Flocculation of Saccharomyces cerevisiae is dependent on activation of Slt2 and Rlm1 regulated by the cell wall integrity pathway

• Published in: Molecular microbiology 2019-10, Vol.112 (4), p.1350-1369
• Main Authors: Sariki, Santhosh Kumar, Kumawat, Ramesh, Singh, Vikash, Tomar, Raghuvir Singh
• Format: Article
• Language: English
• Subjects: Activation; Baking yeast; Binding sites; Cell surface; Cell walls; Chemical treatment; Clonal deletion; Cytology; Flocculation; Gene expression; Gene regulation; Genes; Integrity; MAP kinase; Molecular modelling; Morphology; Mutants; Occupancy; Organic chemistry; Phosphorylation; Promoters; Saccharomyces cerevisiae; Survival; Transcription; Yeasts
• ISSN: 0950-382X; 1365-2958
• DOI: 10.1111/mmi.14375

Summary Flocculation is an essential characteristic of yeast cells required for survival under adverse conditions. The multicellular structure (flocs) of yeast provides a suitable microenvironment to enhance the chances of survival during stress conditions. Although the signaling events triggering flocculation have been studied earlier, molecular mechanisms remain elusive. In the present study, we used flocculating sen1 mutants to identify the mechanism of flocculation. Based on the abnormal cell surface morphology and constitutive phosphorylation of Slt2p in flocculating sen1 mutant cells, we hypothesized if flocculation was regulated by the cell wall integrity (CWI) pathway. Up‐regulation of FLO genes in wild‐type cells was observed upon the activation of CWI pathway either by chemical treatment or by deleting Slt2 phosphatase (Msg5). Our study with Slt2 mutants reveals that the active state of Slt2 is indispensable for flocculation. Deletion of either SLT2 or RLM1 leads to reduced flocculation. Furthermore, we observed overlapping binding sites for Rlm1 and Tup1 at the promoters of almost all the FLO genes. Finally, we show higher Rlm1 and lower Tup1 occupancy at the promoters of FLO1 and FLO5 in flocculating cells. Altogether we demonstrate that CWI MAPK (Slt2) pathway uses a non‐catalytic mechanism to activate the transcription of FLO genes. The flocculation of yeast cells is regulated by Rlm1p and a Cyc8‐Tup1p repressor complex. Under normal conditions, FLO genes are switched OFF due to nucleosome positioning and binding of repressor complex. However, under stress conditions, FLO genes are switched ON due to nucleosome disruption, eviction of repressor complex and subsequent binding of Rlm1, activated by CWI signaling pathway.