Dependence on pH of substrate binding to a mutant lactose carrier, lacYun, in Escherichia coli. A model for H+/lactose symport.

The lacYun gene, which encodes a lactose carrier showing the uncoupled phenotype of substrate transport in Escherichia coli [Wilson, Kusch & Kashket (1970) Biochem. Biophys. Res. Commun. 40, 1409-1414], was cloned on a plasmid vector, pBR322. The binding of a substrate, p-nitrophenyl alpha-galactoside, to the lacYun carrier in membranes from the strain harbouring the lacYun clone showed a pH-dependence different from its binding to the wild-type lactose carrier. This finding indicated that the lacYun mutation confers higher affinity for H+ on the carrier, exerting its effect on the less efficient dissociation of substrate inside cells. The result coincides with the proposal [Yamato & Rosenbusch (1983) FEBS Lett. 151, 102-104] that the proton affecting the substrate binding is the coupling proton of the proton/lactose symport reaction, which allows only the ordered mechanism of binding of substrate to an H+-carrier binary complex. From the simplest model of the symport reaction, constructed on the basis of these results, the coupling site of energy in the carrier cycle of the transport reaction can be identified at the substrate-dissociation step inside cells.