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.