title: Direct measurement of lactose/proton symport in Escherichia coli membrane vesicles: further evidence for the involvement of histidine residue(s).
abstract: Addition of lactose to Escherichia coli ML 308-225 membrane vesicles under nonenergized conditions induces transient alkalinization of the medium, and the initial rate of proton influx is stimulated by valinomycin and abolished by nigericin or carbonyl cyanide m-chlorophenylhydrazone. A functional lac y gene product is absolutely required as the effect is not observed in ML 308-225 vesicles treated with N-ethylmaleimide nor with vesicles from uninduced Escherichia coli ML 30. Furthermore, the magnitude of the phenomenon is enhanced about 3-fold in vesicles from Escherichia coli T206, which contain amplified levels of the lac carrier protein. Kinetic parameters for lactose-induced proton influx are the same as those determined for lactose-facilitated diffusion, and quantitative comparison of the initial rates of the two fluxes indicates that the stoichiometry between protons and lactose is 1:1. Treatment of ML 308-225 vesicles with diethyl pyrocarbonate causes inactivation of lactose-induced proton influx. Remarkably, however, treatment with the histidine reagent enhances the rate of lactose-facilitated diffusion in a manner suggesting that the altered lac carrier catalyzes lactose influx without the symport of protons. The results are consistent with the hypothesis that acylation of a histidyl residue(s) in the lac carrier protein dissociates lactose influx from proton influx and indicate that this residue(s) play(s) an important role in the pathway of proton translocation.
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism*; D004047:Diethyl Pyrocarbonate / Q000494:pharmacology; D004058:Diffusion; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007785:Lactose / Q000378:metabolism* / Q000494:pharmacology; D026901:Membrane Transport Proteins / Q000378:metabolism*; D009004:Monosaccharide Transport Proteins; D011522:Protons; D013329:Structure-Activity Relationship; D027981:Symporters
chemical_list: D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D011522:Protons; D027981:Symporters; D006639:Histidine; C021499:lactose permease; D007785:Lactose; D004047:Diethyl Pyrocarbonate