Transport of maltodextrins through maltoporin: a single-channel study.
Transport of sugars through maltoporin channels reconstituted into planar lipid membranes has traditionally been addressed using multichannel preparations. Here we show that single-channel experiments offer new possibilities to reveal molecular details of the interaction between the sugar and the channel. We analyze time-resolved transient interruptions in the maltoporin ionic current in the presence of differently sized maltodextrins. We find for all studied sugars, from maltotriose to maltoheptaose, that only one sugar molecule is required to completely block one of the pores in the maltoporin trimer. The probability of simultaneous blockage of different pores increases with sugar concentration in a manner that demonstrates their mutual independence. The maltoporin channel is asymmetric and, added from one side only, predominantly inserts in an oriented manner. The asymmetry of the channel structure manifests itself in two ways. First, it is seen as an asymmetrical response to applied voltage at otherwise symmetrical conditions; second, as asymmetrical rates of sugar entry into the channel with asymmetrical (one-sided) sugar addition. Importantly, we find that the sugar residence time in the pore does not depend on which side the sugar is added. This voltage-dependent time is the same for symmetrical, cis, or trans sugar addition. This observation suggests that once a sugar molecule is captured by the "greasy slide" of the channel, it spends enough time there to "forget" from what entrance it was captured. This also means that the blockage events studied here represent sugar translocation events, and not just binding at and release from the same entrance of the channel.
Kullman
Lisen
L
Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-0924 USA.
Winterhalter
Mathias
M
Bezrukov
Sergey M
SM
D001425:Bacterial Outer Membrane Proteins; D050260:Carbohydrate Metabolism; D002462:Cell Membrane / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D007477:Ions; D008051:Lipid Bilayers / Q000378:metabolism; D050356:Lipid Metabolism; D009844:Oligosaccharides / Q000494:pharmacology; D011134:Polysaccharides / Q000737:chemistry*; D018272:Porins; D011485:Protein Binding; D021381:Protein Transport; D011991:Receptors, Virus / Q000737:chemistry* / Q000378:metabolism*; D013997:Time Factors
D001425:Bacterial Outer Membrane Proteins; D007477:Ions; D008051:Lipid Bilayers; D009844:Oligosaccharides; D011134:Polysaccharides; D018272:Porins; D011991:Receptors, Virus; C016532:maltoporins; C016549:maltohexaose; C008315:maltodextrin