COPPER-DEPENDENT DEGRADATION OF THE SACCHAROMYCES-CEREVISIAE PLASMA-MEMBRANE COPPER TRANSPORTER CTR1P IN THE APPARENT ABSENCE OF ENDOCYTOSIS

The cell surface protein repertoire needs to be regulated in response to changes in the extracellular environment. In this study, we investi gate protein turnover of the Saccharomyces cerevisiae plasma membrane copper transporter Ctr1p, in response to a change in extracellular cop per levels. As Ctr1p mediates high affinity uptake of copper into the cell, modulation of its expression is expected to be involved in coppe r homeostasis. We demonstrate that Ctr1p is a stable protein when cell s are grown in low concentrations of copper, but that exposure of cell s to high concentrations of copper (10 mu M) triggers degradation of c ell surface Ctr1p. This degradation appears to be specific for Ctr1p a nd does not occur with another yeast plasma membrane protein tested. I nternalization of some Ctr1p can be seen when cells are exposed to cop per. However, yeast mutant strains defective in endocytosis (end3, end 4 and chc1-ts) and vacuolar degradation (pep4) exhibit copper-dependen t Ctr1p degradation, indicating that internalization and delivery to t he vacuole is not the principal mechanism responsible for degradation. In addition, a variant Ctr1p with a deletion in the cytosolic tail is not internalized upon exposure of cells to copper, but is nevertheles s degraded. These observations indicate that proteolysis at the plasma membrane most likely explains copper-dependent turnover of Ctr1p and point to the existence of a novel pathway in yeast for plasma membrane protein turnover.