This domain is found at the N-terminal end of CRISPR-associated nuclease/helicase Cas3 subtype I-F/YPEST from Pseudomonas aeruginosa and similar sequences predominantly found in proteobacteria. In the type I-F CRISPR-Cas system, Cas3 is fused with Cas2 at the N terminus. This entry represents the Cas2 domain of Cas3, which shows a four-stranded beta-sheet and two alpha-helices, in a structure that resembles that of Cas2 from the Escherichia coli. It appears N- terminal to the histidine-aspartate (HD)-type nuclease domain (Pfam:PF18019) [1,2].

1: Structural basis of Cas3 inhibition by the bacteriophage protein AcrF3. Wang X, Yao D, Xu JG, Li AR, Xu J, Fu P, Zhou Y, Zhu Y; Nat Struct Mol Biol. 2016;23:868-870. PMID:27455460

2: A CRISPR evolutionary arms race: structural insights into viral anti-CRISPR/Cas responses. Wang J, Ma J, Cheng Z, Meng X, You L, Wang M, Zhang X, Wang Y; Cell Res. 2016;26:1165-1168. PMID:27585537