Regulation of cytokine-independent survival kinase (CISK) by the Phox homology domain and phosphoinositides

PKB/Akt and serum and glucocorticoid-regulated kinase (SGK) family kinases are important downstream targets of phosphatidylinositol 3 (PI-3) kinase an d have been shown to mediate a variety of cellular processes, including cel l growth and survival. Although regulation of Akt can be achieved through s everal mechanisms, including its phosphoinositide-binding Pleckstrin homolo gy (PH) domain, how SGK kinases are targeted and regulated remains to be el ucidated. Unlike Akt, cytokine-independent survival kinase (CISK)/SGK3 cont ains a Phox homology (PX) domain. PX domains have been implicated in severa l cellular events involving membrane trafficking. However, their precise fu nction remains unknown. We demonstrate here that the PX domain of CISK inte racts with phosphatidylinositol (PtdIns)(3,5)P-2, PtdIns(3,4,5)P-3, and to a lesser extent PtdIns(4,5)P-2. The CISK PX domain is required for targetin g CISK to the endosomal compartment. Mutation in the PX domain that abolish ed its phospholipid binding ability not only disrupted CISK localization, b ut also resulted in a decrease in CISK activity in vivo. These results sugg est that the PX domain regulates CISK localization and function through its direct interaction with phosphoinositides. Therefore, CISK and Akt have ev olved to utilize different lipid binding domains to accomplish a similar me chanism of activation in response to PI-3 kinase signaling.