A phage-encoded sponge protein suppresses bacterial TIR-Caspase immune signaling

Triggering immune response through generation of signal molecules is a common immune strategy across all domains of life. In the bacterial type IV Thoeris anti-phage system, a Toll/interleukin-1 receptor (TIR)-domain protein produces adenosine 5'-diphosphate-cycloN7:1"-ribose (N7-cADPR) as the immune signal to activate a Caspase-like effector. Here, we identified an inhibitor of type IV Thoeris through a phage mating assay that allows a sensitive phage to acquire anti-defense genes from related resistant phages. The inhibitor (hereafter TadIV-1) functions as a sponge that sequesters the N7-cADPR signal to inhibit Caspase activation. Structural analyses of TadIV-1 indicate a distinctive signal binding mechanism, wherein the binding pocket comprises its N-terminal flexible loop. In addition, phages lacking TadIV-1 can escape type IV Thoeris sensing through mutation in the capsid vertex protein. Collectively, this work expands the phage anti-defense arsenal with a unique immune signal sequestration mechanism and provides insights into phage invasion recognition mechanism of type IV Thoeris.