A Copper-sensing Transcription Factor Regulates Iron Uptake Genes in Schizosaccharomyces pombe

Abstract

Copper and iron serve essential functions as catalytic co-factors in a wide variety of critical cellular enzymes. Studies in yeast have demonstrated an absolute dependence upon copper acquisition for proper assembly and function of the iron transport machinery. We have cloned genes for a high affinity copper transporter (Ctr4) and copper-sensing transcription factor (Cuf1) from Schizosaccharomyces pombe. Interestingly, the primary structure of Ctr4 and a putative human high affinity copper transport protein, hCtr1, suggests that they are derived from a fusion of the functionally redundant but structurally distinct Ctr1 and Ctr3 copper transporters from Saccharomyces cerevisiae. Furthermore, although Cuf1 activates ctr4 ؉ gene expression under copper starvation conditions, under these same conditions Cuf1 directly represses expression of genes encoding components of the iron transport machinery. These studies have identified an evolutionary step in which copper transport modules have been fused, and describe a mechanism by which a copper-sensing factor directly represses expression of the iron uptake genes under conditions in which the essential copper co-factor is scarce.

Biochemical Characterization of the Human Copper Transporter Ctr1

“…The yeast high affinity copper transporters and the human and mouse Ctr1 copper transporters are rich in methionine and histidine residues within an amino-terminal hydrophilic region. The methionine-rich motifs, repeated eight times in S. cerevisiae Ctr1 and five times in the S. pombe Ctr4 amino terminus, are arranged as the consensus Met-X-X-Met-X-Met (10,12). Furthermore, these and a number of other conserved residues in the transmembrane domains are identified in the plasma membrane copper transporters in yeast, mice, and humans (data not shown).…”

The CaCTR1 gene is required for high-affinity iron uptake and is transcriptionally controlled by a copper-sensing transactivator encoded by CaMAC1

“…3a; Altschul et al, 1997). This motif is found in several copper-responsive transcription factors, including Mac1p and Ace1p of S. cerevisiae (Jungmann et al, 1993;Szczypka & Thiele, 1989), Amt1p of Candida glabrata (Zhou & Thiele, 1991), Cuf1p of Schizosaccharomyces pombe (Labbe et al, 1999) and Crf1p of Yarrowia lipolytica (Garcia et al, 2002). In each corresponding organism, the motif facilitates the binding of protein to DNA in the presence of copper or silver (Furst & Hamer, 1989;.…”

Biochemical and Genetic Analyses of Yeast and Human High Affinity Copper Transporters Suggest a Conserved Mechanism for Copper Uptake

“…With the exception of yeast Ctr3, all Ctr family members are rich in methionine residues within the amino-terminal hydrophilic portion of the protein (18). It has been suggested that these residues, arranged as MXXM or MXM (Mets motifs), could be involved in extracellular copper binding (10), but experimental evidence supporting a role for these methionine-rich sequences in copper transport is currently lacking.…”

“…Genes encoding high affinity (K m 1-5 M) copper transport proteins have been identified in S. cerevisiae, Schizosaccharomyces pombe (18,19), mammals (20,21), and plants (22). With the exception of yeast Ctr3, all Ctr family members are rich in methionine residues within the amino-terminal hydrophilic portion of the protein (18).…”