Rho stimulates tyrosine phosphorylation of focal adhesion kinase, p130 and paxillin.

The small GTP-binding protein Rho rapidly stimulates the formation of focal adhesions and actin stress fibres when microinjected into serum-starved Swiss 3T3 fibroblasts. This response is inhibited by tyrosine kinase inhibitors. Addition of growth factors such as lysophosphatidic acid and bombesin to Swiss 3T3 cells stimulates a similar response, which is dependent on endogenous Rho proteins. To investigate signalling events regulated by Rho, we have scrape loaded Rho into serum-starved cells. Activated Rho stimulates the tyrosine phosphorylation of a number of proteins, including three proteins known to localise to focal adhesions, pp125FAK, p130 and paxillin. Rho-induced phosphorylation of pp125FAK, p130 and paxillin is observed in the absence of stress fibre formation and is, therefore, independent of Rho-induced actin polymerisation. We propose that the tyrosine kinase, pp125FAK, and the putative adapter proteins, paxillin and p130, are components of a Rho-regulated signal transduction pathway, and that these protein tyrosine phosphorylation events are likely to be important for the regulation of focal adhesion formation.