AKT1 Can Be Activated via Distinct Phosphorylation-Based Mechanisms Free

Phosphorylation of AKT1 drives downstream signaling and cancer cell proliferation, and AKT1 inhibitors are in clinical development. In the classic model of AKT1 regulation, AKT1 is recruited to the plasma membrane by PIP3 where it is phosphorylated on the C-terminus (Ser473) by mTORC2 and on the activation loop (Thr308) by PDK1, leading to enhanced kinase activity. AKT1 can also be phosphorylated on Ser477 and Thr479, and dual phosphorylation of these noncanonical sites has been suggested to mimic Ser473 phosphorylation. However, analysis of AKT1 regulation has been limited by the lack of methods for purification of specific phospho-modified AKT1. To address this limitation, Chu and colleagues used expressed protein ligation to generate 18 specific AKT1 phospho forms. These semisynthetic proteins were used to determine the structure and mechanisms by which AKT phosphorylation controls its activity. Phosphorylation of Ser473 promoted AKT1 activation via an interaction between the C-tail and the pleckstrin homology (PH) kinase domain linker that relieved AKT1 autoinhibition mediated by the PH domain. In contrast, dual phosphorylation of Ser477 and Thr479 produced an alternative active kinase conformation where AKT1 was activated via an interaction of the activation loop that suppresses PH domain–mediated autoinhibition and weakens the affinity for PIP3. Taken together, these findings suggest distinct modes of AKT activation mediated by specific phosphorylation events, and these findings may have implications for the development of AKT inhibitors in cancer. Moreover, the expressed protein ligation approach may facilitate further studies to broaden understanding of the effects of post-translational modifications on AKT1.

Chu N, Salguero AL, Liu AZ, Chen Z, Dempsey DR, Ficarro SB, et al. Akt kinase activation mechanisms revealed using protein semisynthesis. Cell 2018;174:897–907.e14.

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