This family includes the Type IV secretory pathway VirB3 protein, that is found associated with bacterial inner and outer membranes [2]. The family also includes the conjugal transfer protein TrbD family that contains a nucleotide binding motif and may provide energy for the export of DNA or the export of other Trb proteins [3].

1: Structural characterization of the virB operon on the hairy-root-inducing plasmid A4. Liang Y, Aoyama T, Oka A; DNA Res 1998;5:87-93. PMID:9679196

2: Membrane location of the Ti plasmid VirB proteins involved in the biosynthesis of a pilin-like conjugative structure on Agrobacterium tumefaciens. Shirasu K, Kado CI; FEMS Microbiol Lett 1993;111:287-294. PMID:8405938

3: The conjugal transfer system of Agrobacterium tumefaciens octopine-type Ti plasmids is closely related to the transfer system of an IncP plasmid and distantly related to Ti plasmid vir genes. Alt-Morbe J, Stryker JL, Fuqua C, Li PL, Farrand SK, Winans SC; J Bacteriol 1996;178:4248-4257. PMID:8763954