title: The glucose transporter of Escherichia coli. Mutants with impaired translocation activity that retain phosphorylation activity.
abstract: The glucose transporter of the bacterial phosphotransferase system couples translocation with phosphorylation of the substrate in a 1:1 stoichiometry. It is a complex consisting of a transmembrane subunit (IIGlc) and a hydrophilic subunit (IIIGlc). Both subunits are transiently phosphorylated. IIIGlc is phosphorylated at a histidyl residue by the cytoplasmic phosphoryl carrier protein phospho-heat-stable phosphoryl carrier protein; IIGlc is phosphorylated at a cysteinyl residue by phospho-IIIGlc. The IIGlc subunit consists of two domains. The N-terminal hydrophobic domain is presumed to span the membrane several times; the C-terminal cytoplasmic domain includes the phosphorylation site. IIGlc phosphorylates glucose and methyl-alpha-D-glucopyranoside in transit across the inner membrane but can also phosphorylate intracellular glucose. Ten mutants resistant against extracellular toxic methyl-alpha-D-glucopyranoside yet capable of phosphorylating intracellular glucose were isolated. Strong impairment of transport activity in these mutants was accompanied by only a slight decrease of phosphorylation activity. Amino acid substitutions occurred at six sites that are clustered in three presumably hydrophilic loops in the transmembrane domain of IIGlc: M17T, M17I, G149S, K150E, S157F, H339Y, and D343G. We presume that the three polypeptide segments are directly involved in sugar translocation and/or binding but are of little importance for phosphorylation activity, folding, and membrane localization of IIGlc.
authors.affiliation: Department of Biology, Philipps University, Marburg, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Buhr
authors.forename: G A
authors.initials: GA
authors.lastname: Daniels
authors.forename: B
authors.initials: B
authors.lastname: Erni
mesh_terms: D000595:Amino Acid Sequence; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D008757:Methylglucosides / Q000378:metabolism; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000235:genetics* / Q000378:metabolism; D009154:Mutation; D010766:Phosphorylation; D010770:Phosphotransferases / Q000378:metabolism; D010957:Plasmids; D012689:Sequence Homology, Nucleic Acid
chemical_list: D004269:DNA, Bacterial; D008757:Methylglucosides; D009004:Monosaccharide Transport Proteins; D010770:Phosphotransferases; D005947:Glucose