TONB PROTEIN AND ENERGY TRANSDUCTION BETWEEN MEMBRANES

TonB protein couples cytoplasmic membrane electrochemical potential to active transport of iron-siderophore complexes and vitamin B12 throug h high-affinity outer membrane receptors of Gram-negative bacteria. Th e mechanism of energy transduction remains to be determined, but impor tant concepts have already begun to emerge. Consistent with its functi on, TonB is anchored in the cytoplasmic membrane by its uncleaved amin o terminus while largely occupying the periplasm. Both the connection to the cytoplasmic membrane and the amino acid sequences of the anchor are essential for activity. TonB directly associates with a number of envelope proteins, among them the outer membrane receptors and cytopl asmic membrane protein ExbB. ExbB and TonB interact through their resp ective transmembrane domains. ExbB is proposed to recycle TonB to an a ctive conformation following energy transduction to the outer membrane . TonB most likely associates with the outer membrane receptors throug h its carboxy terminus, which is required for function. In contrast, t he novel proline-rich region of TonB can be deleted without affecting function. A model that incorporates this information, as well as tempe red speculation, is presented.