Probing sugar translocation through maltoporin at the single channel level.
Sugar permeation through maltoporin of Escherichia coli, a trimer protein that facilitates maltodextrin translocation across outer bacterial membranes, was investigated at the single channel level. For large sugars, such as maltohexaose, elementary events of individual sugar molecule penetration into the channel were readily observed. At small sugar concentrations an elementary event consists of maltoporin channel closure by one third of its initial conductance in sugar-free solution. Statistical analysis of such closures at higher sugar concentrations shows that all three pores of the maltoporin channel transport sugars independently. Interestingly, while channel conductance is only slightly asymmetric showing about 10% higher values at -200 mV than at +200 mV (from the side of protein addition), asymmetry in dependence of the sugar binding constant on the voltage polarity is about 20 times higher. Combining our data with observations made with bacteriophage-lambda we conclude that the sugar residence time is much more sensitive to (and is decreased by) voltages that are negative from the intra-cell side of the bacterial membrane.
Laboratory of Physical and Structural Biology, NICHD, National Institutes of Health, Bethesda, MD 20892-0924, USA. bezrukov@helix.nih.gov
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Bezrukov
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Kullman
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Winterhalter
D001425:Bacterial Outer Membrane Proteins; D001693:Biological Transport, Active; D050260:Carbohydrate Metabolism; D002462:Cell Membrane / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D007473:Ion Channels / Q000378:metabolism; D007700:Kinetics; D008051:Lipid Bilayers; D008564:Membrane Potentials; D018272:Porins / Q000378:metabolism*; D011991:Receptors, Virus / Q000378:metabolism*
D001425:Bacterial Outer Membrane Proteins; D007473:Ion Channels; D008051:Lipid Bilayers; D018272:Porins; D011991:Receptors, Virus; C016532:maltoporins