Fig. 1.

Schematic depicting subunits and subassemblies of the A. tumefaciens VirB/VirD4 type IV secretion system (T4SS). Bottom: The 11 VirB subunits are synthesized from the virB operon and VirD4 from the separate virD operon. Lower: The VirB/VirD4 subunits are inserted in the inner membrane (IM) or delivered to the periplasm (P) with general topologies/locations as indicated. Upper: The subunits form a network of interactions resulting in four functional subassemblies: i) T4CP substrate receptor, ii) inner membrane translocase (IMC), iii) outer membrane complex (OMC)/core complex, and iv) extracellular pilus. The T4CP, IMC, and OMC interact to form the substrate translocation channel (red arrow). During translocation, the T-DNA forms formaldehyde-crosslinkable contacts with the 6 VirB/VirD4 subunits listed [38]. The IMC and OMC, together with the VirB1 transglycosylase, mediate assembly of the conjugative pilus.