Tanaka Natsuki, Fujiwara Takashi, Tomioka Rie, Krämer Ute, Kawachi Miki, Maeshima Masayoshi
Laboratory of Cell Dynamics, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan.
Department of Plant Physiology, Ruhr-University Bochum, D-44801 Bochum, Germany.
Plant Cell Physiol. 2015 Mar;56(3):510-9. doi: 10.1093/pcp/pcu194. Epub 2014 Dec 15.
The vacuolar Zn(2+)/H(+) antiporter of Arabidopsis thaliana, AtMTP1, has a long cytosolic histidine-rich loop. A mutated AtMTP1 in which the first half of the loop (His-half) was deleted exhibited a 11-fold higher transport velocity in yeast cells. Transgenic lines overexpressing the His-half-deleted AtMTP1 in the loss-of-function mutant were evaluated for growth and metal content in the presence of various zinc concentrations. These overexpressing lines (35S-AtMTP1 and 35S-His-half lines) showed high tolerance to excess concentrations of zinc at 150 µM, as did the wild type, compared with the loss-of-function line. The His-half AtMTP1 transported cobalt in a heterologous expression assay in yeast, but the cumulative amount of cobalt in 35S-His-half plants was not increased. Moreover, the accumulation of calcium and iron was not changed in plants. Under zinc-deficient conditions, growth of 35S-His-half lines was markedly suppressed. Under the same conditions, the 35S-His-half lines accumulated larger amounts of zinc in roots and smaller amounts of zinc in shoots compared with the other lines, suggesting an abnormal accumulation of zinc in the roots of 35S-His-half lines. As a result, the shoots may exhibit zinc deficiency. Taken together, these results suggest that the His-loop acts as a sensor of cytosolic zinc to maintain an essential level in the cytosol and that the dysfunction of the loop results in an uncontrolled accumulation of zinc in the vacuoles of root cells.
拟南芥的液泡型锌离子/氢离子反向转运蛋白AtMTP1具有一个较长的富含组氨酸的胞质环。其中该环前半部分（组氨酸半段）缺失的突变型AtMTP1在酵母细胞中的转运速度提高了11倍。对在功能缺失突变体中过表达组氨酸半段缺失型AtMTP1的转基因株系在不同锌浓度条件下的生长和金属含量进行了评估。与功能缺失株系相比，这些过表达株系（35S-AtMTP1和35S-组氨酸半段株系）在150µM锌过量浓度下表现出与野生型一样的高耐受性。组氨酸半段AtMTP1在酵母的异源表达试验中能转运钴，但35S-组氨酸半段植株中钴的累积量并未增加。此外，植株中钙和铁的积累没有变化。在缺锌条件下，35S-组氨酸半段株系的生长受到明显抑制。在相同条件下，与其他株系相比，35S-组氨酸半段株系在根中积累了更多的锌而在地上部积累了较少的锌，这表明35S-组氨酸半段株系的根中锌积累异常。结果，地上部可能表现出锌缺乏。综上所述，这些结果表明组氨酸环作为胞质锌的传感器以维持胞质中必需的锌水平，并且该环功能失调导致根细胞液泡中锌的无控制积累。
