Mao Xiaoqing, Kim Byung-Eun, Wang Fudi, Eide David J, Petris Michael J
Department of Biochemistry, University of Missouri, Columbia, Missouri 65211, USA.
J Biol Chem. 2007 Mar 9;282(10):6992-7000. doi: 10.1074/jbc.M610552200. Epub 2007 Jan 3.
Zinc is an essential nutrient. Genetic evidence for this nutritional requirement in humans is the zinc deficiency disease, acrodermatitis enteropathica. This disorder is caused by mutations in hZIP4 (SLC39A4), a zinc importer required for zinc uptake in enterocytes and other cell types. Studies in mice have demonstrated that levels of the mZIP4 mRNA are reduced by elevated dietary zinc, resulting in a decreased abundance of the ZIP4 protein at the plasma membrane. Moreover, studies in cultured cells have demonstrated that low micromolar concentrations of zinc stimulate the endocytosis of the mZIP4 protein resulting in a reduction in cellular zinc uptake. In this study, we demonstrate an additional level of hZIP4 regulation involving ubiquitination and degradation of this transporter in elevated zinc concentrations. Mutational analysis identified a cytoplasmic histidine-rich domain that was essential for ubiquitin-dependent degradation of ZIP4 and protection against zinc toxicity. However, this motif was dispensable for zinc-induced endocytosis. These findings indicate that ubiquitin-mediated degradation of the ZIP4 protein is critical for regulating zinc homeostasis in response to the upper tier of physiological zinc concentrations, via a process that is distinct from zinc-stimulated endocytosis.
锌是一种必需营养素。人类这种营养需求的遗传学证据是锌缺乏症——肠病性肢端皮炎。这种疾病是由hZIP4（SLC39A4）突变引起的，hZIP4是肠上皮细胞和其他细胞类型摄取锌所必需的锌转运体。对小鼠的研究表明，饮食中锌含量升高会降低mZIP4 mRNA的水平，导致质膜上ZIP4蛋白的丰度降低。此外，对培养细胞的研究表明，低微摩尔浓度的锌会刺激mZIP4蛋白的内吞作用，导致细胞锌摄取减少。在本研究中，我们证明了hZIP4调控的另一个层面，即在锌浓度升高时，该转运体的泛素化和降解。突变分析确定了一个富含胞质组氨酸的结构域，该结构域对于ZIP4的泛素依赖性降解和抵御锌毒性至关重要。然而，该基序对于锌诱导的内吞作用是可有可无的。这些发现表明，ZIP4蛋白的泛素介导降解对于通过一个不同于锌刺激内吞作用的过程来响应生理锌浓度上限时调节锌稳态至关重要。
