Type IV secretion system, TraD, DNA-binding domain <p> The plasmid conjugative coupling protein TraD (also known as TrwB) is a basic integral inner-membrane nucleoside-triphosphate-binding protein. It is the structural prototype for the type IV secretion system coupling proteins, a family of proteins essential for macromolecular transport between cells [<cite idref="PUB00044522"/>]. This protein forms hexamers from six structurally very similar protomers [<cite idref="PUB00028411"/>]. This hexamer contains a central channel running from the cytosolic pole (formed by the all-alpha domains) to the membrane pole ending at the transmembrane pore shaped by 12 transmembrane helices, rendering an overall mushroom-like structure. The TrwB all-alpha domain appears to be the DNA-binding domain of the structure. </p>