The Saccharomyces cerevisiae copper transport protein (Ctr1p). Biochemical characterization, regulation by copper, and physiologic role in copper uptake.

The CTR1 gene of Saccharomyces cerevisiae encodes a protein required for high affinity copper uptake. The protein is expressed on the plasma membrane, is heavily glycosylated with O-linkages, and exists as an oligomer in vivo. The transcript abundance is strongly regulated by copper availability, being induced by copper deprivation and repressed by copper excess. Regulation occurs at very low, nontoxic levels of available copper and is independent of ACE1, the trans-inducer of yeast metallothionein. Expression of Ctr1p is limiting for copper uptake, since overexpression from a 2 mu high copy number plasmid increases copper uptake. Mutations in CTR1 result in altered cellular responses to extracellular copper, demonstrating a physiologic role for CTR1 in the delivery of copper to the cytosol. A copper-dependent reporter gene construct, CUP1-lacZ, is not expressed in CTR1 mutants to the same level as in wild-type strains, and Cu,Zn superoxide dismutase activity is deficient in these mutants. The growth arrest that occurs in CTR1 mutants grown aerobically in copper-deficient media is attributable to the defect in Cu,Zn superoxide dismutase activity.