University of Wuerzburg, Institute for Molecular Plant Physiology and Biophysics, Julius von Sachs Platz 2, D-97082 Wuerzburg, Germany.
Plant J. 2011 Oct;68(1):129-36. doi: 10.1111/j.1365-313X.2011.04672.x. Epub 2011 Jul 27.
The vacuolar membrane is involved in solute uptake into and release from the vacuole, which is the largest plant organelle. In addition to inorganic ions and metabolites, large quantities of protons and sugars are shuttled across this membrane. Current models suggest that the proton gradient across the membrane drives the accumulation and/or release of sugars. Recent studies have associated AtSUC4 with the vacuolar membrane. Some members of the SUC family are plasma membrane proton/sucrose symporters. In addition, the sugar transporters TMT1 and TMT2, which are localized to the vacuolar membrane, have been suggested to function in proton-driven glucose antiport. Here we used the patch-clamp technique to monitor carrier-mediated sucrose transport by AtSUC4 and AtTMTs in intact Arabidopsis thaliana mesophyll vacuoles. In the whole-vacuole configuration with wild-type material, cytosolic sucrose-induced proton currents were associated with a proton/sucrose antiport mechanism. To identify the related transporter on one hand, and to enable the recording of symporter-mediated currents on the other hand, we electrophysiologically characterized vacuolar proteins recognized by Arabidopsis mutants of partially impaired sugar compartmentation. To our surprise, the intrinsic sucrose/proton antiporter activity was greatly reduced when vacuoles were isolated from plants lacking the monosaccharide transporter AtTMT1/TMT2. Transient expression of AtSUC4 in this mutant background resulted in proton/sucrose symport activity. From these studies, we conclude that, in the natural environment within the Arabidopsis cell, AtSUC4 most likely catalyses proton-coupled sucrose export from the vacuole. However, TMT1/2 probably represents a proton-coupled antiporter capable of high-capacity loading of glucose and sucrose into the vacuole.
液泡膜参与溶质从液泡中摄取和释放，液泡是植物最大的细胞器。除了无机离子和代谢物外，大量的质子和糖被转运穿过该膜。目前的模型表明，膜两侧的质子梯度驱动糖的积累和/或释放。最近的研究将 AtSUC4 与液泡膜联系起来。SUC 家族的一些成员是质膜质子/蔗糖共转运体。此外，定位于液泡膜的糖转运蛋白 TMT1 和 TMT2 被认为在质子驱动的葡萄糖反向转运中发挥作用。在这里，我们使用膜片钳技术在完整的拟南芥质膜泡中监测 AtSUC4 和 AtTMTs 介导的载体蔗糖转运。在用野生型材料进行的全泡构型中，细胞质中蔗糖诱导的质子电流与质子/蔗糖反向转运机制有关。一方面为了鉴定相关转运体，另一方面为了记录共转运体介导的电流，我们对部分糖区室化受损的拟南芥突变体识别的液泡蛋白进行了电生理特性分析。令我们惊讶的是，当从缺乏单糖转运蛋白 AtTMT1/TMT2 的植物中分离液泡时，内在的蔗糖/质子反向转运活性大大降低。在该突变背景下瞬时表达 AtSUC4 导致质子/蔗糖共转运活性。从这些研究中，我们得出结论，在拟南芥细胞的自然环境中，AtSUC4 很可能催化质子偶联的蔗糖从液泡中输出。然而，TMT1/2 可能代表一种质子偶联的反向转运体，能够将葡萄糖和蔗糖高效地装入液泡。
