Evidence for Cu(II) reduction as a component of copper uptake by Saccharomyces cerevisiae.

The yeast Saccharomyces cerevisiae contains a plasma membrane reductase activity associated with the gene product of the FRE1 locus. This reductase is required for Fe(III) uptake by this yeast; transcription from FRE1 is repressed by iron (Dancis, A., Klausner, R. D., Hinnebusch, A. G., and Barriocanal, J. G. (1990) Mol. Cell. Biol. 10, 2294-2301). We show here that Cu(II) is equally efficient at repressing FRE1 transcription and is an excellent substrate for the Fre1p reductase. This reductase activity is required for 50-70% of the uptake of 64Cu by wild type cells. Under conditions of low Fre1-dependent activity, cells retain 30-70% of Cu(II) reductase activity but only 8-25% of Fe(III) reductase activity. While Fre1p-dependent activity is 100% inhibitable by Pt(II), this residual Cu(II) reduction is insensitive to this inhibitor. The data suggest the presence of a Fre1p-independent reductase activity in the yeast plasma membrane which is relatively specific for Cu(II) and which supports copper uptake in the absence of FRE1 expression. The gene product of MAC1, which is required for regulation of FRE1 transcription, is also required for expression of Cu(II) reduction activity. This is due in part to its role in the regulation of FRE1; however, it is required for expression of the putative Cu(II) reductase, as well. Similarly, a gain-of-function mutation, MAC1up1, which causes elevated and unregulated transcription from FRE1 and elevated Fe(III) reduction and 59Fe uptake exhibits a similar phenotype with respect to Cu(II) reduction and 64Cu uptake. Ascorbate, which reduces periplasmic Cu(II) to Cu(I), suppresses the dependence of 64Cu uptake on plasma membrane reductase activity as is the case for ascorbate-supported 59Fe uptake. The close parallels between Cu(II) and Fe(III) reduction, and 64Cu and 59Fe uptake, strongly suggest that Cu(II) uptake by yeast involves a Cu(I) intermediate. This results in the reductive mobilization of the copper from periplasmic chelating agents, making the free ion available for translocation across the plasma membrane.