Structural basis of Gabija anti-phage defence and viral immune evasion.

Bacteria encode hundreds of diverse defense systems that protect from viral infection and inhibit phage propagation 1-5 . Gabija is one of the most prevalent anti-phage defense systems, occurring in >15% of all sequenced bacterial and archaeal genomes 1,6,7 , but the molecular basis of how Gabija defends cells from viral infection remains poorly understood. Here we use X-ray crystallography and cryo-EM to define how Gabija proteins assemble into an ~500 kDa supramolecular complex that degrades phage DNA. Gabija protein A (GajA) is a DNA endonuclease that tetramerizes to form the core of the anti-phage defense complex. Two sets of Gabija protein B (GajB) dimers dock at opposite sides of the complex and create a 4:4 GajAB assembly that is essential for phage resistance in vivo. We show that a phage-encoded protein Gabija anti-defense 1 (Gad1) directly binds the Gabija GajAB complex and inactivates defense. A cryo-EM structure of the virally inhibited state reveals that Gad1 forms an octameric web that encases the GajAB complex and inhibits DNA recognition and cleavage. Our results reveal the structural basis of assembly of the Gabija anti-phage defense complex and define a unique mechanism of viral immune evasion.