Abstract A090: Identify molecular mechanisms that regulate the GMP-AMP synthase (cGAS) activity

Abstract

Abstract While checkpoint inhibitors are achieving stunning success in cancer treatment, one possible approach to improve the response rate is to enhance the initiation of anti-tumor immune response. Recently studies showed that the cytosolic DNA sensing pathway is essential for the innate immune response in dendritic cells to radiation treated tumor cells. Cytosolic DNA, as a danger signal, is sensed by the cyclic GMP-AMP synthase (cGAS), which produces the second messenger, 2'3'-cyclic AMP-GMP (cGAMP) to activate STING-mediated Type I interferon response. Despite the critical role of cGAS in anti-tumor immunity, how its activity is regulated remains poorly understood. To explore cGAS-regulating mechanisms in cells, we used mass spectrometry to study cGAS and its interacting partners during the course of DNA sensing. The results indicate that cGAS is phosphorylated at multiple serine and threonine residues. Mutagenesis experiments showed that the phosphorylation at Serine305 can potently inhibit the cGAMP-synthesis activity. Structural analysis illustrated that this phosphorylation site locates at the entrance of the active site, therefore the negatively charged phosphate group may sterically block the access of substrates, ATP and GTP. This serine is highly conserved in mammalian species, and locates within a cyclin kinase motif. Indeed, purified human CDK1-cyclin B complex, the dominant mitosis driver, can specifically phosphorylate this serine in vitro. The results suggest that the cGAS activity may coordinate with the cell cycle, and implies that the phosphorylation may limit the immunogenicity of tumor radiation therapy. Note: This abstract was not presented at the conference. Citation Format: Tuo Li, Zhijian J. Chen. Identify molecular mechanisms that regulate the GMP-AMP synthase (cGAS) activity [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A090.