From Lipid Homeostasis to Differentiation: Old and New Functions of the Zinc Cluster Proteins Ecm22, Upc2, Sut1 and Sut2

Abstract

Zinc cluster proteins are a large family of transcriptional regulators with a wide range of biological functions. The zinc cluster proteins Ecm22, Upc2, Sut1 and Sut2 have initially been identified as regulators of sterol import in the budding yeast Saccharomyces cerevisiae. These proteins also control adaptations to anaerobic growth, sterol biosynthesis as well as filamentation and mating. Orthologs of these zinc cluster proteins have been identified in several species of Candida. Upc2 plays a critical role in antifungal resistance in these important human fungal pathogens. Upc2 is therefore an interesting potential target for novel antifungals. In this review we discuss the functions, mode of actions and regulation of Ecm22, Upc2, Sut1 and Sut2 in budding yeast and Candida.

Keywords: Candida; anaerobic; antifungal; budding yeast; filamentation; mating; sterol biosynthesis; sterol uptake; transcription factor; zinc cluster protein.

Figure 1

Structures of DNA-binding domains of zinc cluster proteins. (A) Crystal structure of the Gal4-DNA complex (PDB 1D66); and (B) crystal structure of the Ppr1 DNA-binding domain (PDB 1PYI). Both proteins form dimers. The two subunits are shown in yellow and pink. Grey spheres represent Zn²⁺ ions.

Figure 2

Domain structures of Ecm22, Upc2, Sut1 and Sut2. Shown are proteins from the budding yeast S. cerevisiae (S.c.) and C. albicans (C.a.). Asterisks denote gain-of-function mutations of Ecm22 and Upc2. The red asterisk of C. albicans Upc2 represents several distinct amino acid substitutions between residue 642 and 648.

Figure 3

Crystal structure of the Upc2 lipid-binding domain. The lipid-binding domain forms dimers shown in green and turquoise (PDB 4N9N). To improve diffraction quality for crystallographic studies, a T4 lysozyme has been inserted between helix 5 and 6 (shown in light blue) [14]. The hydrophobic pockets are shown by the black ellipses.

Figure 4

Regulation of Upc2 activity through lipid sensing. (A) When ergosterol (red spheres) levels are high it binds to the lipid-binding domain (LBD) of Upc2 and keeps the protein in the cytoplasm, possibly because the lipid-binding domain masks the NLS that lies adjacent to the Zn(II)₂Cys₆ motif (Figure 2) (Zn). (B) When sterol levels drop ergosterol no longer binds to Upc2. This might trigger a conformational change of the protein, which results in a translocation to the nucleus. In the nucleus Upc2 induces genes involved in sterol biosynthesis, sterol uptake and general adaptations to anaerobic conditions.

Figure 5

Transcriptional regulation of DAN1. (A) SUT1 and UPC2 are partially repressed by Rox1 in the presence of oxygen. The anaerobic gene DAN1 is not expressed due to chromatin-mediated repression. (B) Under anaerobic conditions, nucleosomes are released from the DAN1 promoter following histone deacetylation by Rpd3. Rox1 repression is lifted, which results in increased expression of UPC2 and SUT1. Upc2 induces DAN1 expression through direct binding with the promoter, whereas Sut1 might inactivate the co-repressor complex Cyc8–Tup1. Experiments that led to this model have largely been done with DAN1. However, these mechanisms might also apply to the regulation of other anaerobic genes.

Figure 6

Transcriptional regulation of filamentation. (A) Under nutrient-rich conditions, SUT1 and SUT2 are expressed and partially repress UPC2 and target genes that have a role in filamentation; (B) when cells grow on a semisolid medium with limited nutrients, Ste12 becomes activated, which represses SUT1 and SUT2. This results in induction of the Sut1/Sut2 target genes including UPC2, which in turn leads to increased expression of Ecm22/Upc2 targets. The combined action of increased levels of Sut1/Sut2 and Ecm22/Upc2 targets might then trigger a switch to filamentous growth. Arrows indicate induction of gene expression, T-bars represent repression.