Structural domains comprising this superfamily share the structure of the type IV secretion system proteins VirB10, TraB and TrbI. All of this are part of the different DNA transfer systems the tumour-inducing (Ti) plasmid of Agrobacterium tumefaciens encodes where the virB operon is required for DNA transfer to the plant host and the tra/trb systems are required for conjugal transfer of the Ti plasmid between cells of Agrobacterium PMID:12169609.

The trbI gene encodes an intrinsic inner-membrane protein that is 128-amino-acids long. Although it is not essential for conjugational transfer, the TrbI protein greatly increases conjugational efficiency as it is believed to influence the kinetics of pilus outgrowth and/or retraction PMID:1355084.