This family includes the toxic component HepT of a type II toxin-antitoxin (TA) system, which has RNase activity. These proteins contain a HEPN (higher eukaryotes and prokaryotes nucleotide-binding) domain and are neutralised through tri-AMPylation by the cognate antitoxin MntA, containing a MNT (minimal nucleotidyltransferase) domain [1, 2, 3]. HepT-MnA form an heterooctamer (at a 2:6 ratio), a rare organisation for this kind of TA systems. HepT dimerises and enables the formation of a deep cleft at the HEPN-domain interface, containing the RX4-6H motif (where X is any amino acid and the residue immediately after the conserved R is typically a polar amino acid) as the active site that functions as an RNA-cleaving RNase. This type II TA system regulates cell motility and confers plasmid stability [2]. Due to the prevalence of these HEPN/MNT modules in bacteria and archaea, it has been suggested that these TA systems may also play a role in the environmental adaptation to extreme habitats [3].

1: Identification and characterization of a HEPN-MNT family type II toxin-antitoxin in Shewanella oneidensis. Yao J, Guo Y, Zeng Z, Liu X, Shi F, Wang X; Microb Biotechnol. 2015;8:961-973. PMID:26112399

2: Structure-function analyses reveal the molecular architecture and neutralization mechanism of a bacterial HEPN-MNT toxin-antitoxin system. Jia X, Yao J, Gao Z, Liu G, Dong YH, Wang X, Zhang H; J Biol Chem. 2018;293:6812-6823. PMID:29555683

3: Novel polyadenylylation-dependent neutralization mechanism of the HEPN/MNT toxin/antitoxin system. Yao J, Zhen X, Tang K, Liu T, Xu X, Chen Z, Guo Y, Liu X, Wood TK, Ouyang S, Wang X; Nucleic Acids Res. 2020;48:11054-11067. PMID:33045733