Mechanism of the melibiose porter in membrane vesicles of Escherichia coli.
The melibiose transport system of Escherichia 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 [Kaczorowski, G. J., & Kaback, H. R. (1979) Biochemistry 18, 3691], efflux and exchange of TMG occur at the same rate, implying that the rates of the two 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. It is proposed that the sodium gradient (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 sodium 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.