title: Sugar recognition by CscB and LacY.
abstract: The sucrose permease (CscB) and lactose permease (LacY) of Escherichia coli belong to the oligosaccharide/H(+) symporter subfamily of the major facilitator superfamily, and both catalyze sugar/H(+) symport across the cytoplasmic membrane. Thus far, there is no common substrate for the two permeases; CscB transports sucrose, and LacY is highly specific for galactopyranosides. Determinants for CscB sugar specificity are unclear, but the structural organization of key residues involved in sugar binding appears to be similar in CscB and LacY. In this study, several sugars containing galactopyranosyl, glucopyranosyl, or fructofuranosyl moieties were tested for transport with cells overexpressing either CscB or LacY. CscB recognizes not only sucrose but also fructose and lactulose, but glucopyranosides are not transported and do not inhibit sucrose transport. The findings indicate that CscB exhibits practically no specificity with respect to the glucopyranosyl moiety of sucrose. Inhibition of sucrose transport by CscB tested with various fructofuranosides suggests that the C(3)-OH group of the fructofuranosyl ring may be important for recognition by CscB. Lactulose is readily transported by LacY, where specificity is directed toward the galactopyranosyl ring, and the affinity of LacY for lactulose is similar to that observed for lactose. The studies demonstrate that the substrate specificity of CscB is directed toward the fructofuranosyl moiety of the substrate, while the specificity of LacY is directed toward the galactopyranosyl moiety.
authors: Department of Physiology, University of California Los Angeles, Los Angeles, California 90095-7327, United States.
authors: Junichi
authors: J
authors: Sugihara
authors: Irina
authors: I
authors: Smirnova
authors: Vladimir
authors: V
authors: Kasho
authors: H Ronald
authors: HR
authors: Kaback
mesh_terms: D000478:Alkylation / Q000187:drug effects; D000817:Anilino Naphthalenesulfonates / Q000494:pharmacology; D001665:Binding Sites / Q000187:drug effects; D001667:Binding, Competitive; D001692:Biological Transport / Q000187:drug effects; D003545:Cysteine / Q000737:chemistry; D004187:Disaccharides / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005632:Fructose / Q000031:analogs & derivatives / Q000378:metabolism; D005697:Galactosides / Q000378:metabolism; D005960:Glucosides / Q000378:metabolism; D006027:Glycosides / Q000378:metabolism*; D007700:Kinetics; D007792:Lactulose / Q000031:analogs & derivatives / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008968:Molecular Conformation; D009004:Monosaccharide Transport Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013439:Sulfhydryl Reagents / Q000494:pharmacology; D027981:Symporters / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D000817:Anilino Naphthalenesulfonates; D004187:Disaccharides; D029968:Escherichia coli Proteins; D005697:Galactosides; D005960:Glucosides; D006027:Glycosides; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D011994:Recombinant Proteins; D013439:Sulfhydryl Reagents; D027981:Symporters; D005632:Fructose; D007792:Lactulose; C056379:cscB protein, E coli; C021310:2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid; D003545:Cysteine