Summary: The metabolism of copper in the yeast has been studied with respect to the distribution and stability to exchange of newly arrived Cu. Cells pre-incubated with 10 μm-Cu accumulated Cu into two pools distinguishable by cellular locale and lability to exchange with extracellular cold copper. One pool was non-exchangeable and was localized to protoplasts. Size-exclusion chromatography of a soluble cell (protoplast) extract showed that this Cu was associated with up to four species. Two were identified as copper metallothionein and Cu,Zn superoxide dismutase based on comparisons of chromatograms derived from strains in which the genes for these two proteins had been deleted. A third species was identified as copper-glutathione based on chromatographic and biochemical assays. A second pool was exchangeable and was localized to the cell wall. In contrast to its rapid copper-stimulated exchange (1/2 % 1 min), this pool exhibited only slow efflux (10% Cu loss per 60 min). Zn did not stimulate the loss of Cu from this pool indicating that it was selective for copper. This pool was released into the supernatant upon protoplast formation and was found in the cell wall debris obtained when cells were mechanically disrupted. This Cu eluted in the void volume (peak P) of the column used to size-fractionate copper-binding species. The metal in P was exchangeable and However, the corresponding chromatographic fraction obtained from copper-naive cells when labelled could bind less than 20 % of the Cu bound to it indicating that the deposition of copper in this pool was primarily cell-dependent. In fact, this deposition was shown to be dependent on the cellular reduction of medium sulphate or sulphite to the level of sulphide, or on the addition of sulphide to the Cu uptake buffer. Cu in the non-exchangeable protoplast pool was not mobilized by cellular sulphide generation, indicating that cellular sulphide generation did not causally lead to the partitioning of Cu to the cell wall pool. The data indicate that the appearance of copper sulphide(s) on the cell wall in is gratuitous and does not represent a sulphide-based mechanism of copper resistance in this yeast.


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