Abstract LB-326: Structural basis of lipid-binding and regulation in PI3Kα

PI3Kα is a well-established target for the development of novel cancer therapeutics. Currently, all inhibitors in clinical trials target the highly conserved ATP-binding site, which has made the development of PI3K selective inhibitors difficult, with many inhibitors displaying cross-reactivity against protein kinases. We have used X-ray crystallography and fluorescence quenching studies to characterize the lipid-binding site of PI3Kα and the structural basis of regulation by nSH2. A newly determined p110α/niSH2 crystal structure is the first to reveal the nSH2 domain in complex with wild-type p110α, allowing investigation of the mechanisms of nSH2 regulation. Key interactions between the nSH2 domain and the activation loop suggest a mechanism by which the kinase domain is kept in an inactive conformation until activation by phosphopeptide binding. Key differences in nSH2 domain binding to p110α were identified between the wild-type and oncogenic mutant, p110αH1047R. Increased buried surface area and two unique salt-bridges are suggestive of tighter regulatory control in the wild-type PI3Kα compared with the oncogenic mutant. A second structure reveals the details of PI3K binding to a lipid substrate mimetic, diC4-PIP2. Unexpectedly, we found an additional lipid-binding site, and this striking observation was confirmed by fluorescence quenching experiments. The identification of multiple lipid binding sites provides additional targets that may enable more selective inhibition among the various isoforms, or even between mutant and wild-type forms of PI3Kα. Citation Format: Michelle S. Miller, Oleg Schmidt-Kittler, David M. Bolduc, Evan T. Brower, Daniele Chaves-Moreira, Marc Allaire, Kenneth W. Kinzler, Ian G. Jennings, Philip E. Thompson, Philip A. Cole, L. Mario Amzel, Bert Vogelstein, Sandra B. Gabelli. Structural basis of lipid-binding and regulation in PI3Kα. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-326. doi:10.1158/1538-7445.AM2014-LB-326