Mechanism of the melibiose porter in membrane vesicles of Escherichia coli

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Abstract
The melibiose transport system of E. coli catalyzes sodium-methyl 1-thio-.beta.-D-galactopyranoside (TMG) symport, and the cation is required not only for respiration-driven active transport but also for binding of substrate to the carrier in the absence of energy and for carrier-mediated TMG efflux. As opposed to the proton-.beta.-galactoside symport system, efflux and exchange of TMG occur at the same rate, implying that the rates of the 2 processes are limited by a common step, most likely the translocation of substrate across the membrane. Furthermore, the rate of exchange, as well as efflux, is influenced by imposition of a membrane potential (.DELTA..PSI.; interior negative), suggesting that the ternary complex between sodium, TMG and the porter may bear a net positive charge. Consistently, energization of the vesicles leads to a large increase in the Vmax for TMG influx, with little or no change in the apparent Km of the process. The sodium Na (Na+out > Na+in) and the .DELTA..PSI. (interior negative) may affect different steps in the overall mechanism of active TMG accumulation in the following manner: the Na gradient causes an increased affinity for TMG on the outer surface of the membrane relative to the inside and the .DELTA..PSI. facilitates a reaction involved with the translocation of the positively charged ternary complex to the inner surface of the membrane.