Signal transduction through the GTP-binding proteins Rac and Rho

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Abstract
Actin reorganization is an early response to many extra- cellular factors. In Swiss 3T3 fibroblasts, the Ras-related GTP-binding proteins Rho and Rac act as key signal trans- ducers in these responses: Rho is required for growth factor-induced formation of stress fibres and focal adhesions, whereas membrane ruffling is regulated by Rac proteins. Several proteins that act as GTPase activating proteins (GAPs) for Rho-related proteins have been iden- tified, and these could act either as targets or down-regu- lators of Rho or Rac in cells. In vitro, the GAP domain of pl90 has a striking preference for Rho as a substrate, and when microinjected into Swiss 3T3 cells it inhibits stress fibre formation but not membrane ruffling induced by growth factors. BcrGAP acts on Rac but not Rho in vitro, and specifically inhibits membrane ruffling in vivo. Finally, RhoGAP acts preferentially on the Rho-related protein G25K/Cdc42Hs in vitro, but can inhibit Rho-mediated responses in vivo. These results suggest that p!90, Bcr and RhoGAP play specific roles in signalling pathways through different Rho family members. The mechanisms underlying Rho-regulated stress fibre formation have been investigated further by analysing the role of other signals known to be activated by lysophos- phatidic acid (LPA). Neither activation of PK-C, increased intracellular Ca2+, decreased cAMP levels or Ras activa- tion appear to mediate stress fibre formation. However, LPA stimulates tyrosine phosphorylation of a number of proteins, including the focal adhesion kinase, pp125FAK, and genistein, a tyrosine kinase inhibitor, prevents this increase in tyrosine phosphorylation. Genistein also inhibits LPA- and Rho-induced stress fibre formation, implying that a tyrosine kinase lies downstream of Rho in this signal transduction pathway.