Phosphorylated Calmodulin Promotes PI3K Activation by Binding to the SH2 Domains.

How calmodulin (CaM) acts in KRAS-driven cancers is a vastly important question. CaM binds to and stimulates PI3Kα/Akt signaling, promoting cell growth and proliferation. Phosphorylation of CaM at Tyr99 (pY99) enhances PI3Kα activation. PI3Kα is a lipid kinase. It phosphorylates PIP2 to produce PIP3, to which Akt binds. PI3Kα has two subunits: the regulatory p85 and the catalytic p110. Here, exploiting explicit-solvent MD simulations we unveil key interactions between phosphorylated CaM (pCaM) and the two SH2 domains in the p85 subunit, confirm experimental observations, and uncover PI3Kα's mechanism of activation. pCaMs form strong and stable interactions with both nSH2 and cSH2 domains, with pY99 being the dominant contributor. Despite the high structural similarity between the two SH2 domains, we observe that nSH2 prefers an extended CaM conformation, whereas cSH2 prefers a collapsed conformation. Notably, collapsed CaM is observed after binding of an extended CaM to K-Ras4B. Thus, the more populated extended pCaM conformation targets nSH2 to release its autoinhibition of p110 catalytic sites. This executes the key activation step of PI3Kα. Independently, K-Ras4B allosterically activates p110. These events are at the cell membrane, which contributes to tighten the PI3Kα Ras binding domain/K-Ras4B interaction, to accomplish K-Ras4B allosteric activation, with a minor contribution from cSH2.