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.

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Background: There is ongoing controversy concerning the location and mobility of the N-terminal ␣-helix in VDAC1 during voltage gating. Results: mVDAC1 with the N-terminal ␣-helix cross-linked to ␤-strand 11 forms typical voltage-gated channels. Conclusion:The N-terminal domain of VDAC1 does not move independently during voltage gating. Significance: This study dramatically alters the current view of voltage gating dynamic in VDAC1.The voltage-dependent anion channel (VDAC) governs the free exchange of ions and metabolites between the mitochondria and the rest of the cell. The three-dimensional structure of VDAC1 reveals a channel formed by 19 ␤-strands and an N-terminal ␣-helix located near the midpoint of the pore. The position of this ␣-helix causes a narrowing of the cavity, but ample space for metabolite passage remains. The participation of the N-terminus of VDAC1 in the voltage-gating process has been well established, but the molecular mechanism continues to be debated; however, the majority of models entail large conformational changes of this N-terminal segment. Here we report that the pore-lining N-terminal ␣-helix does not undergo independent structural rearrangements during channel gating. We engineered a double Cys mutant in murine VDAC1 that cross-links the ␣-helix to the wall of the ␤-barrel pore and reconstituted the modified protein into planar lipid bilayers. The modified murine VDAC1 exhibited typical voltage gating. These results suggest that the N-terminal ␣-helix is located inside the pore of VDAC in the open state and remains associated with ␤-strand 11 of the pore wall during voltage gating.The voltage-dependent anion channel (VDAC) 4 serves as the primary conduit between the mitochondria and the rest of the cell, facilitating free exchange of ions and metabolites across the outer mitochondrial membrane. In addition to its metabolic and energetic functions, VDAC has a more complex role, serving as a receptor for molecules and proteins that modulate the organelle's permeability and thereby its function (1-3). This multifunctional channel has been implicated in the metabolic stresses of cancer, cardiovascular disease, and mitochondrialdependent cell death (4 -8); thus, understanding the structure and function of VDAC constitutes a critical objective for basic as well as medical research.Single channel conductance experiments on VDAC1 at low membrane potential (Ͻ30 mV) reveal a high conductance (4.1 Ϯ 0.1 nanosiemens in 1 M KCl) indicative of a large pore, usually referred to as the "open" state of the channel (1). This conformer facilitates the passage of 10 6 ATP molecules (9) and displays a preference for monovalent anions over cations with the anion-to-cation permeability ratio of 2:1 in high salt (10) and 4:1 in physiological salt concentration (11). As voltage is increased (Ͼ30 mV) in either a positive or a negative direction, the channel switches into the lower conducting states (around 2 nanosiemens in 1 M KCl), termed as the "closed" state(s). These states are cation-selective wi...

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 3 3200 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-V V 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. ß