Structural basis of outer membrane biogenesis and cell division by Tol/Pal nanomachinery

TolQRA, a key member of the proton motive force (PMF) family including MotAB and ExbBD, transduces PMF from the inner membrane to the outer bacterial envelope. This mechanism compensates for the absence of conventional energy sources in the outer membrane compartment of Gram-negative bacteria. Here, we present cryo-electron microscopy structures of the TolQRA complex at pH 5.4 and pH 8.0, resolved at 3.18 and 3.60 angstroms, respectively. Our findings revealed that TolQRA has a stoichiometry of 5:2:5, with key residues mediating interactions between TolQ, TolR, and TolA. Notably, the nanomachine has appeared to exhibit an asymmetric arrangement, which may be consistent with a two-gate mechanism for proton translocation and energy transfer. These insights illuminate the mechanism of energy transduction in TolQRA, offering parallels with the ExbBD-TonB and MotAB systems. Furthermore, this work provides a foundation for the development of innovative therapeutics that target the critical TolQRA complex.