PI(3,4,5)P3 Engagement Restricts Akt Activity to Cellular Membranes

Protein kinase B/Akt regulates cellular metabolism, survival, and proliferation in response to hormones and growth factors. Hyperactivation of Akt is frequently observed in cancer, while Akt inactivation is associated with severe diabetes. Here, we investigated the molecular and cellular mechanisms that maintain Akt activity proportional to the activating stimulus. We show that binding of phosphatidylinositol-3,4,5-trisphosphate (PIP3) or PI(3,4)P2 to the PH domain allosterically activates Akt by promoting high-affinity substrate binding. Conversely, dissociation from PIP3 was rate limiting for Akt dephosphorylation, dependent on the presence of the PH domain. In cells, active Akt associated primarily with cellular membranes. In contrast, a transforming mutation that uncouples kinase activation from PIP3 resulted in the accumulation of hyperphosphorylated, active Akt in the cytosol. Our results suggest that intramolecular allosteric and cellular mechanisms cooperate to restrict Akt activity to cellular membranes, thereby enhancing the fidelity of Akt signaling and the specificity of downstream substrate phosphorylation. [Display omitted] •Active Akt is associated exclusively with PIP3- or PI(3,4)P2-containing membranes•Allosteric activation by PIP3 and PI(3,4)P2 permits substrate binding•PH domain-mediated autoinhibition promotes dephosphorylation of Akt•Mutation of autoinhibitory interface drives hyperactivation of Akt in the cytosol Ebner et al. show that the membrane lipid PI(3,4,5)P3 allosterically activates Akt kinase by relieving an intramolecular autoinhibitory mechanism. PH domain-mediated autoinhibition blocks substrate binding and promotes Akt dephosphorylation. In cells, phosphorylated Akt is exclusively associated with PI(3,4,5)P3- or PI(3,4)P2-containing membranes, ensuring that Akt activity remains proportional to the activating stimulus.