adhesion assembly. Thus, pp41/43(FRNK) functions as a negative inhibit or of adhesion signaling and provides a tool to dissect the mechanism by which pp125(FAK) promotes cell spreading. We report here that the i nhibitory effects of pp41/43(FRNK) expression can be rescued by the co -overexpression of wild-type pp125(FAK) and partially rescued by catal ytically inactive variants of pp125(FAK). However, coexpression of an autophosphorylation site mutant of pp125(FAK), Which fails to bind the SH2 domain of pp60(c-Src), or a mutant that fails to bind paxillin di d not promote cell spreading. In contrast, expression of pp41/43(FRNK) and pp60(c-Src) reconstituted cell spreading and tyrosine phosphoryla tion of paxillin but did so without inducing tyrosine phosphorylation of pp125(FAK). These data provide additional support for a model where by pp125(FAK) acts as a ''switchable adaptor'' that recruits pp60(c-Sr c) to phosphorylate paxillin, promoting cell spreading. In addition, t hese data point to tyrosine phosphorylation of paxillin as being a cri tical step in focal adhesion assembly.