Gitan R S, Luo H, Rodgers J, Broderius M, Eide D
Nutritional Sciences Program, University of Missouri-Columbia, Columbia, Missouri 65211, USA.
J Biol Chem. 1998 Oct 30;273(44):28617-24. doi: 10.1074/jbc.273.44.28617.
The ZRT1 gene encodes the transporter responsible for high affinity zinc uptake in yeast. ZRT1 is transcribed in zinc-limited cells and its transcription is repressed in zinc-replete cells. In this report, we describe a second, post-translational mechanism that regulates ZRT1 activity. In zinc-limited cells, ZRT1 is a stable, N-glycosylated plasma membrane protein. Exposure to high levels of extracellular zinc triggers a rapid loss of ZRT1 uptake activity. Our results demonstrate that this inactivation occurs through zinc-induced endocytosis of the protein and its subsequent degradation in the vacuole. Mutations that inhibit the internalization step of endocytosis also inhibited zinc-induced ZRT1 inactivation and the major vacuolar proteases were required to degrade ZRT1 in response to zinc. Furthermore, immunofluorescence microscopy showed that ZRT1 is localized to the plasma membrane in zinc-limited cells and that the protein is transferred to the vacuole via an endosome-like compartment upon exposure to zinc. ZRT1 inactivation is a relatively specific response to zinc; cadmium and cobalt ions trigger the response but less effectively than zinc. Moreover, zinc does not alter the stability of several other plasma membrane proteins. Therefore, zinc-induced ZRT1 inactivation is a specific regulatory system to shut off zinc uptake activity in cells exposed to high extracellular zinc levels thereby preventing overaccumulation of this potentially toxic metal.
ZRT1基因编码负责酵母中高亲和力锌摄取的转运蛋白。ZRT1在锌限制的细胞中被转录，而其转录在锌充足的细胞中受到抑制。在本报告中，我们描述了另一种调节ZRT1活性的翻译后机制。在锌限制的细胞中，ZRT1是一种稳定的、N-糖基化的质膜蛋白。暴露于高水平的细胞外锌会引发ZRT1摄取活性的快速丧失。我们的结果表明，这种失活是通过锌诱导的该蛋白的内吞作用及其随后在液泡中的降解而发生的。抑制内吞作用内化步骤的突变也抑制了锌诱导的ZRT1失活，并且主要的液泡蛋白酶是响应锌而降解ZRT1所必需的。此外，免疫荧光显微镜显示，ZRT1在锌限制的细胞中定位于质膜，并且在暴露于锌时该蛋白通过类似内体的区室转移至液泡。ZRT1失活是对锌的一种相对特异性的反应；镉和钴离子也会引发该反应，但效果不如锌。此外，锌不会改变其他几种质膜蛋白的稳定性。因此，锌诱导的ZRT1失活是一种特异性的调节系统，用于在暴露于高细胞外锌水平的细胞中关闭锌摄取活性，从而防止这种潜在有毒金属的过度积累。
