Abstract:

<p>Prokaryotic CRISPR-Cas adaptive immune systems utilize sequence-specific RNA-guided endonucleases&nbsp;to defend against infection by viruses, bacteriophages, and mobile elements, while these foreign genetic elements evolve diverse anti-CRISPR proteins to overcome the CRISPR-Cas-mediated defense of the host. Recently, AcrIIA2 and AcrIIA4, encoded by Listeria monocytogene prophages, were shown to block the endonuclease activity of type II-A Streptococcus pyogene Cas9 (SpyCas9). We now report the crystal structure of AcrIIA4 in complex with single-guide RNA-bound SpyCas9, thereby establishing that AcrIIA4 preferentially targets critical residues essential for PAM duplex recognition, as well as blocks target DNA access to key catalytic residues lining the RuvC&nbsp;pocket. These structural insights, validated by&nbsp;biochemical assays on key mutants, demonstrate that AcrIIA4 competitively occupies both PAM-interacting and non-target DNA strand cleavage catalytic pockets. Our studies provide insights into anti-CRISPR-mediated suppression mechanisms for inactivating SpyCas9, thereby broadening the applicability of CRISPR-Cas regulatory tools for genome editing.</p>