Fig. 1: The Ton and Tol systems.
From: Cryo-EM structures of the E. coli Ton and Tol motor complexes
Schematic representation of the Tol and Ton systems. Tol is represented on the left: five TolQ subunits assemble at the inner membrane (IM) to form a pentamer that defines a central hydrophobic pore in which a dimer of TolR subunits resides. The TolA subunit has one transmembrane (TM) domain, a very elongated periplasmic linker with predicted helical domains, and a folded C-terminal domain that crosses the peptidoglycan layer (PG) and interacts with the TolB-Pal complex at the outer membrane (OM). The proton gradient, or pmf, at the IM is used as an energy source by the TolQR subcomplex. The force generated is transmitted by TolA to displace TolB from Pal, allowing Pal to interact with the PG. The architecture of the Ton system, right, is similar: ExbB and ExbD are homologous to TolQ and TolR and have the same architecture. The elongated periplasmic linker of TonB crosses the PG layer and the C-terminal domain of TonB interacts with TonB Dependent Transporters (TBDTs) at the OM. The force generated by the proton translocation at the ExbBD subcomplex is transmitted by TonB to open a channel in the TBDT, allowing diffusion of the bound nutrient in the periplasmic space. The red arrow on TolA shows the location where the TEV site was introduced.
