Kaback Hr

Structure of sugar-bound LacY

Significance The lactose permease of Escherichia coli (LacY), a model for the major facilitator superfamily, catalyzes the symport of a galactopyranoside and an H+ across the membrane by a mechanism in which the sugar-binding site in the middle of the protein becomes alternately accessible to either side of the membrane. However, all X-ray structures thus far show LacY in an inward-facing conformation with a tightly sealed periplasmic side. Significantly, by using a double-Trp mutant, we now describe an almost occluded, outward-open conformation with bound sugar, confirming more than two decades of biochemical and biophysical findings. We also present evidence that protonated LacY specifically binds D-galactopyranosides, inducing an occluded state that can open to either side of the membrane. Here we describe the X-ray crystal structure of a double-Trp mutant (Gly46→Trp/Gly262→Trp) of the lactose permease of Escherichia coli (LacY) with a bound, high-affinity lactose analog. Although thought to be arrested in an open-outward conformation, the structure is almost occluded and is partially open to the periplasmic side; the cytoplasmic side is tightly sealed. Surprisingly, the opening on the periplasmic side is sufficiently narrow that sugar cannot get in or out of the binding site. Clearly defined density for a bound sugar is observed at the apex of the almost occluded cavity in the middle of the protein, and the side chains shown to ligate the galactopyranoside strongly confirm more than two decades of biochemical and spectroscopic findings. Comparison of the current structure with a previous structure of LacY with a covalently bound inactivator suggests that the galactopyranoside must be fully ligated to induce an occluded conformation. We conclude that protonated LacY binds d-galactopyranosides specifically, inducing an occluded state that can open to either side of the membrane.

Real-time conformational changes in LacY

Significance The lactose permease from Escherichia coli (LacY), a model for the major facilitator superfamily, catalyzes the symport of a galactopyranoside and an H+ across the membrane by a mechanism in which the sugar-binding site in the middle of the protein becomes alternately accessible to either side of the membrane. The global conformational change is dissected into events that occur on the cytoplasmic and periplasmic aspects of LacY. Rates of individual steps are measured directly during opening or closing of periplasmic or cytoplasmic cavities by utilizing changes in Trp-bimane fluorescence with LacY in a phospholipid membrane. The findings provide a better understanding of the alternating access mechanism. Galactoside/H+ symport across the cytoplasmic membrane of Escherichia coli is catalyzed by lactose permease (LacY), which uses an alternating access mechanism with opening and closing of deep cavities on the periplasmic and cytoplasmic sides. In this study, conformational changes in LacY initiated by galactoside binding were monitored in real time by Trp quenching/unquenching of bimane, a small fluorophore covalently attached to the protein. Rates of change in bimane fluorescence on either side of LacY were measured by stopped flow with LacY in detergent or in proteoliposomes and were compared with rates of galactoside binding. With LacY in proteoliposomes, the periplasmic cavity is tightly sealed and the substrate-binding rate is limited by the rate of opening of this cavity. Rates of opening, measured as unquenching of bimane fluorescence, are 20–30 s−1, independent of sugar concentration and essentially the same in detergent or in proteoliposomes. On the cytoplasmic side of LacY in proteoliposomes, slow bimane quenching (i.e., closing of the cavity) is observed at a rate that is also independent of sugar concentration and similar to the rate of sugar binding from the periplasmic side. Therefore, opening of the periplasmic cavity not only limits access of sugar to the binding site of LacY but also controls the rate of closing of the cytoplasmic cavity.