title: The periplasmic maltose-binding protein modifies the channel-forming characteristics of maltoporin.
abstract: Maltoporin, a protein spanning Escherichia coli outer membranes, modifies electrical conductance of membranes due to its channel-forming properties. This observation was made by conductance measurements across planar bilayers which were derived from unextracted, isolated outer membrane vesicles using a porin-deficient E. coli strain. Alternatively, proteoliposomes reconstituted with detergent-solubilized homogeneous maltoporin and phospholipids were used. With either membrane preparation, channel conductance was observed, although no discrete conductance levels were detected. The presence of lipopolysaccharide, a bacterial glycolipid, was not required, nor did it affect channel activity. In the presence of the water-soluble periplasmic maltose-binding protein, conductance fluctuations occurred in discrete steps, demonstrating opening and closing events of channels. Multiple step sizes (1/3, 2/3 and 1 ns in 1 M KCl) in single channel traces suggest cooperative opening and closing of up to three channels. The action of maltose-binding protein is highly asymmetrical, and its affinity to maltoporin is very high (KD = 1.5 X 10(-7) M). Association of maltose-binding protein to maltoporin shifts, for a given polarity, the equilibrium between open and closed states in favour of closed states. This result matches earlier in vivo studies, and supports the physiological significance of the observations made.
mesh_terms: D018528:ATP-Binding Cassette Transporters; D000494:Allosteric Regulation; D001426:Bacterial Proteins / Q000502:physiology*; D001692:Biological Transport; D002352:Carrier Proteins / Q000502:physiology*; D002463:Cell Membrane Permeability; D004553:Electric Conductivity; D004926:Escherichia coli / Q000502:physiology; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D008565:Membrane Proteins / Q000502:physiology*; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; D018272:Porins; D011485:Protein Binding
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; C461619:MalE protein, E coli; D058619:Maltose-Binding Proteins; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; D018272:Porins; C461599:maltose transport system, E coli