This is a RuvC nuclease domain found in type V CRISPR-associated protein Cas12a (Cpf1), used for genome editing applications. These proteins carry out endoribonuclease activity for processing its own guide RNAs and RNA-guided DNase activity for target DNA cleavage. The C-terminal region of Cas12a carries the RUVC domain, NUC domain Pfam:PF18510 and the arginine-rich bridge helix (BH). Both the NUC and BH domains are nested in the RuvC domain. Mutations in the RuvC domain impair cleavage of both strands in a target DNA duplex, while a mutation in the Nuc domain impaired target strand cleavage only. This indicates that the DNA nuclease active sites are located at the interface of the RuvC and Nuc domains and that cleavage of the non-target DNA strand by the RuvC domain is a prerequisite for target strand cleavage by the Nuc domain [1].

1: Structural Basis for Guide RNA Processing and Seed-Dependent DNA Targeting by CRISPR-Cas12a. Swarts DC, van der Oost J, Jinek M; Mol Cell. 2017;66:221-233. PMID:28431230