Site-exposure model for proton-lactose symport in Escherichia coli.

A model is presented for the lactose-proton co-transporter of E. coli. Either proton translocation inwards or galactoside translocation outwards brings about the exposure of galactoside binding sites externally. This alternation in the exposure of the galactoside binding site to either side of the membrane is viewed as the fundamental event in coupled uptake, rather than affinity changes for galactoside.The transporter is proposed to function as a dimer, exhibiting two forms corresponding to the “cis” and the “trans” orientation of the two galactosyl binding sites. A galactoside or a proton gradient brings about conversion of the sites from the “trans” to the “cis” configuration. The two forms can be experimentally differentiated by the accessibility of non-transportable substrate analogs to the galactosyl binding sites.