title: Structure and mechanism of the lactose permease of Escherichia coli.
abstract: Membrane transport proteins that transduce free energy stored in electrochemical ion gradients into a concentration gradient are a major class of membrane proteins. We report the crystal structure at 3.5 angstroms of the Escherichia coli lactose permease, an intensively studied member of the major facilitator superfamily of transporters. The molecule is composed of N- and C-terminal domains, each with six transmembrane helices, symmetrically positioned within the permease. A large internal hydrophilic cavity open to the cytoplasmic side represents the inward-facing conformation of the transporter. The structure with a bound lactose homolog, beta-D-galactopyranosyl-1-thio-beta-D-galactopyranoside, reveals the sugar-binding site in the cavity, and residues that play major roles in substrate recognition and proton translocation are identified. We propose a possible mechanism for lactose/proton symport (co-transport) consistent with both the structure and a large body of experimental data.
authors.affiliation: Department of Biological Sciences, Imperial College London, London SW7 2AZ, UK.
authors.forename: Jeff
authors.initials: J
authors.lastname: Abramson
authors.forename: Irina
authors.initials: I
authors.lastname: Smirnova
authors.forename: Vladimir
authors.initials: V
authors.lastname: Kasho
authors.forename: Gillian
authors.initials: G
authors.lastname: Verner
authors.forename: H Ronald
authors.initials: HR
authors.lastname: Kaback
authors.forename: So
authors.initials: S
authors.lastname: Iwata
mesh_terms: D019943:Amino Acid Substitution; D001665:Binding Sites; D001692:Biological Transport; D002462:Cell Membrane / Q000201:enzymology; D003460:Crystallization; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry* / Q000201:enzymology; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D017136:Ion Transport; D007785:Lactose / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D009004:Monosaccharide Transport Proteins; D009154:Mutation; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011522:Protons; D013379:Substrate Specificity; D027981:Symporters; D013862:Thiogalactosides / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D011522:Protons; D027981:Symporters; D013862:Thiogalactosides; C031803:thiodigalactoside; C021499:lactose permease; D007785:Lactose