Conjugal transfer, TrbG/VirB9/CagX (IPR010258)

Short name: Conjugal_tfr_TrbG/VirB9/CagX

Family relationships


Several bacterial pathogens utilise conjugation machines to export effector molecules during infection. Such systems are members of the type IV or 'adapted conjugation' secretion family. The prototypical type IV system is the Agrobacterium tumefaciens T-DNA transfer machine, which delivers oncogenic nucleoprotein particles to plant cells. Other pathogens, including Bordetella pertussis, Legionella pneumophila, Brucella spp. and Helicobacter pylori (Campylobacter pylori), use type IV machines to export effector proteins to the extracellular milieu or the mammalian cell cytosol.

Conjugation machines of Gram-negative bacteria consist of two surface structures, the mating channel through which the DNA transfer intermediate and proteins are translocated and the conjugal pilus for contacting recipient cells. Various conjugative pili have been visualised, but to date there is no ultrastructural information about the mating channel. Recent work on the A. tumefaciens T-DNA transfer system has focused on identifying interactions among the VirB protein subunits and defining steps in the transporter assembly pathway. There are three functional groups of VirB proteins: proteins localised exocellularly forming the T-pilus or other adhesive structures; mating-channel components; and cytoplasmic membrane ATPases. Although all of these proteins probably assemble as a supramolecular complex, as yet there is no direct evidence for a physical association between the conjugative pilus and the mating channel.

Several lines of evidence suggest that VirB6-VirB10 are probable channel subunits. VirB6, a highly hydrophobic protein, is thought to span the cytoplasmic membrane several times and presently is the best candidate for a channel-forming protein. VirB7, an outer membrane lipoprotein, interacts with itself and with VirB9 via disulphide bonds between unique reactive cysteines present in each protein. The VirB7-VirB9 heterodimer localises at the outer membrane and plays a critical role in stabilising other VirB proteins during assembly of the transfer machine. VirB9 is also required for formation of chemically crosslinked VirB10 oligomers probably corresponding to homotrimers [PMID: 10920394].

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.