The plasmid conjugative coupling protein TrwB forms hexamers from six structurally very similar protomers [1]. This hexamer contains a central channel running from the cytosolic pole (made up by the AADs) to the membrane pole ending at the transmembrane pore shaped by 12 transmembrane helices, rendering an overall mushroom-like structure. The TrwB_AAD (all-alpha domain) domain appears to be the DNA-binding domain of the structure. TrwB, a basic integral inner-membrane nucleoside-triphosphate-binding protein, is the structural prototype for the type IV secretion system coupling proteins, a family of proteins essential for macromolecular transport between cells and export [2].

1: The bacterial conjugation protein TrwB resembles ring helicases and F1-ATPase. Gomis-Ruth FX, Moncalian G, Perez-Luque R, Gonzalez A, Cabezon E, de la Cruz F, Coll M; Nature. 2001;409:637-641. PMID:11214325

2: Conjugative plasmid protein TrwB, an integral membrane type IV secretion system coupling protein. Detailed structural features and mapping of the active site cleft. Gomis-Ruth FX, Moncalian G, de la Cruz F, Coll M; J Biol Chem. 2002;277:7556-7566. PMID:11748238