Turn motif phosphorylation negatively regulates activation loop phosphorylation in Akt

Area of Science:

• Cellular signaling
• Molecular biology
• Biochemistry

Background:

• Akt (protein kinase B) is crucial for cellular processes like metabolism, proliferation, and survival.
• Akt activation involves phosphorylation of its activation loop (A-loop) and hydrophobic motif (HM) by PDK1 and mTORC2.
• The role of threonine in the turn motif (TM) phosphorylation by mTORC2 in Akt activation requires further investigation.

Purpose of the Study:

• To investigate the role of TM phosphorylation in Akt activation, specifically its impact on A-loop phosphorylation.
• To elucidate the regulatory mechanisms controlling Akt phosphorylation in a model system.

Main Methods:

• Utilized starfish oocytes as a model system to study exogenous human Akt (hAkt) phosphorylation.
• Inhibited TM and HM phosphorylation using an anti-starfish TOR antibody.
• Employed alanine substitutions at phosphorylation sites to assess the impact of TM phosphorylation.
• Utilized okadaic acid (OA) to inhibit A-loop phosphatase.

Main Results:

• TM phosphorylation negatively regulates A-loop phosphorylation, contrary to expectations.
• TM phosphorylation increases Akt susceptibility to A-loop dephosphorylation, potentially by reducing sensitivity to PDK1.
• HM phosphorylation enhances A-loop phosphorylation, contributing to full Akt activation.
• TM phosphorylation's negative regulation was functionally observed in meiosis resumption.

Conclusions:

• TM phosphorylation acts as a negative regulator of Akt A-loop phosphorylation, uncovering a novel regulatory pathway.
• Akt activation is a complex process involving both positive (HM) and negative (TM) regulatory phosphorylation events.
• These findings offer new insights into the intricate control mechanisms governing Akt signaling.