Cas9 proteins are abundant across the bacterial kingdom, but vary widely in both sequence and size. All known Cas9 enzymes contain an HNH domain that cleaves the DNA strand complementary to the guide RNA sequence (target strand), and a RuvC nuclease domain required for cleaving the noncomplementary strand (non-target strand), yielding double-strand DNA breaks (DSBs). The crystal structures of type II-A and II-C Cas9 proteins highlight the features in Cas9 enzymes that support their function as RNA-guided endonucleases. Cas9 enzymes adopt a bilobed architecture composed of a nuclease lobe containing juxtaposed RuvC and HNH nuclease domains and a variable alpha-helical lobe likely to be involved in nucleic acid binding. The RuvC domain forms the structural core of the nuclease lobe, a six-stranded beta sheet surrounded by four alpha helices, with all three conserved subdomains (I, II, III) contributing catalytic residues to the active site [1].

1: Structures of Cas9 endonucleases reveal RNA-mediated conformational activation. Jinek M, Jiang F, Taylor DW, Sternberg SH, Kaya E, Ma E, Anders C, Hauer M, Zhou K, Lin S, Kaplan M, Iavarone AT, Charpentier E, Nogales E, Doudna JA; Science. 2014;343:1247997. PMID:24505130