Metabolic and developmental functions of the transcription factor Gcn4p of Saccharomyces cerevisiae

Abstract

The bakers’ yeast Saccharomyces cerevisiae executes two well established pathways, the ‘General Amino Acid Control’ (GAAC) and the ‘Unfolded Protein Response’ (UPR), which are in contrast to mammals not essential but enable yeast cells to adapt to environmental changes and different stress conditions. The bZIP transcription factor Gcn4p represents the global key regulator of the GAAC and herein regulates transcription of numerous metabolic genes of amino acid or purine biosynthesis in response to amino acid starvation. Gcn4p is also involved in the regulation of the developmental cell-surface flocculin Flo11p, which is required for diploid pseudohyphae formation and for adhesion upon nutrient starvation. This dual function as metabolic and developmental activator could be separated by a Gcn4p variant carrying a single amino acid substitution in its leucine zipper, Gcn4pL267S. This mutation abolishes FLO11 expression and results in a reduced but sufficient transcriptional activity for the induction of amino acid biosynthetic genes. Gcn4pL267S is impaired in homodimer formation and presents a significantly more stable protein compared to wild type Gcn4p. A helix breaker substitution in Leu253 results in a transcriptionally inactive, but highly stable protein variant. This is due to a feedback circuit between transcriptional activity of Gcn4p and its own stability, which depends on the Gcn4p-controlled cyclin Pcl5p. Gcn4pL253G reduces the expression of Pcl5p and therefore its own degradation. Hac1p plays an important role in the yeast UPR system and represents a bZIP transcription factor, alike Gcn4p. This work presents first evidence for a so far unknown function of Hac1p in the GAAC. Hac1p is not only able to activate Gcn4p specific target genes, but also FLO11 expression is reduced in yeast cells deleted for HAC1 and diploids can neither grow adhesively when starved for amino acids nor develop pseudohyhae upon nitrogen starvation. Promoter analysis of FLO11 identified a promoter element influenced by both, Hac1p and Gcn4p, in response to amino acid starvation that was previously identified to confer regulation by amino acid starvation. Transcription factor specific stress situations result in repression of the respective antagonist. First results indicate novel evidence in Hac1p regulation, which might be of clinical interest due to the involvement of the UPR system in tumorogenesis in mammal ian.