Structural mechanism of 3′3′-cGAMP-induced filamentation and phospholipid hydrolysis by CapV in bacterial antiphage defense

The cyclic-oligonucleotide-based antiphage signaling system (CBASS) protects bacteria from phage infection. In Vibrio cholerae , phage infection activates CD-NTase DncV to produce 3′3′-cGAMP, which triggers phospholipase CapV to degrade phosphatidylethanolamine and phosphatidylglycerol, the major phospholipids in the inner-membranes, thereby inducing cell death. However, how 3′3′-cGAMP activates CapV was unclear. Here we present crystal structures of inactive Acinetobacter baumannii CapV in apo and 3′3′-cGAMP-bound forms, along with cryo-EM structures of activated CapV-3′3′-cGAMP complex, with or without substrate dioleoylphosphatidyl-ethanolamine (DOPE). Apo-CapV forms symmetric dimers in a “closed” state. 3′3′-cGAMP binding drives lateral polymerization of dimers into filament assembly, inducing an “open” state that exposes the active site and substrate-binding cleft. DOPE binding further shifts CapV to an “ajar” state, where a Y-shaped cleft positions DOPE for hydrolysis via a conserved Ser/Asp catalytic dyad. This 3′3′-cGAMP-induced filamentation mirrors activation mechanisms of TIR-STING, TIR-SAVED, and mammalian STING, revealing a conserved signaling pattern across immune systems.