Structural Superimposition

Abstact

CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) systems provide powerful adaptive immunity against phage infection. In response, phages use anti-CRISPR (Acr) proteins to evade CRISPR immunity. The few type III Acrs identified so far show conditional effectiveness in countering type III immunity or rely on unknown or poorly understood inhibitory mechanisms. Here we report the discovery of AcrIIIA2, a type III-A Acr encoded by Streptococcus thermophilus phages. Biochemical and structural analyses reveal that phage AcrIIIA2 co-opts host enolase, a highly abundant glycolysis enzyme, to form a ternary complex with the S. thermophilus type III-A (Csm) CRISPR ribonucleoprotein complex, obstructing its immune responses. The enolase-chaperoned AcrIIIA2 blocks the initial step of phage RNA binding, thereby preventing downstream type III anti-phage immune responses. Enolase participates in the anti-immune response by serving as an essential structural scaffold, stabilizing Acr-CRISPR interactions. These findings uncover a new anti-defence strategy that exploits a well-conserved host factor to block CRISPR immunity.