A cytosolic domain of the yeast Zrt1 zinc transporter is required for its post-translational inactivation in response to zinc and cadmium

Nutrient metals such as zinc are both essential to life and potentially toxic if overaccumulated by cells. Nonessential 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.