Accumulation of metal-binding peptides in fission yeast requires hmt2(+)

The fission yeast Schizosaccharomyces pombe detoxifies cadmium by synthesiz ing phytochelatins, peptides of the structure (gamma -GluCys)(n)Gly, which bind cadmium and mediate its sequestration into the vacuole. The fission ye ast protein HMT2, a mitochondrial enzyme that can oxidize sulphide, appears to be essential for tolerance to multiple forms of stress, including expos ure to cadmium. We found that the hmt2(-) mutant is unable to accumulate no rmal levels of phytochelatins in response to cadmium, although the cells po ssess a phytochelatin synthase that is active in vitro. Radioactive pulse-c hase experiments demonstrated that the defect lies in two steps: the synthe sis of phytochelations and the upregulation of glutathione production. Phyt ochelatins, once formed, are stable. hmt2- cells accumulate high levels of sulphide and, when exposed to cadmium, display bright fluorescent bodies co nsistent with cadmium sulphide. We propose that the precipitation of free c admium blocks phytochelatin synthesis in vivo, by preventing upregulation o f glutathione production and formation of the cadmium-glutathione thiolate required as a substrate by phytochelatin synthase. Thus, although sulphide is required for phytochelatin-mediated metal tolerance, aberrantly high sul phide levels can inhibit this pathway. Precise regulation of sulphur metabo lism, mediated in part by HMT2, is essential for metal tolerance in fission yeast.