ParD is the antitoxin of a bacterial toxin-antitoxin gene pair. The cognate toxin is ParE in, Pfam:PF05016. The family contains several related antitoxins from Cyanobacteria, Proteobacteria and Actinobacteria. Antitoxins of this class carry an N-terminal ribbon-helix-helix domain, RHH, that is highly conserved across all type II bacterial antitoxins, which dimerises with the RHH domain of a second VapB molecule. A hinge section follows the RHH, with an additional pair of flexible alpha helices at the C-terminus. This C-terminus is the toxin-binding region of the dimer, and so is specific to the cognate toxin, whereas the RHH domain has the specific function of lying across the RNA-binding groove of the toxin dimer and inactivating the active-site - a more general function of all type II antitoxins.

1: Toxin-antitoxin loci are highly abundant in free-living but lost from host-associated prokaryotes. Pandey DP, Gerdes K; Nucleic Acids Res. 2005;33:966-976. PMID:15718296

2: Interaction specificity, toxicity and regulation of a paralogous set of ParE/RelE-family toxin-antitoxin systems. Fiebig A, Castro Rojas CM, Siegal-Gaskins D, Crosson S; Mol Microbiol. 2010;77:236-251. PMID:20487277

3: A conserved mode of protein recognition and binding in a ParD-ParE toxin-antitoxin complex. Dalton KM, Crosson S; Biochemistry. 2010;49:2205-2215. PMID:20143871