Structural Superimposition

Abstact

Protease-mediated activation of immune effectors is an evolutionarily conserved mechanism. This study identifies a widespread trypsin-MBL (metallo-beta-lactamase) module as a core effector in diverse antiviral bacterial immune systems, such as Hachiman, AVAST and Argonaute. Focusing on the Hachiman-associated trypsin-MBL system, we show that trypsin*HamAB protease activity is inhibited by ATP, while MBL is an autoinhibited DNase with two insertion loops obstructing its catalytic site. Upon infection, trypsin*HamAB senses foreign DNA and hydrolyzes ATP, activating trypsin-like activity, which specifically cleaves MBL at the insertion loops to release repression. The activated MBL depletes DNA and arrests host cell growth. Cryo-electron microscopy structures of trypsin*HamAB-DNA reveal that DNA binding and ATP hydrolysis trigger HamAB oligomerization and trypsin-like domain release, enabling its activation. Our work elucidates a conserved immune mechanism wherein proteolytic activation of a nuclease enables robust immunity against phage while multilayered controls prevent self-toxicity, expanding the repertoire of immune processes governed by regulatory proteolysis.