This is an alpha-helical lobe domain found in Cas9 proteins. 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. Amino acid residues located in both the nuclease and alpha-helical lobe clefts are highly conserved within type II-A Cas9 proteins [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