The beta and gamma subunits of heterotrimeric GTP-binding proteins (Gbetagamma) were found to bi-directionally regulate the UV-induced activation of p38 and c-Jun NH(2)-terminal kinase, and the UV-induced activation of p38 was reported to enhance the resistance of normal keratinocytes to apoptosis. However, the signaling pathway downstream of Gbetagamma for this UV-induced p38 activation is not known. Thus, we examined the role of the Rho GTPase family in the regulation of UV-induced p38 activation by Gbetagamma. We found that overexpression of Gbetagamma increased the UV-induced activation of Cdc42 and that overexpression of constitutively active V12 Cdc42 increased the UV-induced p38 activation. Transfection of dominant negative N17 Cdc42 or small interfering RNA for Cdc42 blocked UV-induced p38 activation mediated by Gbetagamma in COS-1 and HaCaT cells. UV-induced p38 activation by Gbetagamma was blocked by overexpression of dominant negative p21-activated kinase (PAK)-interacting exchange factor beta (betaPix), and wild type betaPix stimulated the UV-induced p38 activation, which was blocked by N17 Cdc42. Gbetagamma increased the UV-induced activation of Ras, and the overexpression of V12 Ras increased UV-induced p38 activation, which was blocked by dominant negative betaPix. UV-induced p38 activation was inhibited by N17 Ras and a farnesyltransferase inhibitor, manumycin A. Gbetagamma also increased the UV-induced phosphorylation of the epidermal growth factor receptor (EGFR), and the UV-induced p38 activation was blocked by an EGFR kinase inhibitor, AG1478. From these results, we conclude that Gbetagamma mediates UV-induced activation of p38 in a Cdc42-dependent way and that EGFR, Ras, and betaPix act sequentially upstream of Cdc42 in COS-1 and HaCaT cells.