Activation of the Akt-related cytokine-independent survival kinase requires interaction of its phox domain with endosomal phosphatidylinositol 3-phosphate

Protein kinases of the Akt and related serum- and glucocorticoid-regulated kinase (SGK) families are major downstream mediators of phosphatidylinosito l (Pl) 3-kinase signaling to many cellular processes including metabolic fl ux, membrane trafficking, and apoptosis. Activation of these kinases is tho ught to occur at the plasma membrane through their serine and threonine pho sphorylation by the phosphoinositide-dependent kinase 1 (PDK1) protein kina se, which interacts with membrane 3'-polyphosphoinositides through its plec kstrin homology (PH) domain. Here, we demonstrate that the SGK family membe r cytokine-independent survival kinase (CISK) binds strongly and selectivel y to the monophosphoinositide Pl(3)P through its phox homology (PX) domain. Comparing native green fluorescent protein-CISK (EGFP-CISK) to a mutant EG FP-CISK (Y51A) that displays attenuated binding to Pl(3)P reveals that this interaction is both necessary and sufficient for its localization to early endosome antigen (EEA1)positive endosomes. Furthermore, early endosome ass ociation of expressed epitope-tagged CISK in COS cells directed by binding of its PX domain to Pl(3)P is required for activation of the CISK protein k inase by both insulin-like growth factor-1 and epidermal growth factor. Tak en together, these results reveal a critical role of endosomal Pl(3)P in th e signal transmission mechanism whereby this survival kinase is activated i n response to P13-kinase stimulation by growth factors.