A MOLECULAR MECHANISM FOR ENERGY COUPLING IN A MEMBRANE-TRANSPORT PROTEIN, THE LACTOSE PERMEASE OF ESCHERICHIA-COLI

A mechanism for the coupled translocation of substrate and H+ by the l actose permease of Escherichia coli is proposed, based on a variety of experimental observations, The permease is composed of 12 alpha-helic al rods that traverse the membrane with the N and C termini on the cyt oplasmic face. Four residues are irreplaceable with respect to couplin g, and the residues are paired-Arg-302 (helix IX) with Glu-325 (helix X) and His-322 (helix X) with Glu-269 (helix VIII), In an adjacent reg ion of the molecule at the interface between helices VIII and V is the substrate translocation pathway, Because of this arrangement, interfa cial changes between helices VIII and V are transmitted to the interfa ce between helices IX and X and vice versa, Upon ligand binding, a str uctural change at the interface between helices V and VIII disrupts th e interaction between Glu-269 and His-322, Glu-269 displaces Glu-325 f rom Arg-302, and Glu-325 is protonated. Simultaneously, protonated Glu -325 becomes inaccessible to water, which drastically increases its pK (a), In this configuration, the permease undergoes a freely reversible conformational change that corresponds to translocation of the ternar y complex, To return to ground state after release of substrate, the A rg-302-Glu-325 interaction must be reestablished, which necessitates l oss of H+ from Glu-325, The H+ is released into a water-filled crevice between helices IX and X which becomes transiently accessible to both sides of the membrane due to a change in helix tilt, where it is acte d upon equally by either the membrane potential or the pH gradient acr oss the membrane.