title: Evidence for a role of helix IV in connecting cation- and sugar-binding sites of Escherichia coli melibiose permease.
abstract: To improve the structural organization model of melibiose permease, we assessed the individual contributions of the N-terminal tryptophans to the transporter fluorescence variations induced by the binding of cations and beta-configured sugars, by replacement of the six N-terminal tryptophans by phenylalanines and the study of the signal changes. Only two mutations, W116F located in helix IV and W128F located in the cytoplasmic loop 4-5, impair permease activity. The intrinsic fluorescence spectroscopy analysis of the other mutants suggests that W54, located in helix II, W116, and W128 are mostly responsible for the cation-induced fluorescence variations. These tryptophans, W116 and W128, would also be responsible for the beta-galactoside-induced fluorescence changes observed in the N-terminal domain of the transporter. The implication of W116 and W128 in both the cation- and beta-galactoside-induced fluorescence variations led us to investigate in detail the effects of their mutations on the functional properties of the permease. The results obtained suggest that the domains harboring the two tryptophans, or the residues themselves, play a critical role in the mechanism of Na(+)/sugar symport. Taken together, the results presented in this paper and previous results are consistent with a fundamental role of helix IV in connecting cation- and sugar-binding sites of the melibiose permease.
authors.affiliation: Laboratoire des Membranes Cellulaires (Bat. Jean Maetz), Université de Nice/Sophia-Antipolis, CNRS (ERS 1253), LRC-CEA 16V, 06238 Villefranche Sur Mer Cedex 1, France.
authors.forename: E
authors.initials: E
authors.lastname: Cordat
authors.forename: G
authors.initials: G
authors.lastname: Leblanc
authors.forename: I
authors.initials: I
authors.lastname: Mus-Veteau
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D001665:Binding Sites; D001692:Biological Transport / Q000187:drug effects; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000494:pharmacology; D002414:Cations, Monovalent / Q000378:metabolism* / Q000494:pharmacology; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D008553:Melibiose / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008756:Methylgalactosides / Q000378:metabolism / Q000494:pharmacology; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics; D009598:Nitrophenylgalactosides / Q000378:metabolism; D009994:Osmolar Concentration; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011510:Proteolipids / Q000187:drug effects / Q000378:metabolism; D011887:Raffinose / Q000378:metabolism / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013379:Substrate Specificity; D027981:Symporters; D013862:Thiogalactosides / Q000378:metabolism / Q000494:pharmacology; D014364:Tryptophan / Q000235:genetics / Q000378:metabolism; D014443:Tyrosine / Q000235:genetics / Q000378:metabolism
chemical_list: D002241:Carbohydrates; D002414:Cations, Monovalent; D026901:Membrane Transport Proteins; D008756:Methylgalactosides; D011510:Proteolipids; D027981:Symporters; D013862:Thiogalactosides; C020485:proteoliposomes; C009468:thiomethylgalactoside; D009598:Nitrophenylgalactosides; C059200:4-nitrophenylgalactoside; D014443:Tyrosine; D014364:Tryptophan; C021548:melibiose permease; D008553:Melibiose; D012964:Sodium; D011887:Raffinose