Evidence for a novel role of copper-zinc superoxide dismutase in zinc metabolism

The LYS7 gene in Saccharomyces cerevisiae encodes a protein (yCCS) that delivers copper to the active site of copper-zinc superoxide dismutase (CuZn-SOD, a product of the SOD1 gene). In yeast lacking Lys7 (lys7Delta), the SOD1 polypeptide is present but inactive. Mutants lacking the SOD1 polypeptide (sod1Delta) and lys7Delta yeast show very similar phenotypes, namely poor growth in air and aerobic auxotrophies for lysine and methionine. Here, we demonstrate certain phenotypic differences between these strains: 1) lys7Delta cells are slightly less sensitive to paraquat than sod1Delta cells, 2) EPR-detectable or "free" iron is dramatically elevated in sod1Delta mutants but not in lys7Delta yeast, and 3) although sod1Delta mutants show increased sensitivity to extracellular zinc, the lys7Delta strain is as resistant as wild type. To restore the SOD catalytic activity but not the zinc-binding capability of the SOD1 polypeptide, we overexpressed Mn-SOD from Bacillus stearothermophilus in the cytoplasm of sod1Delta yeast. Paraquat resistance was restored to wild-type levels, but zinc was not. Conversely, expression of a mutant CuZn-SOD that binds zinc but has no SOD activity (H46C) restored zinc resistance but not paraquat resistance. Taken together, these results strongly suggest that CuZn-SOD, in addition to its antioxidant properties, plays a role in zinc homeostasis.