title: Oriented channels reveal asymmetric energy barriers for sugar translocation through maltoporin of Escherichia coli.
abstract: Sugar transport through maltoporin of Escherichia coli was investigated. This protein facilitates maltooligosaccharide translocation via a binding site in the channel. Because incorporation of the protein into the bilayer results in randomly orientated channels, we re-examined the postulated symmetric translocation model by reconstitution of maltoporin under an externally applied field. Upon binding of bacteriophage lambda, which exploit surface-exposed loops of maltoporin as the receptor, sugar permeation, but not the ion current, was blocked. Thus using the phage-to-probe orientation we were able to show that the channels were approximately 80% directionally inserted into the bilayer. Moreover, asymmetry of the channel was revealed because sugar entrance through the 'open' periplasmic side of maltoporin was similarly reduced. Here a new asymmetrical two-barrier model is presented. Based on liposome-swelling assays and current-fluctuation analysis we conclude that the periplasmic side of the porin shows a two- to threefold higher energy barrier than the extracellular loop-side of the channels.
authors.affiliation: Department of Microbiology, Biozentrum, University of Basel, Switzerland. vangelder@ubaclu.unibas.ch
authors.forename: P
authors.initials: P
authors.lastname: Van Gelder
authors.forename: F
authors.initials: F
authors.lastname: Dumas
authors.forename: J P
authors.initials: JP
authors.lastname: Rosenbusch
authors.forename: M
authors.initials: M
authors.lastname: Winterhalter
mesh_terms: D001425:Bacterial Outer Membrane Proteins; D010582:Bacteriophage lambda / Q000378:metabolism; D001693:Biological Transport, Active / Q000187:drug effects; D050260:Carbohydrate Metabolism; D002463:Cell Membrane Permeability / Q000187:drug effects; D004492:Edetic Acid / Q000494:pharmacology; D004553:Electric Conductivity; D004734:Energy Metabolism; D004926:Escherichia coli / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008051:Lipid Bilayers / Q000378:metabolism; D008320:Maltose / Q000378:metabolism; D008954:Models, Biological; D009844:Oligosaccharides / Q000378:metabolism; D019897:Periplasm / Q000187:drug effects / Q000378:metabolism; D018272:Porins; D011991:Receptors, Virus / Q000378:metabolism*; D014312:Trisaccharides / Q000378:metabolism
chemical_list: D001425:Bacterial Outer Membrane Proteins; D008051:Lipid Bilayers; D009844:Oligosaccharides; D018272:Porins; D011991:Receptors, Virus; D014312:Trisaccharides; C016532:maltoporins; C016549:maltohexaose; C008317:maltotriose; D008320:Maltose; D004492:Edetic Acid