The structural heterogeneity of AKT autoinhibition

Abstract

AKT is key to controlling cell growth through the PI3K/AKT/mTOR pathway. In the cytosol, in the absence of stimulus, AKT is autoinhibited to prevent uncontrolled activation. Increased AKT activity contributes to tumor growth by phosphorylating numerous downstream targets. Relieving the autoinhibition is a prerequisite for full activation, which occurs through C-terminal tail phosphorylation by mTOR, followed by activation loop phosphorylation by PDK1. However, the atomic-level mechanisms by which AKT autoinhibition persists in the cytosol and the phosphorylation (posttranslational modifications) allosterically shift AKT to its open conformation, which may serve as drug targets, remain unclear. Here, we performed explicit molecular dynamics simulations to explore the conformational ensembles of AKT in these different states. Our unbiased results show how the variable loops of the PH domain contribute to the PH-mediated AKT autoinhibition. Autoinhibited states are commonly only marginally stable, populating function-related shallow metastable wells with relatively similar energies and low kinetic barriers, making them receptive to regulation. The conformational heterogeneity of AKT's autoinhibitory interface is susceptible to regulation, including by phosphorylation, but also by activating mutations and allosteric inhibitors. As to activation by phosphorylation, allosteric communication between the phosphorylated C-terminal tail and the PH domain of AKT promotes the release of the PH domain from the kinase domain, independent of PIP3. Our results clarify how mutations and phosphorylation can impact autoinhibition and resistance to allosteric inhibitors, highlighting how metastable states can contribute to cellular regulation. Heterogeneous population with low stability and low kinetic barriers can be a useful attribute of living cells.