Isoform‐Selective Regulation of Akt1 and Akt2 by Interdomain Interaction

Abstract

The serine/threonine protein kinase PKB/Akt is a key signal transducer in a wide range of cellular responses and a prominent therapeutic target. The three highly homologous isoforms of Akt regulate critical and distinct physiological functions in cell growth, proliferation, and metabolism. However, the mechanisms that determine isoform-specific Akt signaling remain largely unclear. Here we examined the mechanisms by which Akt1 and Akt2, the most broadly expressed isoforms, are differentially activated upon growth factor stimulation. Using a newly developed FRET-based reporter of Akt2 activation (ReAktion2) along with a previously developed reporter of Akt1 activation (ReAktion), we found that growth factor triggers a conformational change in both Akt isoforms at the plasma membrane, which is critically dependent on activation-loop phosphorylation. Interestingly, the allosteric pan-Akt inhibitor, MK-2206, appears to selectively induce a conformational change in Akt2, but not Akt1 at the plasma membrane. This prompts us to further examine whether inter-domain interactions differ between Akt1 and Akt2. Our preliminary results suggest Akt1 activation enhances the interaction between its PH and linker-catalytic domains, which is dependent on phosphorylation of T308 within the catalytic domain, whereas this induced interdomain interaction is absent in Akt2. Furthermore, the different interdomain interaction of Akt isoforms may lead to different membrane departure, as single cell analysis of plasma membrane translocation revealed that Akt2 accumulates at the plasma membrane to a greater extent than Akt1. Taken together, our results suggest that distinct modes of interactions between the PH and linker-catalytic domains modulate the isoform-specific activation of Akt.