Abstract 15889: C-Terminal Modifications of Akt Kinase Boost Cellular Responses to Insulin by Inhibiting Activation Loop Dephosphorylation

Introduction: The Akt kinase isoforms (Akt1, Akt2 and Akt3) are activated downstream of the insulin receptor, and exert unique effects on cell growth, survival and metabolism. We recently described a novel ATP-dependent "dephosphorylation-resistance cage" in Akt kinases that controls access of cellular phosphatases to dephosphorylate the Akt activation loop (T308 in Akt1). Hypothesis: Here, we describe that, when Akt is localized at the cell membrane, intramolecular interactions of Akt C-terminal sequences with the hydrophobic groove of the kinase domain protect the phosphorylated activation loop (pT308 in Akt1) from cellular phosphatases. Methods and Results: Charged amino acid replacements of Akt C-terminal phosphorylation sites regulated by MTORC2 (Ser473) and by cyclin-dependent kinase 2 (Ser477) increased phospho-T308 protection from cellular phosphatases regardless of subcellular location. Functionally, these phosphatase-resistant Akt variants were refractory to ceramide-dependent dephosphorylation and amplified insulin-dependent T308 phosphorylation in a regulated fashion (Figure 1). Conclusions: Collectively, these results suggest that modulating phosphatase sensitivity of Akt activation loops via C-terminal hydrophobic groove may provide a platform to develop novel Akt agonists.