Bezrukov S M, Kullman L, Winterhalter M
Laboratory of Physical and Structural Biology, NICHD, National Institutes of Health, Bethesda, MD 20892-0924, USA.
FEBS Lett. 2000 Jul 7;476(3):224-8. doi: 10.1016/s0014-5793(00)01753-1.
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.
研究了糖通过大肠杆菌麦芽糖孔蛋白（一种促进麦芽糊精跨细菌外膜转运的三聚体蛋白）的单通道水平渗透情况。对于大的糖类，如麦芽六糖，很容易观察到单个糖分子进入通道的基本事件。在低糖浓度下，一个基本事件是麦芽糖孔蛋白通道关闭至其在无糖溶液中初始电导的三分之一。对更高糖浓度下此类关闭事件的统计分析表明，麦芽糖孔蛋白通道的所有三个孔独立运输糖类。有趣的是，虽然通道电导仅略有不对称，在 -200 mV 时的值比在 +200 mV 时（从添加蛋白的一侧）高约 10%，但糖结合常数对电压极性的依赖性不对称性约高 20 倍。将我们的数据与用噬菌体 λ 所做的观察结果相结合，我们得出结论，糖的停留时间对细菌膜细胞内侧为负的电压更为敏感（且会因之而减少）。
