VapB is the antitoxin of a bacterial toxin-antitoxin gene pair. The cognate toxin is VapC, Pfam:PF05016. The family contains several related antitoxins from Cyanobacteria and Actinobacterial families. 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 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: Comprehensive functional analysis of Mycobacterium tuberculosis toxin-antitoxin systems: implications for pathogenesis, stress responses, and evolution. Ramage HR, Connolly LE, Cox JS; PLoS Genet. 2009;5:e1000767. PMID:20011113