REGULATION OF PROTEIN-KINASE-C-ZETA BY PI-3-KINASE AND PDK-1

Background: Protein kinase C zeta (PKC zeta) is a member of the PKC fa mily of enzymes and is involved in a wide range of physiological proce sses including mitogenesis, protein synthesis, cell survival and trans criptional regulation. PKC zeta has received considerable attention re cently as a target of phosphoinositide 3-kinase (PI 3-kinase), althoug h the mechanism of PKC zeta activation is, as yet, unknown. Recent rep orts have also shown that the phosphoinositide-dependent protein kinas e-l (PDK-1), which binds with high affinity to the PI 3-kinase lipid p roduct phosphatidylinositol-3,4,5-trisphosphate (Ptdlns-3,4,5-P-3), ph osphorylates and potently activates two other PI 3-kinase targets, the protein kinases Akt/PKB and p70S6K, We therefore investigated whether PDK-1 is the kinase that activates PKC zeta, Results: In vivo, PI 3-k inase is both necessary and sufficient to activate PKC zeta. PDK-1 pho sphorylates and activates PKC zeta in vivo, and we have shown that thi s is due to phosphorylation of threonine 410 in the PKC zeta activatio n loop. In vitro, PDK-1 phosphorylates and activates PKC zeta in a Ptd lns-3,4,5-P-3-enhanced manner. PKC zeta and PDK-1 are associated in vi vo, and membrane targeting of PKC zeta renders it constitutively activ e in cells, Conclusions: Our results have identified PDK-1 as the kina se that phosphorylates and activates PKC zeta in the PI 3-kinase signa ling pathway. This phosphorylation and activation of PKC zeta by PDK-1 is enhanced in the presence of Ptdlns-3,4-5-P-3. Consistent with the notion that PKCs are enzymes that are regulated at the plasma membrane , a membrane-targeted PKC zeta is constitutively active in the absence of agonist stimulation. The association between PKC zeta and PDK-1 re veals extensive cross-talk between enzymes in the PI 3-kinase signalin g pathway.