Allosteric activation mechanism of the type VII CRISPR-Cas system.

Type VII CRISPR-Cas system, evolutionarily associated with type III systems, utilizes a Cascade complex formed by Cas5 and catalytically inactive Cas7 copies for target RNA binding, but instead incorporates a specialized Cas14 ribonuclease for target cleavage. Here, we report a high-quality cryo-EM structure at the target engagement state with a shortened crRNA and elucidate how the recruited Cas14 captures the target RNA and undergoes target-mediated activation. The signature Cas14 is homologous to eukaryotic CPSF73 and prokaryotic RNase J, comprising two conserved subdomains, MβL and β-CASP. Different from canonical type III systems, 5'-end target RNA, rather than 3'-end, is bent into the positively charged binding channel formed by the two subdomains to access the conserved catalytic pocket on Cas14. Two special structural features, α1 helix from Cas7 and α10 helix from Cas14, promote the bent target RNA docking into the catalytic pocket of Cas14 nuclease in concert. A dual-functional loop, displaced by the entering target RNA, induces a closed-to-open transition between the two subdomains for nuclease activation. More importantly, the flipped dual-functional loop also maintains the stabilization of incoming target RNA. Altogether, our work provides a more comprehensive understanding of type VII system mechanism, laying a mechanistic foundation for RNA-targeting tool development.