The study investigates the role of Cam1, a CARF effector with a transmembrane domain, in type III CRISPR-Cas immunity. Cam1, found in *Nitrosococcus halophilus*, is structurally and functionally characterized, revealing that it binds cyclic oligoadenylate (cA) second messengers and forms a tetrameric pore on the bacterial membrane. In *Staphylococcus aureus*, Cam1 mediates growth arrest and membrane depolarization upon activation by cA, preventing viral propagation without causing lysis. This mechanism is essential for defense against phage infection, especially when the viral target is expressed late in the lytic cycle. The findings highlight the evolutionary flexibility of type III CRISPR-Cas systems, which can utilize effectors beyond nucleic acid degradation to provide robust immunity.The study investigates the role of Cam1, a CARF effector with a transmembrane domain, in type III CRISPR-Cas immunity. Cam1, found in *Nitrosococcus halophilus*, is structurally and functionally characterized, revealing that it binds cyclic oligoadenylate (cA) second messengers and forms a tetrameric pore on the bacterial membrane. In *Staphylococcus aureus*, Cam1 mediates growth arrest and membrane depolarization upon activation by cA, preventing viral propagation without causing lysis. This mechanism is essential for defense against phage infection, especially when the viral target is expressed late in the lytic cycle. The findings highlight the evolutionary flexibility of type III CRISPR-Cas systems, which can utilize effectors beyond nucleic acid degradation to provide robust immunity.