title: A nontransportable substrate for lactose permease.
abstract: A substrate for lactose permease of Escherichia coli was synthesized that binds to the protein with a relatively high affinity, but is not transported to any detectable extent. This substrate, 6'-[(N-phenylalanylphenylalanyl)amino]hexyl 1-thio-beta-D-galactoside, is a peptide galactoside composed of a bulky aromatic dipeptide that is linked to galactose via an aminohexyl spacer. Binding of the peptide galactoside to lactose permease in cytoplasmic membranes was determined in a competition assay yielding a dissociation constant of 150 microM. Transport was measured by a counterflow assay using lipid vesicles with reconstituted lactose permease. An upper limit for the rate constant of transport was obtained as 0.02 s-1, 3 orders of magnitude smaller than the value for lactose.
authors.affiliation: Max-Planck-Institut für Biologie, Abteilung Membranbiochemie, Tübingen, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Seibert
authors.forename: W
authors.initials: W
authors.lastname: Dörner
authors.forename: F
authors.initials: F
authors.lastname: Jähnig
mesh_terms: D001692:Biological Transport; D004151:Dipeptides / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D005697:Galactosides / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008956:Models, Chemical; D009004:Monosaccharide Transport Proteins; D011510:Proteolipids / Q000378:metabolism; D027981:Symporters; D013862:Thiogalactosides / Q000378:metabolism*
chemical_list: C094412:6'-((N-phenylalanylphenylalanyl)amino)hexyl 1-thiogalactoside; D004151:Dipeptides; D029968:Escherichia coli Proteins; D005697:Galactosides; C050221:LacY protein, E coli; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D011510:Proteolipids; D027981:Symporters; D013862:Thiogalactosides; C020485:proteoliposomes; C021499:lactose permease