Orthosteric and allosteric effects of anti-CRISPR II-C1 inhibition on GeoCas9 from integrated structural biophysics

Anti-CRISPRs (Acrs) are small protein inhibitors of CRISPR-Cas effectors that originate from the translated genetic material of bacteriophage. Harnessing the natural ability of Acrs to bind and disrupt CRISPR-Cas editing can provide enhanced spatiotemporal control of gene editing. Recent studies have revealed diverse structures and functions of Acrs, however, atomistic studies of the specific molecular mechanisms behind Acr inhibition are lacking. Here, we reveal how structure, function, and dynamics govern AcrIIC1 inhibition of Cas9 from G. stearothermophilus (GeoCas9) via its HNH nuclease domain. An X-ray crystal structure of the GeoHNH-AcrIIC1complex reveals a conserved binding interface at the catalytic site and disruption of crucial electrostatic contacts known to modulate the thermostability of GeoCas9. AcrIIC1 binding also rewires the intrinsic dynamics of the GeoHNH domain, stimulates millisecond motions that are absent from the unliganded nuclease, and attenuates the guide RNA affinity of GeoCas9. Subsequent AcrIIC1 mutations in residues at its crystallographic binding interface uncouple Acr binding from inhibition, providing new insight into mechanism by which AcrIIC1 acts on GeoCas9.