The cyclic oligonucleotide-based anti-phage signaling system (CBASS) is a family of defense system in prokaryotes 1, 2 . Composed of a cyclic GMP-AMP synthase (cGAS) and CBASS-associated proteins, CBASS utilizes cyclic oligonucleotides to activate antiviral immunity 3–6 . One major group of CBASS-associated proteins are homologs of eukaryotic E2 ubiquitin-conjugating enzymes. However, the function of E2 in CBASS remains elusive. Here, we report that a bacterial E2 enzyme regulates cGAS by imitating the entire ubiquitination cascade. This includes the processing of the cGAS C-terminus, conjugation of cGAS to a cysteine residue, ligation of cGAS to a lysine residue, cleavage of the isopeptide bond, and poly-cGASylation. The poly-cGASylation fully activates cGAS to produce cGAMP, which acts as an antiviral signal and leads to cell death. Our findings reveal unique regulatory roles of E2 in CBASS and provide insights into the origin of the ubiquitin system.

Ubiquitin-like cGAS chain formation by a super enzyme activates anti-phage response