The Schizosaccharomyces pombe Cuf1 is composed of functional modules from two distinct classes of copper metalloregulatory transcription factors

Abstract

In fission yeast, the genes encoding proteins that are components of the copper transporter family are controlled at the transcriptional level by the Cuf1 transcription factor. Under low copper availability, Cuf1 induces expression of the copper transporter genes. In contrast, sufficient levels of copper inactivate Cuf1 and expression of its target genes. Our study reveals that Cuf1 harbors a putative copper-binding motif, Cys-X-Cys-X3-Cys-X-Cys-X2-Cys-X2-His, within its carboxyl-terminal region to sense changing environmental copper levels. Binding studies reveal that the amino-terminal 174-residue segment of Cuf1 expressed as a fusion protein in Escherichia coli specifically interacts with the cis-acting copper transporter promoter element CuSE (copper-signaling element). Within this region, the first 61 amino acids of Cuf1 exhibit more overall homology to the Saccharomyces cerevisiae Ace1 copper-detoxifying factor (from residues 1 to 63) than to Mac1, its functional ortholog. Consistently, we demonstrate that a chimeric Cuf1 protein bearing the amino-terminal 63-residue segment of Ace1 complements cuf1Δ null phenotypes. Furthermore, we show that Schizosaccharomyces pombe cuf1Δ mutant cells expressing the full-length S. cerevisiae Ace1 protein are hypersensitive to copper ions, with a concomitant up-regulation of CuSE-mediated gene expression in fission yeast. Taken together, these studies reveal that S. cerevisiae Ace1 1-63 is functionally exchangeable with S. pombe Cuf1 1-61, and the nature of the amino acids located downstream of this amino-terminal conserved region may be crucial in dictating the type of regulatory response required to establish and maintain copper homeostasis.