Energy-dependent masking of substrate binding sites of the lactose permease of Escherichia coli

A method was devised to measure the number of specific substrate binding sites of lactose permease in membrane preparations derived from mechanically disrupted Escherichia coli.The method consists of incubation with radioactive thiodigalactoside (galactosyl β-d-thiogalactoside, TDG) followed by precipitation with 80% saturated (NH₄)₂SO₄ and washing with the same solution.The measurement gave reproducible results, easy to correct for a moderate nonspecific binding, but active transport, when it occurred, resulted in excess counts.The radioactivity bound to the pellet was shown to depend on the presence of intact lac y gene product.Addition of ascorbate and phenazine methosulfate (PMS) stimulated active transport into the membrane vesicles. This could be inhibited by cyanide and by uncoupling agents and under these conditions the number of available binding sites was strongly diminished, while the inhibitors alone did not bring about a similar decrease.The decrease of available substrate binding sites was reversed by removal of oxygen or by washing out the respiratory substrates.The decrease in available binding sites is interpreted as reflecting one of the energy coupling steps which during in vivo active transport prevents the mobile carrier from being available for outflux, but the detailed interpretation of the reported results raises a number of problems connected with the energy cycle of active transport