Photoinactivation of the beta-galactoside transport system in Escherichia coli membrane vesicles with an impermeant azidophenylgalactoside.

2'-N-(2-Nitro-4-azidophenyl) aminoethyl-1-thio-beta-D-galactopyranoside (APG2) is a competitive inhibitor of lactose transport in membrane vesicles isolated from Escherichia coli ML 308-225, exhibiting an apparent Ki of 30 to 40 muM, but is not transported. When irradiated with visible light in the presence of D-lactate, APG2 irreversibly inactivates the lac transport system. Imposition of a membrane potential (positive outside) by valinomycin-induced potassium efflux also causes APG2 photoinactivation. Strikingly, photoinactivation is not observed in the absence of D-lactate or a potassium diffusion gradient. Kinetic studies of the inactivation process yield a KD of 35 muM. Since lactose protects against the inactivation, it is apparent that these effects are specific for the lac transport system. The results show that APG2 inactivates from the outer surface of the vesicle membrane and support the previous hypothesis that the lac carrier protein is unable to bind external substrate in the absence of energy coupling.