THE CONSERVED PROLINE-RICH MOTIF IS NOT ESSENTIAL FOR ENERGY TRANSDUCTION BY ESCHERICHIA-COLI TONB PROTEIN

TonB protein functions as an energy transducer, coupling cytoplasmic m embrane electrochemical potential to the active transport of vitamin B -12 and Fe(III)-siderophore complexes across the outer membrane of Esc herichia coli and other Gram-negative bacteria. Accumulated evidence i ndicates that TonB is anchored in the cytoplasm, but spans the peripla smic space to interact physically with outer membrane receptors. ft ha s been presumed that this ability is caused by a conserved (Glu-Pro)n- (Lys-Pro)m repeat motif, predicted to assume a rigid, linear conformat ion of sufficient length to reach the outer membrane. Based on in vitr o studies with synthetic peptides and purified FhuA outer membrane rec eptor, it has been suggested that this region contains a site that dir ectly binds outer membrane receptors and is essential for energy trans duction. We have found a TonB lacking the (Glu-Pro)n-(Lys-Pro)m repeat motif (TonBDELTA(66-100)). TonBDELTA(66-100) is fully capable of irre versible phi80 adsorption, except under physiological circumstances wh ere the periplasmic space is expanded. Based on the ability of TonBDEL TA(66-1 00) to interact with outer membrane receptors and components o f the energy transduction apparatus under normal physiological conditi ons, it is evident that the TonB prolinerich region has no role in ene rgy transduction other than to provide a physical extension sufficient to reach the outer membrane.