Ishihara Kaori, Yamazaki Tomohiro, Ishida Yoko, Suzuki Tomoyuki, Oda Kimimitsu, Nagao Masaya, Yamaguchi-Iwai Yuko, Kambe Taiho
Division of Integrated Life Science, Graduate School of Biostudies, Kyoto University, Kyoto 606-8502, Japan.
J Biol Chem. 2006 Jun 30;281(26):17743-50. doi: 10.1074/jbc.M602470200. Epub 2006 Apr 24.
Zinc transporters play important roles in a wide range of biochemical processes. Here we report an important function of ZnT5/ZnT6 hetero-oligomeric complexes in the secretory pathway. The activity of human tissue-nonspecific alkaline phosphatase (ALP) expressed in ZnT5(-)ZnT7(-/-) cells was significantly reduced compared with that expressed in wild-type cells as in the case of endogenous chicken tissue-nonspecific ALP activity. The inactive human tissue-nonspecific ALP in ZnT5(-)ZnT7(-/-) cells was degraded by proteasome-mediated degradation without being trafficked to the plasma membrane. ZnT5(-)ZnT7(-/-) cells showed exacerbation of the unfolded protein response as did the wild-type cells cultured under a zinc-deficient condition, revealing that both complexes play a role in homeostatic maintenance of secretory pathway function. Furthermore, we showed that expression of ZnT5 mRNA was up-regulated by the endoplasmic reticulum stress in various cell lines. The up-regulation of the hZnT5 transcript was mediated by transcription factor XBP1 through the TGACGTGG sequence in the hZnT5 promoter, and this sequence was highly conserved in the ZnT5 genes of mouse and chicken. These results suggest that zinc transport into the secretory pathway is strictly regulated for the homeostatic maintenance of secretory pathway function in vertebrate cells.
锌转运体在广泛的生化过程中发挥着重要作用。在此我们报道了ZnT5/ZnT6异源寡聚复合物在分泌途径中的一项重要功能。与野生型细胞中表达的人组织非特异性碱性磷酸酶（ALP）活性相比，ZnT5(-)ZnT7(-/-)细胞中表达的该酶活性显著降低，内源性鸡组织非特异性ALP活性的情况也是如此。ZnT5(-)ZnT7(-/-)细胞中无活性的人组织非特异性ALP通过蛋白酶体介导的降解被降解，而不会被转运到质膜。ZnT5(-)ZnT7(-/-)细胞与在缺锌条件下培养的野生型细胞一样，显示出未折叠蛋白反应加剧，这表明这两种复合物在分泌途径功能的稳态维持中都发挥作用。此外，我们表明在各种细胞系中，内质网应激会上调ZnT5 mRNA的表达。hZnT5转录本的上调是由转录因子XBP1通过hZnT5启动子中的TGACGTGG序列介导的，并且该序列在小鼠和鸡的ZnT5基因中高度保守。这些结果表明，为了在脊椎动物细胞中稳态维持分泌途径功能，进入分泌途径的锌转运受到严格调控。
