Characterization and functional significance of Msc2p & Zrg17p, which form a zinc transport complex in the endoplasmic reticulum of Saccharomyces cerevisiae

Abstract

We demonstrate a novel interaction between two members of the cation diffusion facilitator (CDF) family in Saccharomyces cerevisiae: Msc2p and a newly recognized CDF family member, Zrg17p. Both Msc2p and Zrg17p have been previously implicated in zinc homeostasis in yeast. In particular, ZRG17 was previously identified as a zinc regulated gene controlled by the zinc-responsive Zap1p transcription factor. We show that both Msc2p and Zrg17p are localized to the endoplasmic reticulum (ER) when expressed at normal levels. Zinc deficiency in yeast induces the unfolded protein response (UPR), a system normally activated by unfolded ER proteins. UPR induction in low zinc is exacerbated in msc2 and zrg17 mutants. Genetic and biochemical evidence indicate that this UPR induction is due to genuine ER dysfunction. Notably, ER-associated protein degradation (ERAD) is defective in zinc-limited msc2 mutants. Msc2p and Zrg17p physically interact, as determined by co-immunoprecipitation. Therefore, we propose that Msc2p and Zrg17p form a zinc transport complex in the ER membrane to maintain the function of this compartment. Zinc deficiency also upregulates the mammalian ER stress response, indicating a conserved requirement for zinc in ER function among eukaryotes. Lastly, ZnT5 and ZnT6, the closest mammalian homologues to Msc2p and Zrg17p, may also functionally interact, suggesting that interactions between CDF members may be a common phenomenon.We demonstrate a novel interaction between two members of the cation diffusion.