Crystal Structure of an Anti-CRISPR Protein, AcrIF2, and its Interaction With Type I-F Cas Proteins

CRISPRs and Cas proteins form a microbial adaptive immune system against phage infection. To counteract this defense mechanism, phages evolved anti-CRISPR (Acr) proteins that inhibit the CRISPR-Cas systems. Here, we report the crystal structure of AcrIF2, an Acr protein inactivating the type I-F CRISPR-Cas system in Pseudomonas aeruginosa. In the type I-F systems, multiple Cas proteins (Csy1-4) constitute a surveillance complex with a guide RNA for target recognition. We demonstrated that AcrIF2 binds strongly to Csy1-Csy2 subcomplexes from two Xanthomonas species. In the crystal structure of AcrIF2, the arrangement of Asp and Glu side chains resembles the negative charge distribution of the dsDNA backbone, confirming that AcrIF2 is a dsDNA mimic blocking target DNA recognition by the CRISPR surveillance complex. Our results corroborate the inhibitory mechanism employed by AcrIF2 and suggest its broad specificity against diverse type I-F CRISPR-Cas systems.