FIG. 7.

Model of TonB-dependent energy transduction. ExbB and ExbD may exist as a heterohexamer complex (three ExbB and three ExbD) in the cytoplasmic membrane. ExbB and ExbD probably interact through transmembrane segments and together may provide a protein channel for the transmembrane segment of TonB. Step 1. TonB is energized by passage of a proton through the proposed ExbBD heterohexamer (25). Step 2. TonB either shuttles to or becomes more strongly associated with the outer membrane (28) and physically contacts a TonB-dependent outer membrane receptor (OMR) (40), triggering a conformational change in the OMR (19) such that ligand is pumped into the periplasmic space (36, 48). Step 3. ExbB and ExbD cycle TonB that is in a tight association with the outer membrane back to a high-affinity interaction with the cytoplasmic membrane energy transduction complex, allowing the energy transduction process to be repeated (28).