Advances in understanding Ton and Tol system motor proteins

The Ton and Tol-Pal systems are molecular machines that are essential for survival of Gram-negative bacteria.Both use the energy derived from the proton gradient at the inner membrane to generate force on protein components at the outer membrane. Ton and Tol share extensive homology, but they fulfill different functions: Ton is involved in the active transport of essential nutrients from the extracellular media into the cell, while Tol maintains the outer membrane integrity and participates in the cell division process. Despite decades of biochemical and biophysical studies, the molecular mechanism coupling the proton gradient at the inner membrane with the propagation of force and movement to the outer membrane is not understood. In this review, we discuss the recent high-resolution structures obtained for both systems, and how these structures fit with existing mechanistic models.