Surplus Zinc Is Handled by Zym1 Metallothionein and Zhf Endoplasmic Reticulum Transporter in Schizosaccharomyces pombe

Homeostatic mechanisms prevent the accumulation of free zinc in the cytoplasm, raising questions regarding where surplus zinc is stored and how it is delivered to and from these stores. A genetic screen for zinc hypersensitivity inSchizosaccharomyces pombe identified a missense mutation truncating Zhf, an endoplasmic reticulum transporter. These cells were ∼5-fold more zinc-sensitive than other independent mutants. The targeted disruption of zhf prevented growth on low zinc medium and caused hypersensitivity to elevated zinc/cobalt but resistance to cadmium. The exposure to elevated zinc but not copper also promotes the accumulation of transcripts encoding a metallothionein designated Zym1. The Sty1 pathway is required for maximal zym1 expression but is not obligatory for zinc perception. The targeted disruption of zym1impaired cadmium tolerance but only slightly impaired zinc tolerance, whereas zym1 overexpression substantially rescued zinc hypersensitivity of zhf− cells. Four equivalents of zinc were displaced from Zym1 by up to 12 equivalents ofp-(hydroxymercuri)phenylsulphonate. Zym1 thiols react rapidly with 5,5′-dithiobis-(2-nitrobenzoic acid) compared with bacterial zinc metallothionein (6.8 and 0.2 × 10−4s−1, respectively). Zym1 is unlike known fungal metallothioneins that are induced by and sequester copper but not zinc. Less zinc but normal cadmium was accumulated by zym1Δ, consistent with zinc sequestration by Zym1 in vivo.