Structural Studies of the Complex Between Akt-in and the Akt2-PH Domain Suggest that the Peptide Acts as an Allosteric Inhibitor of the Akt Kinase

Serine/threonine kinase Akt plays a central role in the regulation of cell survival and proliferation. Hence, the search for Akt specific inhibitors constitutes an attractive strategy for anticancer therapy. We have previously demonstrated that the proto-oncogene TCL1 coactivates Akt upon binding to its Plekstrin Homology Domain, and we proposed a model for the structure of the complex TCL1:Akt2-PHD. This model led to the rational design of Akt-in, a peptide inhibitor spanning the A s-strand of human TCL1 that binds Akt2 PH domain and inhibits the kinase activation. In the present report, we used NMR spectroscopy to determine the 3D structure of the peptide free in solution and bound to Akt2-PHD. NMR chemical shift mapping was used to determine the imprint of Akt-in on the PH domain; whereas peptide Ala-scanning revealed which peptide residues were involved in the interaction. Together with the solution structure of Akt2-PHD, these results allowed us to dock Akt-in on the PH domain. The docked complex suggests that while Akt-in binds Akt2-PHD in a region overlapping the binding site of TCL1, its mode of interaction is markedly different. Moreover, the affinity was disappointingly low, contrary to that published previously. Besides providing a description of the interaction between Akt-in an Akt2 PH domain, the present work brings additional clues on the putative peptide mode of action. Instead of behaving as an analog of PtdIns, as previously suggested, Akt-in might act as an "allosteric" inhibitor, maintaining the full-length kinase in its "closed" inactive conformation, rather than disturbing the membrane anchorage of its "open" active conformation.