Mn2+ Directly Activates cGAS and Structural Analysis Suggests Mn2+ Induces a Noncanonical Catalytic Synthesis of 2′3′-cGAMP

• Published in: Cell reports (Cambridge) 2020-08, Vol.32 (7), p.108053, Article 108053
• Main Authors: Zhao, Zhen, Ma, Zhixing, Wang, Bo, Guan, Yukun, Su, Xiao-Dong, Jiang, Zhengfan
• Format: Article
• Language: English
• Subjects: 2’3’-cGAMP; catalytic synthesis; cGAS; innate immunity; Mn2; type I-IFNs
• ISSN: 2211-1247; 2211-1247
• DOI: 10.1016/j.celrep.2020.108053

DNA binding allosterically activates the cytosolic DNA sensor cGAS (cyclic GMP-AMP [cGAMP] synthase) to synthesize 2′3′-cGAMP, using Mg2+ as the metal cofactor that catalyzes two nucleotidyl-transferring reactions. We previously found that Mn2+ potentiates cGAS activation, but the underlying mechanism remains unclear. Here, we report that Mn2+ directly activates cGAS. Structural analysis reveals that Mn2+-activated cGAS undergoes globally similar conformational changes to DNA-activated cGAS but forms a unique η1 helix to widen the catalytic pocket, allowing substrate entry and cGAMP synthesis. Strikingly, in Mn2+-activated cGAS, the linear intermediates pppGpG and pGpA take an inverted orientation in the active pocket, suggesting a noncanonical but accelerated cGAMP cyclization without substrate flip-over. Moreover, unlike the octahedral coordination around Mg2+, the two catalytic Mn2+ are coordinated by triphosphate moiety of the inverted substrate, independent of the catalytic triad residues. Our findings thus uncover Mn2+ as a cGAS activator that initiates noncanonical 2′3′-cGAMP synthesis. [Display omitted] •Mn2+ directly activates cGAS, independent of dsDNA, to synthesize 2′3′-cGAMP•Mn2+-activated cGAS adopts similar overall conformation to dsDNA-activated cGAS•The inverted intermediate product orientation suggests a noncanonical cGAMP synthesis•The catalytic triad of cGAS is not involved in coordinating Mn2+ ions Zhao et al. find that Mn2+ directly activates cGAS in a dsDNA-independent manner. Their findings uncover a Mn2+ elicited enzymatic activation of cGAS and the subsequent cGAMP synthesis, which may be applied to other cGAS/DncV-like nucleotidyltransferases (CD-NTases).