Binding of phosphatidylinositol 3,4,5-trisphosphate to the pleckstrin homology domain of protein kinase B induces a conformational change

Protein kinase B (PKB/Akt) is a key regulator of cell growth, proliferation and metabolism. It possesses an N-terminal pleckstrin homology (PH) domain that interacts with equal affinity with the second messengers PtdIns(3,4,5)P₃ and PtdIns(3,4)P₂, generated through insulin and growth factor-mediated activation of phosphoinositide 3-kinase (PI3K). The binding of PKB to PtdIns(3,4,5)P₃/PtdIns(3,4)P₂ recruits PKB from the cytosol to the plasma membrane and is also thought to induce a conformational change that converts PKB into a substrate that can be activated by the phosphoinositide-dependent kinase 1 (PDK1). In this study we describe two high-resolution crystal structures of the PH domain of PKBα in a noncomplexed form and compare this to a new atomic resolution (0.98 Å, where 1 Å=0.1 nm) structure of the PH domain of PKBα complexed to Ins(1,3,4,5)P₄, the head group of PtdIns(3,4,5)P₃. Remarkably, in contrast to all other PH domains crystallized so far, our data suggest that binding of Ins(1,3,4,5)P₄ to the PH domain of PKB, induces a large conformational change. This is characterized by marked changes in certain residues making up the phosphoinositide-binding site, formation of a short α-helix in variable loop 2, and a movement of variable loop 3 away from the lipid-binding site. Solution studies with CD also provided evidence of conformational changes taking place upon binding of Ins(1,3,4,5)P₄ to the PH domain of PKB. Our data provides the first structural insight into the mechanism by which the interaction of PKB with PtdIns(3,4,5)P₃/PtdIns(3,4)P₂ induces conformational changes that could enable PKB to be activated by PDK1.