Regulation of protein kinase C ζ by PI 3-kinase and PDK-1

Abstract Background: Protein kinase C ζ (PKC ζ ) is a member of the PKC family of enzymes and is involved in a wide range of physiological processes including mitogenesis, protein synthesis, cell survival and transcriptional regulation. PKC ζ has received considerable attention recently as a target of phosphoinositide 3-kinase (PI 3-kinase), although the mechanism of PKC ζ activation is, as yet, unknown. Recent reports have also shown that the phosphoinositide-dependent protein kinase-1 (PDK-1), which binds with high affinity to the PI 3-kinase lipid product phosphatidylinositol-3,4,5-trisphosphate (Ptdlns-3,4,5-P 3 ), phosphorylates 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 ζ . Results: In vivo , PI 3-kinase is both necessary and sufficient to activate PKC ζ . PDK-1 phosphorylates and activates PKC ζ in vivo , and we have shown that this is due to phosphorylation of threonine 410 in the PKC ζ activation loop. In vitro , PDK-1 phosphorylates and activates PKC ζ in a Ptdlns-3,4,5-P 3 -enhanced manner. PKC ζ and PDK-1 are associated in vivo , and membrane targeting of PKC ζ renders it constitutively active in cells. Conclusions: Our results have identified PDK-1 as the kinase that phosphorylates and activates PKC ζ in the PI 3-kinase signaling pathway. This phosphorylation and activation of PKC ζ 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 ζ is constitutively active in the absence of agonist stimulation. The association between PKC ζ and PDK-1 reveals extensive cross-talk between enzymes in the PI 3-kinase signaling pathway.