V-SRC-INDUCED DEGRADATION OF FOCAL ADHESION KINASE DURING MORPHOLOGICAL TRANSFORMATION OF CHICKEN-EMBRYO FIBROBLASTS

Morphological transformation of cells by the v-Src tyrosine kinase is incompletely understood, However, it is independent of nuclear functio ns and probably involves phosphorylation of targets associated with th e cytoskeleton and focal adhesions, structures which tether the cytosk eleton to the points of cell attachment. v-Src activity both stimulate s tyrosine phosphorylation of a tyrosine kinase present in focal adhes ions (focal adhesion kinase or pp125(FAK)) and disrupts focal adhesion s, leading to cell rounding and detachment. However, pp125(FAK) is als o phosphorylated on tyrosine as a result of integrin stimulation which induces quite different biological consequences including the organis ation of focal adhesions when cells spread on fibronectin (reviewed in Schaller and Parsons, 1993). To address this paradox, we examined cha nges in pp125(FAK) during activation and shut-off of temperature sensi tive mutant v-Src proteins that induce varying degrees of transformati on in chick embryo fibroblasts. An efficiently transforming v-Src muta nt initially stimulated pp125(FAK) tyrosine phosphorylation, but induc ed subsequent pp125(FAK) degradation prior to the onset of cell roundi ng and detachment. v-Src mutants which are impaired in their ability t o induce morphological transformation were much less efficient at indu cing degradation of pp125(FAK). Moreover, cell spreading during restit ution of normal morphology did not require detectable tyrosine phospho rylation of pp125(FAK), or its potential substrate paxillin, suggestin g that pp125(FAK) may function more in the turnover of focal adhesions than in their assembly.