Structure and Stoichiometry of the Ton Molecular Motor.

Herve Celia,Nicholas Noinaj,Susan K Buchanan

doi:10.3390/ijms21020375

Abstract

The Ton complex is a molecular motor that uses the proton gradient at the inner membrane of Gram-negative bacteria to generate force and movement, which are transmitted to transporters at the outer membrane, allowing the entry of nutrients into the periplasmic space. Despite decades of investigation and the recent flurry of structures being reported by X-ray crystallography and cryoEM, the mode of action of the Ton molecular motor has remained elusive, and the precise stoichiometry of its subunits is still a matter of debate. This review summarizes the latest findings on the Ton system by presenting the recently reported structures and related reports on the stoichiometry of the fully assembled complex.

Highlights

In addition to the cytoplasmic membrane, Gram-negative bacteria possess an additional outer membrane that acts as an efficient barrier against the environment [1]
The Ton system is specific to Gram-negative bacteria and allows for the transfer of energy from the inner membrane via the Ton complex to receptors found within the outer membrane [2]
It consists of three integral membrane proteins: TonB, ExbB and ExbD [6]

Summary

Introduction

In addition to the cytoplasmic membrane, Gram-negative bacteria possess an additional outer membrane that acts as an efficient barrier against the environment [1]. The Ton complex is a unique inner membrane complex that is powered by the proton motive force (pmf) at the inner membrane, much like the ATP synthase [5] It consists of three integral membrane proteins: TonB, ExbB and ExbD [6]. ExbB and ExbD associate to form the proton translocation part of the motor while the energy derived from the pmf is propagated through the elongated TonB subunit that physically interacts with TonB-dependent transporters (TBDTs) at the outer membrane. This interaction opens a gate through the barrel domain of the TBDT to allow the entry of the bound nutrient into the periplasm (Figure 1). Nature 2016 [7] Celia et al, Nature 2016 [7] Garcia-Herrero et al, 2007 [36]

TonB—The Energy Conduit of the Ton System

The Structure and Stoichiometry of ExbD within the Ton Complex

The Structure and Stoichiometry of ExbB within the Ton Complex

Findings

Mechanism for Energy Transduction by the Ton System