Bacteria have evolved a wide array of defence systems to combat phage infection, many of which rely on complex signalling systems and large protein complexes to function1. Here we describe a 164-residue prophage-encoded protein that defends bacteria by sensing conserved oligomeric components of phage assembly. This protein, called ring interacting pore 1 (Rip1), is activated by the portal or small terminase proteins of infecting phages—oligomeric ring-shaped complexes that are essential for virion maturation. Rip1 uses these phage protein ring complexes as a template to assemble into membrane-disrupting pores that inhibit phage virion assembly and cause premature death of the host cell. Rip1 homologues are widely distributed across bacteria and provide robust defence against diverse phages. This study reveals a strategy by which a small defence protein integrates both sensing and effector activity by exploiting a conserved feature of viral assembly. The mechanism mirrors eukaryotic pore-forming immunity but is executed by a single protein, offering an evolutionarily streamlined solution to viral detection and defence.
