The lactose permease of Escherichia coli (LacY), a particularly well-studied paradigm for ion gradient driven active transport proteins, is solely responsible for all translocation reactions catalyzed by the galactoside transport system in E. coli. Like many active transport proteins, LacY couples the free energy released from downhill translocation of protons in response to a proton electrochemical gradient to drive the energetically uphill stoichiometric accumulation of D-galactopyranosides.
An X-ray structure of LacY has been solved in an inward-facing conformation, confirming many conclusions derived from various biochemical and biophysical studies. The molecule is composed of N- and C-terminal domains, each with six transmembrane helices, symmetrically positioned within the molecule, and many of the helices are irregularly shaped. A large internal water-filled cavity is exposed to the cytoplasm, and sugar is bound at the two-fold axis of symmetry at the apex of the hydrophilic cavity and in the approximate middle of the molecule. Access to the sugar-binding site is completely blocked from the outer side of the molecule. By combining a large body of experimental data derived from systematic studies of site-directed mutants, residues involved in substrate binding and proton translocation have been identified. Based on the functional properties of the mutants and the X-ray structure, a working model for the mechanism involving alternating access of the binding site to either side of the membrane has been postulated.
Recent experimental results obtained using site-directed alkylation of cysteine-replacement mutants, isothermal calorimetry, site-directed fluorescence and spin labeling provide supporting evidence for the alternating access model.
Guan, L., Mirza, O., Verner, G., Iwata, S. & Kaback, H.R. (2007)
Structural determination of wild-type lactose permease. Proc Natl Acad Sci USA 104, 15294-8.
Smirnova, I., Kasho, V., Choe, J.-Y., Altenbach, C., Hubbell, W. L. & Kaback, H.R. (2007) Sugar binding induces an outward facing conformation of LacY. Proc Natl Acad Sci USA 104, 16504-9.
Guan, L. & Kaback, H.R. (2006) Lessons from Lactose Permease. Annu Rev Biophys Biomol Struct 35, 67-91