HERC5-catalyzed ISGylation potentiates cGAS-mediated innate immunity

The key cytosolic DNA sensor cyclic GMP-AMP (cGAMP) synthetase (cGAS) is essential to elicit anti-infection innate immunity whereas aberrant activation of cGAS by endogenous DNA promotes severe autoimmune diseases, thus the activity of cGAS must be strictly regulated to maintain immune homeostasis. Here we report that the E3 ISG15-protein ligase HERC5 interacts with and catalyzes the cGAS ISGylation robustly, whereas the deconjugating enzyme USP18 removes the ISG15 conjugation of cGAS. The interaction of cGAS and HERC5 depends on the cGAS N-terminal domain and the RRC1-4 and HECT domain of HERC5. Mass spectrometry reveals that the Lys21,187,219,458 are the potential ISGylation sites in human cGAS. Deficiency of ISG15 or the E1 ubiquitin-activating enzyme UBA7 greatly attenuate the downstream gene expression induced by cGAS-STING axis and the antiviral ability in mouse and human cells. Collectively, our study uncovered an important role of the HERC5-facilitated cGAS ISGylaion in promoting antiviral defense.