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Bacterial conjugation is an example of macromolecular trafficking between cells and responsible for the spreading of antibiotic resistance among bacteria. It involves translocation of single-stranded DNA across membranes through a type IV secretion system. A coupling protein links the DNA-processing nucleoprotein complex, the relaxosome, with the transport apparatus during cell mating. In Escherichia coli plasmid R388 such a protein is TrwB, a basic integral inner-membrane nucleoside-triphosphate-binding protein. TrwB is the structural prototype for the type IV secretion system coupling proteins, a family of proteins essential for macromolecular transport between cells and export. The structure of a soluble TrwB variant unveils an elongated molecule with six equivalent protein units featuring a spherical quaternary structure, leaving a central channel. The structures of the non-liganded protein and four different …