Abstract
The cyclic oligonucleotide-based antiphage signaling system (CBASS) is a key defense mechanism that protects bacteria against viral infections, exhibiting functional homology with the eukaryotic cyclic GMP-AMP synthase-stimulator of interferon gene innate immune pathway. The CBASS immune pathway in Vibrio cholerae, a significant human pathogen, positions it as a promising target for novel therapeutic strategies. Here, we report the biochemical and structural characterization of the CdnG-Cap5 CBASS system from V. cholerae, a highly abundant and representative clade G system. Our results elucidate the mechanistic basis of CBASS immunity, from second messenger synthesis to effector activation. We demonstrate that VcCdnG produces 3'2'-cyclic GMP-AMP as a bacterial second messenger, which specifically binds the VcCap5 effector and triggers its tetramerization, leading to cell death. VcCap5 exhibits multiligand sensitivity and dose-responsive behavior, suggesting a sophisticated strategy for threat-level assessment that allows V. cholerae to balance effective antiphage defense. This study provides molecular insights into one of the most widespread CBASS systems and expands our understanding of bacterial immune mechanisms in the ongoing conflict with phages.