Gitan Raad S, Shababi Monir, Kramer Michelle, Eide David J
Department of Nutritional Sciences and Biochemistry, University of Missouri, Columbia, Missouri 65211, USA.
J Biol Chem. 2003 Oct 10;278(41):39558-64. doi: 10.1074/jbc.M302760200. Epub 2003 Jul 31.
Nutrient metals such as zinc are both essential to life and potentially toxic if overaccumulated by cells. Non-essential toxic metals like cadmium can enter cells through the uptake transporters responsible for nutrient metal acquisition. Therefore, in the face of ever changing extracellular metal levels, organisms tightly control their intracellular levels of nutrient metals and prevent accumulation of toxic metals. We show here that post-translational inactivation of the yeast Zrt1 zinc uptake transporter is important for zinc homeostasis. During the transition from zinc-limiting to zinc-replete growth conditions (i.e. zinc shock), the Zrt1 transporter is ubiquitinated, endocytosed, and subsequently degraded in the vacuole. To further understand this process at a molecular level, we mapped a region of Zrt1 required for ubiquitination and endocytosis in response to zinc to a domain located on the intracellular surface of the plasma membrane. This domain is a critical cis-acting component of the metal signaling pathway that controls Zrt1 protein trafficking. Using mutant alleles defective for metal-responsive inactivation, we also show that Zrt1 inactivation may be an important mechanism for preventing cadmium uptake and toxicity in zinc-limited cells.
锌等营养金属对生命至关重要，但如果细胞过度积累则可能具有毒性。镉等非必需有毒金属可通过负责营养金属摄取的转运蛋白进入细胞。因此，面对不断变化的细胞外金属水平，生物体严格控制其细胞内营养金属的水平，并防止有毒金属的积累。我们在此表明，酵母Zrt1锌摄取转运蛋白的翻译后失活对锌稳态很重要。在从锌限制生长条件转变为锌充足生长条件（即锌冲击）期间，Zrt1转运蛋白被泛素化、内吞，随后在液泡中降解。为了在分子水平上进一步了解这一过程，我们将响应锌的泛素化和内吞所需的Zrt1区域定位到位于质膜细胞内表面的一个结构域。该结构域是控制Zrt1蛋白运输的金属信号通路的关键顺式作用成分。使用对金属响应失活有缺陷的突变等位基因，我们还表明Zrt1失活可能是防止锌限制细胞中镉摄取和毒性的重要机制。
