Background: Protein kinase C ζ (PKCζ) 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. PKCζ has received considerable attention recently as a target of phosphoinositide 3-kinase (PI 3-kinase), although the mechanism of PKCζ 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-P₃), 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 PKCζ.

Results:In vivo, PI 3-kinase is both necessary and sufficient to activate PKCζ. PDK-1 phosphorylates and activates PKCζin vivo, and we have shown that this is due to phosphorylation of threonine 410 in the PKCζ activation loop. In vitro, PDK-1 phosphorylates and activates PKCζ in a Ptdlns-3,4,5-P₃-enhanced manner. PKCζ and PDK-1 are associated in vivo, and membrane targeting of PKCζ renders it constitutively active in cells.

Conclusions: Our results have identified PDK-1 as the kinase that phosphorylates and activates PKCζ in the PI 3-kinase signaling pathway. This phosphorylation and activation of PKCζ by PDK-1 is enhanced in the presence of Ptdlns-3,4,5-P₃. Consistent with the notion that PKCs are enzymes that are regulated at the plasma membrane, a membrane-targeted PKCζ is constitutively active in the absence of agonist stimulation. The association between PKCζ and PDK-1 reveals extensive cross-talk between enzymes in the PI 3-kinase signaling pathway.