Energy transduction between membranes. TonB, a cytoplasmic membrane protein, can be chemically cross-linked in vivo to the outer membrane receptor FepA.

TonB, a cytoplasmic membrane protein, couples cytoplasmic membrane protonmotive force to active transport across the outer membrane of Escherichia coli. In vivo cross-linking studies were initiated to analyze TonB interactions with other cell envelope proteins. Four TonB-specific cross-linked complexes were detected with apparent molecular masses of 195, 77, 59, and 43.5 kDa. The 195-kDa complex was shown to contain both TonB and FepA, the outer membrane receptor for the siderophore enterochelin. The 195-kDa complex is absent in strains missing either TonB or FepA and can be detected by either TonB-specific or FepA-specific monoclonal antibodies. This is the first direct in vivo evidence that TonB can span the periplasmic space to interact physically with outer membrane receptors. Consistent with that observation, the outer membrane protease OmpT was shown to play a role in TonB turnover, both in the presence and absence of ExbB results in the rapid degradation of TonB. The absence of OmpT could be used to stabilize TonB in an exbB::Tn10 strain such that steady state levels of TonB protein are identical to a wild-type strain. Under those conditions, the absence of ExbB results in greatly reduced TonB activity, indicating that ExbB plays a direct role in energy transduction and probably secondarily protects TonB protein from proteolysis. The 59-kDa complex was absent in an exbB::Tn10 strain, suggesting either that ExbB is in the complex with TonB or that ExbB is required to form the 59-kDa complex. A tolQ nonsense mutation had no effect on the cross-linking profile observed, confirming that its participation in TonB-dependent phenomena is minor and most likely the result of evolutionary cross-talk.