Membrane-bound toxin component of toxin-antitoxin systemCptA is a family of bacterial proteins named for the member of this family, YGFX_ECOLI. YgfX was previously thought to be the toxic part of a toxin-antitoxin module along with the antitoxin, pfam03937 Sdh5. However, studies have shown that, YgfX interferes with correct cell division and morphology. Furthermore, the function of YgfX-SdhE as a TA system could not be demonstrated in either E. coli or Serratia sp. ATCC 39006. YgfX is predicted to have a short N-terminal cytoplasmic domain followed by two transmembrane helices (TMHs) separated by a short periplasmic loop and finally, a larger C-terminal cytoplasmic domain. The TMHs of YgfX are required for activity, but the sequence of the cytoplasmic 13 N-terminal amino acids is not essential. Furthermore, the amino acids W34 and D117 are not required for localization but are necessary for YgfX multimerization, interaction with SdhE, and YgfX activity. It is proposed that the formation of YgfX multimeric membrane-bound proteins are required to enable the interaction with the cytoplasmic SDH assembly factor SdhE. Another study has demonstrated that sdhEygfX (bicistronic operon) affects pig biosynthesis, directly or indirectly, at the level of transcription of the biosynthetic operon (pigA-O). It has also been suggested that, in addition to indirect transcriptional activation of pigA-O, YgfX might facilitate the formation of a terminal pig biosynthetic complex consisting of PigB and PigC.