Loss of Zhf and the tightly regulated zinc-uptake system SpZrt1 inSchizosaccharomyces pombereveals the delicacy of cellular zinc balance

Abstract

Zinc is an essential micronutrient, and yet it can be toxic when present in excess. Zinc acquisition and distribution are dependent on tightly controlled transport of Zn(2+) ions. Schizosaccharomyces pombe represents a second eukaryotic model to study cellular metal homeostasis. In several ways its micronutrient metabolism is fundamentally different from Saccharomyces cerevisiae. We identified the first Zn(2+)-uptake system in S. pombe and named it SpZrt1. Knock-out strains for all three ZIP (Zrt, Irt-like protein) transporters in fission yeast were constructed. Only zrt1Delta cells were unable to grow at low Zn(2+) and showed reduced (65)Zn(2+) uptake. Elemental profiles revealed a strong decrease in zinc accumulation. Cd(2+) ions inhibited uptake but Fe(2+) or Mn(2+) did not. Both mRNA abundance and protein amount are tightly regulated. Zrt1 activity is rapidly shut down upon transfer of zinc-deficient cells to zinc-replete conditions. In cells lacking Zhf, a transporter mediating endoplasmic reticulum storage of zinc, this response is about 100-fold more sensitive. Thus, removal of excess of zinc from the cytosol is largely Zhf dependent. Moreover, cells deficient for both transporters are no longer able to adjust to changing external Zn(2+) concentrations. Optimal growth is restricted to a narrow range of Zn(2+) concentrations, illustrating the fine balance between micronutrient deficiency and toxicity.

Phytochelatin Synthesis Is Essential for the Detoxification of Excess Zinc and Contributes Significantly to the Accumulation of Zinc

“…Zn shock tolerance is expressed as percent cell survival compared to untreated cells. Error bars indicate SD, n = 3. cells are strongly reduced compared to wild-type cells (Boch et al, 2008). This difference was not further aggravated by the loss of functional Pcs or Hmt1.…”

“…Loss of Zhf (Zn homeostasis factor), an endoplasmic reticulum-localized transporter of the cation diffusion facilitator family, results in strong Zn 2+ hypersensitivity (Clemens et al, 2002). Excess Zn can no longer be removed from the cytosol in zhfD cells (Boch et al, 2008). When we compared the growth of the zhfDpcsD strain to zhfD cells, we found a significant decrease in Zn 2+ tolerance correlated with the loss of functional PCS (Fig.…”

“…Interestingly, even in the absence of elevated Zn 2+ zhfD cells accumulated PC2 to a clearly detectable level ( Fig. 2A) (Boch et al, 2008), there is elevated constitutive PC2 accumulation.…”

An outlook on ion signaling and ionome of mycorrhizal symbiosis

“…many fundamental aspects of metal homeostasis, including the first insights into the yeast ionome, have already been elucidated in Saccharomyces cerevisiae (eide et al, 2005). However, in several aspects the S. pombe micronutrient metabolism is fundamentally different from S. cerevisiae and closer to other eukaryotic organisms (Boch et al, 2008). importantly, S. pombe genome was sequenced (wood et al, 2002) Beyond the improvement of the major nutrients n, P and K, the Am fungi can play a key function in supplying micronutrients to the host plant, like Zn and Cu (marschnner and Dell, 1994).…”

“…Transcript profiling and a genetic screen employed to characterize the response of fission yeast cells to zinc deficiency led to identification of Zrt1, a ZiP transporter that is a central component of zinc homeostasis (Dainty et al, 2008). Besides, Zhf transporter mediates endoplasmic reticulum storage of zinc (Boch et al, 2008).…”