NEGATIVE CONTROL OF HEAVY-METAL UPTAKE BY THE SACCHAROMYCES-CEREVISIAE BSD2 GENE

We have previously shown that mutations in the Saccharomyces cerevisia e BSD2 gene suppress oxidative damage in cells lacking superoxide dism utase and also lead to hyperaccumulation of copper ions. We demonstrat e here that bsd2 mutant cells additionally accumulate high levels of c admium and cobalt. By biochemical fractionation and immunofluorescence microscopy, BSD2 exhibited localization to the endoplasmic reticulum, suggesting that BSD2 acts at a distance to inhibit metal uptake from the growth medium. This BSD2 control of ion transport occurs independe ntly of the CTR1 and FET4 metal transport systems. Genetic suppressor analysis revealed that hyperaccumulation of copper and cadmium in bsd2 mutants is mediated through SMF1, previously shown to encode a plasma membrane transporter for manganese. A nonsense mutation removing the carboxyl-terminal hydrophobic domain of SMF1 was found to mimic a smf1 gene deletion by eliminating the copper and cadmium toxicity of bsd2 mutants and also by precluding the bsd2 suppression of superoxide dism utase deficiency. However, inactivation of SMF1 did not eliminate the elevated cobalt levels in bsd2 mutants. Instead, this cobalt accumulat ion was found to be specifically mediated through the SMF1 homologue, SMF2. Hence, BSD2 prevents metal hyperaccumulation by exerting negativ e control over the SMF1 and SMF2 metal transport systems.