Ras-induced serine phosphorylation of the focal adhesion protein paxillin is mediated by the Raf-->MEK-->ERK pathway.

Cells transformed by Ras and Raf display dramatic alterations in cell morphology, adhesion, and intracellular architecture. Consequently, we investigated whether Ras or Raf might influence the behavior of proteins known to be involved in the assembly and integrity of focal adhesion complexes that play a crucial role in many of these processes. We identified Raf-induced serine phosphorylation of the adaptor protein paxillin in a variety of cell types. Raf-induced paxillin serine phosphorylation had no effect on paxillin tyrosine phosphorylation and occurred regardless of whether cells were attached or maintained in suspension. Two sites of serine phosphorylation--S126 and S130--were identified. Mutation of these serines to alanine, either alone or in combination, inhibited the ability of Raf to induce paxillin phosphorylation. These data indicate that paxillin is a target for phosphorylation downstream of the Ras-activated Raf-->MEK pathway. However, we have no evidence to suggest that ERK1/2 are the kinases responsible for Raf-induced paxillin phosphorylation. Furthermore, we did not detect any alterations in the binding of paxillin to a number of focal adhesion proteins following either activation of the Raf-->MEK-->ERK pathway or expression of the S126A/S130A form of paxillin in mammalian cells.