alpha PIX nucleotide exchange factor is activated by interaction with phosphatidylinositol 3-kinase

p21-activated kinase (PAK) is a common effector protein of the small GTPase s Cdc42 and Rac, leading to the activation of downstream mitogen activated protein kinases, PAK also mediates polarized cytoskeletal changes induced b y these GTPases, The recently identified PAK-interacting exchange factor (P IN) acts as a guanine nucleotide exchange factor on Rac, and colocalizes wi th PAK in a focal complex, but little is known about the associated signali ng cascades, including upstream activators of PIX. In this study, we show t hat one of the isoforms of PIX, alpha PIX, is activated by signaling cascad es from the platelet-derived growth factor (PDGF) receptor and EphB2 recept or, and from integrin-induced signaling through phosphatidylinositol 3-kina se (PW-kinase). alpha PIX is activated ba forming a complex with these rece ptors either ria association with PAK and Nck, or direct association with t he p85 regulatory subunit of PU-kinase. Synthetic phosphoinositide and memb rane targeted PW-kinase augmented the alpha PIX activity in vivo. In Xenopu s, aggregates of mesodermal cells derived from embryos microinjected with a lpha PIX significantly increased the peripheral spreading on fibronectin su bstrate in response to PDGF through PI3-kinase. These results indicate that alpha PIX is activated by PI3-kinase, and is involved in the receptor medi ated signaling leading to the activation of the kinase activity of PAK, and the migration of mesodermal cells on extracellular matrix.