This domain family corresponds to the N-terminal of CD-NTase associated protein 4 (Cap4) from bacteria and archaea. CD-NTase-associated protein 4 (Cap4) is a nucleotide second messenger sensor that functions is CBASS immunity. This is the effector domain which has dsDNA nuclease activity, sharing structural homology with type II restriction endonucleases and it has the putative active-site conserved residues required for divalent metal coordination. This domain is inactive as a monomer; after Cap4 signal recognition through its C-terminal SAVED domain (Pfam:PF18145), it oligomerises to closely locate two nuclease effector domains. This domain does not participates in ligand specificity and it has a promiscuous DNA cleavage response [1].

1: CBASS Immunity Uses CARF-Related Effectors to Sense 3'-5'- and 2'-5'-Linked Cyclic Oligonucleotide Signals and Protect Bacteria from Phage Infection. Lowey B, Whiteley AT, Keszei AFA, Morehouse BR, Mathews IT, Antine SP, Cabrera VJ, Kashin D, Niemann P, Jain M, Schwede F, Mekalanos JJ, Shao S, Lee ASY, Kranzusch PJ; Cell. 2020; [Epub ahead of print] PMID:32544385