Catalytic domain, repeat 2, of Deoxyribonuclease II and similar proteins
Catalytic domain, repeat 2, of Deoxyribonuclease II (DNase II, EC 3.1.22.1), an endodeoxyribonuclease with ubiquitous tissue distribution. It is essential for accessory apoptotic DNA fragmentation and DNA clearance during development, as well as in tissue regeneration in higher eukaryotes. Unlike the majority of nucleases, DNase II functions optimally at acidic pH in the absence of divalent metal ion cofactors. It hydrolyzes the phosphodiester backbone of DNA by a single strand cleavage mechanism to generate 3'-phosphate termini. The majority of family members contain an N-terminal signal-peptide leader sequence, which is critical for N-glycosylation and DNase II activity. DNase II is a monomeric nuclease that contains two copies of a variant HKD motif, where the aspartic acid residue is not conserved. The HKD motif (H-x-K-x(4)-D, where x represents any amino acid residue) characterizes the phospholipase D (PLD, EC 3.1.4.4) superfamily. The catalytic center of DNase II is formed by the two variant HKD motifs from the N- and C-terminal domains in a pseudodimeric way. Members of this family are mainly found in metazoans, and vertebrate proteins have been further classified into DNase II alpha and beta (also known as DNase II-like acid DNase, DLAD) subtypes. A few homologs are found in non-metazoan species, but none are found in fungi, plants or prokaryotes, with the sole exception of Burkholderia pseudomallei. Among those homologs, the Caenorhabditis elegans C07B5.5 ORF encoding NUC-1 apoptotic nuclease, the uncharacterized C. elegans crn-6 (cell death related nuclease) gene encoding protein, and the putative gene CG7780 encoding Drosophila DNase II (dDNase II) have similar cleavage activity and specificity to mammalian DNase II enzymes. They may function like an acid DNase implicated in degrading DNA from apoptotic cells engulfed by macrophages. Plancitoxin I, the major lethal factor from the Acanthaster planci venom, is a unique homolog of mammalian DNase II. It has potent hepatotoxicity and the optimum pH for its activity is 7.2, unlike the optimum acidic PH for mammalian DNase II. Some members of this family contain substitutions of conserved residues found in the putative active site, which suggest that these proteins may have diverged from the canonical DNase II activity and may perform other functions.
Comment:Based on similarity with phospholipase D, which functions as a bi-lobed monomer with two catalytic domains. Each domain carries one copy of the conserved HKD motif and two domains form a single active site.
Comment:The HKD signature motif (expanded to H-x-K-x(4)-D-x(6)-G-S-x-N, where x represents any amino acid residue) characterizes the PLD superfamily. The DNase II subfamily carries a variant HKD motif.
Comment:Most residues in the HKD motif are part of the active site.
Jiyao W et al.(2023). "The conserved domain database in 2023.",