Functional lac carrier protein in cytoplasmic membrane vesicles isolated from Escherichia coli: Temperature and pH dependence of dansyl-galactoside binding*

AUTOR(ES)

Therisod, Hélène

RESUMO

6′-(N-Dansyl)aminohexyl-1-thio-β-D-galactopyranoside binds specifically to the lac carrier protein in cytoplasmic membrane vesicles isolated from Escherichia coli. Binding can be induced by substrate oxidation (generation of an electrochemical gradient of protons), by potassium efflux in the presence of valinomycin (generation of a potassium diffusion potential), and by passive, carrier-mediated lactose efflux. We show that in all three cases the number of binding sites is temperature dependent. Binding is maximal and constant above 20°; it decreases between 20° and 10°. Oxidation of substrate (D-lactate) leads to the development of an electrochemical gradient of protons across the membrane (interior negative and alkaline), which is composed of interconvertible electrical and chemical gradients. We show that both the electrical potential across the membrane and the chemical difference in proton concentrations across the membrane are independent of temperature between 5° and 25°. We show that the number of binding sites induced by D-lactate oxidation depends on pH. At both 25° and 5°, the number of binding sites increases from pH 5 to pH 6.5, remains constant between pH 6.5 and 7, and decreases from pH 7 to pH 8. In contrast, the number of binding sites induced by passive, carrier-mediated lactose efflux is independent of pH between pH 5.5 and pH 8.

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