Figure 1.

Schematic of the T4S system. Subunits at the right, identified with the A. tumefaciens VirB/VirD4 nomenclature, assemble as the T4S apparatus/pilus across the Gram-negative cell envelope. Hexameric ATPases establish contacts with the integral inner membrane (IM) subunits to form an inner membrane complex. VirB7, VirB9, and VirB10 form a core complex extending from the IM, periplasm, and outer membrane (OM). A domain of unspecified composition (grey bullet structure) and the pilus assemble within the central chamber of the core complex.