Identification of a novel high affinity copper transport complex in the fission yeast Schizosaccharomyces pombe.

Copper is an essential nutrient that serves as a co-factor for enzymes involved in critical cellular processes including energy generation, peptide hormone maturation, oxidative stress protection, and iron homeostasis. Although genes have been identified from yeast and mammals encoding a homologous subunit of a plasma membrane high affinity copper transporter, the presence of additional subunits that function as part of a copper transport complex has not been reported. We observed that ctr4(+), a previously identified copper transport protein from the fission yeast Schizosaccharomyces pombe, fails to complement bakers' yeast cells defective in high affinity copper transport and fails to be targeted to the plasma membrane. However, selection for S. pombe genes, which, when co-expressed with Ctr4, confer high affinity copper transport to S. cerevisiae cells resulted in the identification of ctr5(+). Both Ctr4 and Ctr5 are integral membrane proteins, are co-regulated by copper levels and the copper-sensing transcription factor Cuf1, physically associate in vivo, are interdependent for secretion to the plasma membrane, and are each essential for high affinity copper transport. These studies in S. pombe identify Ctr4 and Ctr5 as components of a novel eukaryotic heteromeric plasma membrane complex that is essential for high affinity copper transport.