Hamill S, Cloherty E K, Carruthers A
Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester 01655, USA.
Biochemistry. 1999 Dec 21;38(51):16974-83. doi: 10.1021/bi9918792.
The human erythrocyte sugar transporter presents sugar import (e2) and sugar export (e1) sites simultaneously. This study asks whether the sugar transporter exposes only one or multiple import sites. We approached this question by analysis of cytochalasin B binding to the human erythrocyte sugar export site in the presence of sugars that bind to the sugar import site. Extracellular maltose does not enter human erythrocytes. High concentrations of maltose (1-100 mM) inhibit cytochalasin B binding to human red cells. Low concentrations (25-500 microM) increase the level of erythrocyte cytochalasin B binding. Maltose modulation of cytochalasin B binding is mediated by altered affinity of sugar export sites for cytochalasin B. Similar results are obtained with other cell-impermeant inhibitors of sugar uptake. Extracellular D-glucose (a transported sugar) stimulates cytochalasin B binding at low D-glucose concentrations (10-250 microM), but this effect is lost at higher concentrations. Intracellular D-glucose inhibits cytochalasin B binding. Low concentrations of extracellular maltose and other nontransported inhibitors stimulate 3-O-methylglucose uptake in erythrocytes. Higher sugar concentrations (1-100 mM) inhibit transport. These data support the hypothesis that the erythrocyte sugar transporter presents two sugar import sites and at least one sugar export site. This conclusion is consistent with the proposed oligomeric structure of the sugar transporter, a complex of four GluT1 proteins in which each subunit presents a translocation pathway.
人类红细胞糖转运蛋白同时呈现糖进口（e2）和糖出口（e1）位点。本研究探讨糖转运蛋白是仅暴露一个还是多个进口位点。我们通过分析在存在与糖进口位点结合的糖的情况下细胞松弛素B与人红细胞糖出口位点的结合来探讨这个问题。细胞外麦芽糖不进入人类红细胞。高浓度的麦芽糖（1 - 100 mM）抑制细胞松弛素B与人红细胞的结合。低浓度（25 - 500 microM）则增加红细胞细胞松弛素B的结合水平。麦芽糖对细胞松弛素B结合的调节是由糖出口位点对细胞松弛素B的亲和力改变介导的。对于其他糖摄取的细胞不可渗透抑制剂也获得了类似的结果。细胞外D - 葡萄糖（一种可转运的糖）在低D - 葡萄糖浓度（10 - 250 microM）时刺激细胞松弛素B的结合，但在较高浓度时这种作用消失。细胞内D - 葡萄糖抑制细胞松弛素B的结合。低浓度的细胞外麦芽糖和其他非转运抑制剂刺激红细胞对3 - O - 甲基葡萄糖的摄取。较高的糖浓度（1 - 100 mM）抑制转运。这些数据支持红细胞糖转运蛋白呈现两个糖进口位点和至少一个糖出口位点的假说。这一结论与所提出的糖转运蛋白的寡聚结构一致，糖转运蛋白是由四个GluT1蛋白组成的复合物，其中每个亚基都呈现一条转运途径。
