Comparative Functional Features of Plant Potassium HvHAK1 and HvHAK2 Transporters*
Comparative Functional Features of Plant Potassium HvHAK1 and HvHAK2 Transporters*
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DOI：
10.1074/jbc.m108129200
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发表时间：
2001-11
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影响因子：
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通讯作者：
M. Senn;F. Rubio;M. Bañuelos;A. Rodríguez-Navarro
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文献类型：
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作者：
M. Senn;F. Rubio;M. Bañuelos;A. Rodríguez-Navarro
植物K+转运蛋白的HAK家族属于四个相当不同的系统发育簇，虽然大多数的转运蛋白属于集群I或II。真菌和植物HAK转运蛋白的简单系统发育分析表明，原始HAK基因甚至在真菌和植物分化之前就复制了，产生了目前在植物中履行不同功能的转运蛋白。HvHAK 1转运蛋白属于簇I，介导大麦根中的高亲和力K+吸收，但已知簇II转运蛋白HvHAK 2没有功能，其在酵母中没有功能。通过构建HvHAK 1-HAK 2嵌合转运蛋白来研究HvHAK 2的功能，所述嵌合转运蛋白即使当它们仅包含HvHAK 2的短片段时也没有功能。然后，将N末端和第一跨膜结构域中簇II的特征性氨基酸引入HvHAK 1。所有这些变化增加Rb+ Km，引入最小的Na+ Km的变化，这表明HvHAK 2是一个低亲和力，Na+敏感的K+转运蛋白。使用K+缺陷的大肠杆菌突变体，我们功能性地表达HvHAK 2，并发现预测的特征是正确的，以及发现HvHAK 2的细菌表达在pH 5.5下是功能性的，但在7.5下不是。我们讨论是否HvHAK 2可能是一个液泡膜转运蛋白有效的液泡K+耗竭K+饥饿植物。
Plant K+ transporters of the HAK family belong to four rather divergent phylogenetic clusters, although most of the transporters belong to clusters I or II. A simple phylogenetic analysis of fungal and plant HAK transporters suggests that an original HAK gene duplicated even before fungi and plants diverged, generating transporters that at present fulfill different functions in the plant. The HvHAK1 transporter belongs to cluster I and mediates high-affinity K+ uptake in barley roots, but no function is known for the cluster II transporter, HvHAK2, which is not functional in yeast. The function of HvHAK2 was investigated by constructing HvHAK1-HAK2 chimeric transporters, which were not functional even when they included only short fragments of HvHAK2. Then, amino acids characteristic of cluster II in the N terminus and in the first transmembrane domain were introduced into HvHAK1. All of these changes increased the Rb+ K m , introducing minimal changes in the Na+ K m , which suggested that HvHAK2 is a low-affinity, Na+-sensitive K+ transporter. Using a K+-defective Escherichia coli mutant, we functionally expressed HvHAK2 and found that the predicted characteristics were correct, as well as discovering that the bacterial expression of HvHAK2 is functional at pH 5.5 but not at 7.5. We discuss whether HvHAK2 may be a tonoplast transporter effective for vacuolar K+ depletion in K+ starved plants.