Atypical protein kinase C (aPKC) isoenzymes are key modulators of insulin signalling, and their dysfunction correlateswith insulinresistant states in both mice and humans. Despite the engaged interest in the importance of aPKCs to type 2 diabetes, much less is known about the molecular mechanisms that govern their cellular functions than for the conventional and novel PKC isoenzymes and the functionally-related protein kinase B (Akt) family of kinases. Here we show that aPKC is constitutively phosphorylated and, using a genetically-encoded reporter for PKC activity, basally active in cells. Specifically, we show that phosphorylation at two key regulatory sites, the activation loop and turn motif, of the aPKC PKCχ in multiple cultured cell types is constitutive and independently regulated by separate kinases: ribosome-associated mammalian target of rapamycin complex 2 (mTORC2) mediates co-translational phosphorylation of the turn motif, followed by phosphorylation at the activation loop by phosphoinositide-dependent kinase-1 (PDK1). Live cell imaging reveals that global aPKC activity is constitutive and insulin unresponsive, in marked contrast to the insulindependent activation ofAkt monitored by anAkt-specific reporter. Nor does forced recruitment to phosphoinositides by fusing the pleckstrin homology (PH) domain of Akt to the kinase domain of PKCχ alter either the phosphorylation or activity of PKCχ. Thus, insulin stimulation does not activate PKCχ through the canonical phosphatidylinositol-3,4,5-triphosphatemediated pathway that activates Akt, contrasting with previous literature on PKCχ activation. These studies support a model wherein an alternative mechanism regulates PKCχ-mediated insulin signalling that does not utilize conventional activation via agonist-evoked phosphorylation at the activation loop. Rather, we propose that scaffolding near substrates drives the function of PKCχ.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology
- Atypical protein kinase C
- MTOR complex
- Phosphatidylinositol signalling