Zinc Transporters, ZnT5 and ZnT7, Are Required for the Activation of Alkaline Phosphatases, Zinc-requiring Enzymes That Are Glycosylphosphatidylinositol-anchored to the Cytoplasmic Membrane*
Zinc Transporters, ZnT5 and ZnT7, Are Required for the Activation of Alkaline Phosphatases, Zinc-requiring Enzymes That Are Glycosylphosphatidylinositol-anchored to the Cytoplasmic Membrane*
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DOI：
10.1074/jbc.m411247200
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发表时间：
2005-01
影响因子：
4.8
通讯作者：
Tomoyuki Suzuki;Kaori Ishihara;Hitoshi Migaki;W. Matsuura;A. Kohda;K. Okumura;M. Nagao;Y. Yamaguchi-Iwai;T. Kambe
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文献类型：
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作者：
Tomoyuki Suzuki;Kaori Ishihara;Hitoshi Migaki;W. Matsuura;A. Kohda;K. Okumura;M. Nagao;Y. Yamaguchi-Iwai;T. Kambe
许多蛋白质在生物合成-分泌途径中通过与锌结合而正确折叠。几种转运蛋白与锌从细胞质进入分泌室有关，但它们的确切作用尚不清楚。我们在这里报道了两个锌转运蛋白（ZnT5和ZnT7）定位于分泌器官，负责将锌装载到碱性磷酸酶（ALPs）上，碱性磷酸酶是暴露于细胞外部位的糖基磷脂酰肌醇锚定的膜蛋白。在DT40细胞中，ZnT5基因的破坏使ALP活性降低到野生型细胞的45%。ZnT7基因的破坏仅使ALP活性降低20%。两个基因的破坏显著降低ALP活性至<5%。在同时缺乏ZnT5和ZnT7基因的DT40细胞中，过表达人ZnT5或ZnT7基因将ALP活性恢复到与野生型细胞相当的水平。在同时缺乏ZnT5和ZnT7基因的DT40细胞中，失活的ALP蛋白与野生型细胞中的活性ALP蛋白一样被转运到细胞质膜上。因此ZnT5和ZnT7都有助于apo-ALP向holo-ALP的转化。
Numerous proteins are properly folded by binding with zinc during their itinerary in the biosynthetic-secretory pathway. Several transporters have been implicated in the zinc entry into secretory compartments from cytosol, but their precise roles are poorly understood. We report here that two zinc transporters (ZnT5 and ZnT7) localized in the secretory apparatus are responsible for loading zinc to alkaline phosphatases (ALPs) that are glycosylphosphatidylinositol-anchored membrane proteins exposed to the extracellular site. Disruption of the ZnT5 gene in DT40 cells decreased the ALP activity to 45% of that in the wild-type cells. Disruption of the ZnT7 gene lowered the ALP activity only by 20%. Disruption of both genes markedly decreased the ALP activity to <5%. Overexpression of human ZnT5 or ZnT7 in DT40 cells deficient in both ZnT5 and ZnT7 genes recovered the ALP activity to the level comparable to that in the wild-type cells. The inactive ALP protein in DT40 cells deficient in both ZnT5 and ZnT7 genes was transported to cytoplasmic membrane like the active ALP protein in the wild-type cells. Thus both ZnT5 and ZnT7 contribute to the conversion of apo-ALP to holo-ALP.