TonB and the Gram‐negative dilemma

Abstract

TonB protein serves as an energy transducer to couple cytoplasmic membrane energy to high‐affinity active transport of iron siderophores and vitamin B₁₂ across the outer membranes of Gram‐negative bacteria. The biochemical mechanism of the energy transduction remains to be determined, but important details are already known. TonB is targeted to and anchored in the cytoplasmic membrane by a single membrane‐spanning domain and spans the periplasm to physically interact with outer‐membrane receptors of the transport ligands. TonB‐dependent energy transduction is modulated by ExbB protein, which stabilizes TonB, and possibly by several other proteins including ExbC, ExbD, and TolQ. TonB has a relatively short functional half‐life that is accelerated when rates of active transport across the outer membrane are increased. A model that incorporates this information, as well as some tempered speculation, is presented.