Cam1 mediates membrane depolarization to provide phage defense during the Type III CRISPR-Cas immune response

Prokaryotic type III CRISPR-Cas systems provide immunity against viruses and plasmids using CARF protein effectors. Recognition of transcripts of these invaders with sequences that are complementary to crRNA guides leads to the production of cyclic oligoadenylate second messengers, which bind CARF domains and trigger the activity of an effector domain. While most effectors degrade host and invader nucleic acids, some are predicted to contain transmembrane helices (TMH) without an enzymatic function. Whether and how these CARF-TMH fusion proteins facilitate the type III CRISPR-Cas immune response has not been studied. Here we investigate the role of cyclic-oligoadenylate-activated membrane protein 1 (Cam1) during the type III CRISPR immunity. Structural and biochemical analysis reveals that the CARF domains of a Cam1 dimer bind cyclic tetra-adenylate second messengers. In vivo, Cam1 localizes to the membrane, is predicted to form a tetrameric transmembrane pore, and provides defense against viral infection through the induction of membrane depolarization and growth arrest. These results reveal that CRISPR immunity do not always operate through the degradation of nucleic acids, as initially thought, but through a wider range of cellular responses.