Étude de l'homéostasie de la taille chez la levure opportuniste Candida albicans

Authors: Chaillot, Julien
Advisor: Sellam, Adnane
Abstract: Cell size homeostasis is an important process of cell proliferation but the molecular mechanisms are poorly understood. Eukaryotic cells must reach a threshold size before entering the cell cycle, which helps to maintain a constant size over the long term. This process is regulated at the end of the G1 phase, a check point called START. Cell size control has been studied in the model yeast Saccharomyces cerevisiae but has never been studied in pathogenic fungi. In this thesis, we used Candida albicansas a model organism to study the regulation of size in pathogenic yeasts. We have screened heterozygous and homozygous deletion collections of C. albicans to identify genes that control cell size. We analyzed the size distribution of 279 homozygous mutants and 4,348 heterozygous mutants (covering 90% of the genome). We compared our results with different screens performed on the model yeast S.cerevisiae. These comparisons showed that few regulators were conserved between S. cerevisiaeand C. albicans and suggesting that the cell size regulation is evolutionary plastic. For example, dot6 mutant has a small phenotype in C. albicansbut has no size phenotype in S. cerevisiae. We have shown that Dot6 is a transcriptional factor necessary for the activation of ribosome biogenes is genes. Dot6 is also a regulator of START and plays a critical role in adapting size according to the carbon sources available in the medium. We also uncovered a novel stress-independent role of the Hog1/p38 MAPK in size regulation in C. albicansa role that has never been demonstrated in S. cerevisiae. We have shown that Hog1, as well as the entire HOG pathway, are negative regulators of START. We have shown that Hog1 regulates both growth via Sfp1, a major transcriptional regulator of ribosomal biogenesis and ribosomal proteins, and the cell cycle via the SBF complex (Swi4/Swi6), transcriptional factors necessary for the G1/S transition. We also found that Ahr1, a transcription factor with no obvious ortholog in S. cerevisiae, has a role for the adaptation of the size according to the amino acids available in the medium. We have shown that Ahr1 is a negative START regulator and is controlled by the Tor1-Sch9 pathway. In conclusion, our work has permitted to discover new regulators of START, to characterise their function and to map them in different pathways. As the Hog1/p38 pathway is linked to many human pathologies, we think that C.albicansis a useful model to study of this pathway and dissect its role in size control in eukaryotes.
Document Type: Thèse de doctorat
Issue Date: 2019
Open Access Date: 11 July 2019
Permalink: http://hdl.handle.net/20.500.11794/35434
Grantor: Université Laval
Collection:Thèses et mémoires

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