Fig. 4: Conserved network of residues connecting TolA/TonB and TolQ/ExbB TM3, preferential binding of TolA and TonB, and schematic model of Ton assembly and mechanism.
From: Cryo-EM structures of the E. coli Ton and Tol motor complexes
A Left panel: atomic surface representation of TolATEVQR highlighting the conservation of residues (Consurf 23). Middle panel: TolQ chains (A−D) omitted to show the hydrophobic pore and TolR. Network of conserved residues connecting the TM of TolA with the TolQ TM3 is highlighted. Right panel: zoom in of the highlighted region, in cartoon representation. Residues that form the network are shown with spheres. B Same representation as in panel A for TonB-ExbBD. C Predicted structure of the full TonB-ExbBD complex in cartoon representation. Unfolded linkers are dashed lines. The different subunits are colored as in Fig. 3 with the periplasmic domains of TonB in gold, ExbDY black and ExbDZ light grey. D Two schematic representations of the TonB-ExbBD complex, seen from the periplasm. ExbB subunits shown as rounded triangles, TonB (yellow) and ExbD (black and light grey) TMs as circles, ExbD periplasmic domains as ovals, the TonB linker connecting the TM and the D-box with yellow dashed lines, and the D-box as a yellow arrow. The distal C-terminal ___domain of TonB is not represented. The color code is the same as in (C). The upper panel has the D-box bound to the ExbD periplasmic dimer in one orientation, leading to the binding of the TonB TM ___domain either to ExbBE (orange) or ExbBA (blue). In the lower panel the D-box is in the opposite direction, and the TonB TM ___domain binds ExbBC (green). E Hypothetical assembly and mechanistic model for Ton. (1) Monomeric TonB (yellow) diffuses in the cytoplasmic membrane. TBDT (green) in the OM binds a nutrient (purple), inducing the TonB box (green arrow) to extend into the periplasm. (2) TonB C-terminal ___domain binds the TBDT TonB box, and the TonB D-box (yellow arrow) comes close to the ExbD periplasmic dimer (grey and black). (3) TonB D-box binds the ExbD dimer and (4) TonB TM ___domain interacts with ExbB in the membrane (red arrow) forming a full TBDT-TonB-ExbBD complex. (5) TonB-ExbBD lateral diffusion in the membrane pulls on the TonB periplasmic ___domain and reorients the TonB TM ___domain. This change is transmitted to the ExbB TM3 and opens the pentameric pore. Proton translocation through the pore causes the ExbD TM dimer to rotate. (6) ExbD rotation wraps around the TonB periplasmic linker, exerting a pulling force on the TonB C-terminal ___domain towards the periplasm. (7) The pulling force gradually unfolds the TBDT plug ___domain and opens a channel allowing the nutrient to diffuse into the periplasm. The force necessary to further unfold the plug ___domain is greater than the pulling force and TonB detaches from the TonB box, dissipating the tension on the periplasmic linker. TonB-ExbBD reverts to the ground state with proton channel closed, and the TBDT plug ___domain refolds into the barrel.
