P21-Activated (or Not) Protein Kinase

Abstract

Organization of the actin cytoskeleton at focal adhesions is regulated though protein complexes that include the small guanosine triphosphatases (GTPases) Cdc42 and Rac. Such Rho-family GTPases control activity of the p21-activated protein kinase (PAK). Activity of PAK is thought to be regulated in part by its localization to focal adhesions through interaction with the protein PIX (PAK-interacting exchange factor). An obvious regulatory mechanism is, thus, that activation of Cdc42 and Rac1 by PIX could stimulate PAK activity. Although this remains possible, Loo et al. show that there is more to regulation of PAK in such complexes. PIX is anchored to focal adhesions by another protein known as GIT1 (G protein-coupled receptor kinase-interacting target 1). Analysis of the properties of mutant proteins expressed in cultured mammalian cells showed that PAK could be activated without interaction with the small GTPases. Instead, overexpression of GIT1 caused activation. GIT1 is itself a GTPase-activating protein (GAP), and the GAO domain of GIT1, but not its GAP activity, was required to activate PAK. In vitro studies of PAK showed that increasing the concentration of immobilized PAK could cause activation of its kinase activity, thus providing a potential mechanism by which PIX and GIT1 may regulate PAK activity independently of the Rho GTPases. T.-H. Loo, Y.-W. Ng, L. Lim, E. Manser, GIT1 activates p21-activated kinase through a mechanism independent of p21 binding. Mol. Cell. Biol. 24 , 3849-3859 (2004). [Abstract] [Full Text]