Abstract 671: Cryo-electronmicroscopy of PI3K reveals additional regulatory features

Abstract

Abstract Many solid tumors have alterations in phosphoinositide 3-kinase (PI3K) signaling. These can occur through mutation of the p110alpha and p85alpha proteins of PI3Kalpha itself as well as through alterations to upstream receptor tyrosine kinases (RTKs) and phosphatases of PIP3, including PTEN. As a whole, PI3K signaling is one of the most frequently altered in cancer. These mutations lead to activation of the PI3K complex and to constitutive downstream signaling. This signaling allows cells to enter a pro-growth and pro-survival state which is beneficial to the cancer cells. The structure of PI3K has been extensively studied by crystallography and NMR. Here, we present our work on the full-length complex of p110alpha with p85alpha using cryo-EM. When compared with the previous structural studies, cryo-EM shows a similar structure and major structural features are unchanged. However, our cryo-EM studies revealed several additional structural features of the PI3K complex. Cryo-EM showed that SH3, BH and cSH2 domains of p85alpha can bind to p110alpha. When inhibited with Alpelisib, these domains form a stable cover of the active site of the kinase. These domains interact with the helical and kinase domains of p110alpha and the iSH2 domain of p85alpha. The position of these domains restricts access to the kinase active site and may contribute to the inhibition of PI3K activity by Alpelisib. When the PI3K complex binds to phosphorylated RTKs, the nSH2 binds to phosphotyrosine and relieves inhibitory interactions with the helical domain of p110alpha. Using cryo-EM, we have studied this activation by incubating the full-length PI3K complex with synthetic phosphopeptides. 2D cryo-EM class averages showed a dramatic rearrangement of the PI3K complex. In addition to the expected displacement of the nSH2 domain, the ABD and iSH2 domains undergo structural changes. Acknowledgments: This work was supported by the National Cancer Institute of the National Institutes of Health under awards R35 CA197582 (PKV) and R50 CA243899 (JRH). Citation Format: Jonathan R. Hart, Su Yang, Xiao Liu, Yingna Xu, Xinyu Zou, Huibing Zhang, Qingtong Zhou, Tian Xia, Yan Zhang, Dehua Yang, Ming-Wei Wang, Peter K. Vogt. Cryo-electronmicroscopy of PI3K reveals additional regulatory features [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 671.