The protein components are named VirB1 to VirB11 in the A. tumefaciens
T4SS (TrwN to TrwD in R388, see Fig. 1a).
Evidence is accumulating that there is a conserved T4SS core made up of proteins VirB6 to VirB10; these proteins interact with each other and probably assemble a complex that spans both membranes.
In addition, two traffic ATPases are associated with the core complex at the cytoplasmic side: VirB4 and VirB11. The pilus appendage, probably involved in interaction with the recipient cell, is composed of subunits of VirB2 and VirB5 as major and minor components. Finally, a lytic transglycosilase (VirB1) probably helps assembly of the conduit by degrading the peptidoglycan in the periplasm.