Mechanistic insights into the inhibition of the CRISPR-Cas surveillance complex by anti-CRISPR protein AcrIF13.

Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins provide prokaryotes with nucleic acid-based adaptive immunity against infections of mobile genetic elements, including phages. To counteract this immune process, phages have evolved various anti-CRISPR (Acr) proteins which deactivate CRISPR-Cas-based immunity. However, the mechanisms of many of these Acr-mediated inhibitions are not clear. Here, we report the crystal structure of AcrIF13 and explore its inhibition mechanism. The structure of AcrIF13 is unique and displays a negatively charged surface. Additionally, biochemical studies identified that AcrIF13 interacts with the type I-F CRISPR-Cas surveillance complex (Csy complex) to block target DNA recognition and that the Cas5f-8f tail and Cas7.6f subunit of the Csy complex are specific binding targets of AcrIF13. Further mutational studies demonstrated that several negatively charged residues of AcrIF13 and positively charged residues of Cas8f and Cas7f of the Csy complex are involved in AcrIF13-Csy binding. Together, our findings provide mechanistic insights into the inhibition mechanism of AcrIF13 and further suggest the prevalence of the function of Acr proteins as DNA mimics.