Structural basis for the concerted antiphage activity in the SIR2-HerA system

Recently, a novel two-gene bacterial defense system against phages, encoding a SIR2 NADase and a HerA translocase, has been identified. However, the molecular mechanism of the bacterial SIR2-HerA immune system remains unclear. Here, we determine the cryo-EM structures of SIR2, HerA and their complex in different functional states. The SIR2 proteins oligomerize into a dodecameric ring-shaped structure consisting of two layers of interlocked hexamers, in which each SIR2 unit exhibits an auto-inhibited conformation. Distinct from the canonical AAA+ proteins, the HerA hexamer in this antiphage system adopts a split spiral arrangement, resembling the substrate-binding state, which is stabilized by a unique C-terminal extension. SIR2 and HerA proteins assemble into a ~ 1.1 MDa torch-shaped complex to fight against phage infection. Importantly, disruption of the interactions between SIR2 and HerA largely abolishes the antiphage activity. Interestingly, HerA binding alters the oligomer state of SIR2, switching from a 12-mer state to a 14-mer state. On the other hand, binding of SIR2 stimulates the ATPase activity of HerA. Together, our study not only provides a structural basis for the functional communications between SIR2 and HerA proteins, but also unravels a novel concerted antiviral mechanism through nucleotide (NAD+ and ATP) depletion. ### Competing Interest Statement The authors have declared no competing interest.