Type IV secretion-system coupling protein DNA-binding domain
The plasmid conjugative coupling protein TrwB forms hexamers from six structurally very similar protomers. 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.