Activity of yeast nutrient signalling pathways during wine fermentation

Resumen del póster presentado a la XXIX International Yeast Conference Genetics and Molecular Biology (ICYGMB), celebrada en Göteborg (Gothenburg-Sweden) del 18 al 22 de agosto de 2019.The yeast Saccharomyces cerevisiae is the main microorganism carrying out the alcoholic fermentation to convert grape juice to wine. The grape juice is characterized by plenty sugar concentration, in the form of glucose and fructose, that causes hyperosmotic stress, but scarce nitrogen sources, so nitrogen limitation usually produces starvation during most fermentations. These peculiar conditions make the crosstalk between nutrients signalling pathways a critical event not only for cell survival, but also for proper fermentation performance in metabolically active cells. As nitrogen is scarce, the TORC1 pathway must be relevant in the process. TORC1 activity was measured by the phosphorylation state of two of its readouts, the proteins Rps6p and Par32p. The analysis of both TORC1 targets indicates that their branches pathway are activated only during the first hours of winemaking pointing out that signalling of nitrogen shortage may be taking place earlier than expected, when nitrogen is still available. Possibly, other nutrient signalling pathways could be active to promote growth. One of these pathways is the glucose repression pathway, controlled by protein kinase Snf1p, mainly regulating the use of carbon sources alternative to glucose. Snf1p-dependent phosphorylation patterns demonstrate that surprisingly this pathway is also active only during the first hours of winemaking, when glucose and fructose are present at high concentration much longer. The activity of the Ras/cAMp/PKA pathway has also been investigated by measuring cAMP levels, which show a 2.5-fold reduction between the first hours of fermentation and stationary phase cultures, when the pathway is not activated. Stress conditions at the beginning of fermentation may give rise to a certain level of kinase activation, despite the high abundance of sugars in the must. Nitrogen availability also influences the activity of yeast nitrogen catabolic repression (NCR), mediated by the transcription factor Gln3p, which allows cells to repress the use of poor nitrogen sources when optimal nitrogen sources are present. The analysis of Gln3p phosphorylation state along fermentation indicates that NCR seems to be inactive at the beginning of the fermentation, although yeast assimilable nitrogen (YAN) is not low. The coordination between nutrient signalling pathways allows to integrate different inputs, such as nitrogen shortage and high sugar concentration, to respond to the specifically environmental conditions during the alcoholic fermentation of the grape must, which are very harsh.Peer reviewe