Functional Networks Involved in Cell Wall Biosynthesis and the Isoprenoid Pathway in the Yeast Saccharomyces cerevisiae

Ce travail porte sur la reponse de la levure a des composes chimiques, qui de maniere similaire aux antifongiques, alterent la structure parietale et/ou la biosynthese de l'ergosterol. Ces deux voies sont essentielles pour la survie des champignons, la paroi les protege de leur environnement, alors que l'ergosterol garantit un fonctionnement correct de la membrane plasmique. Pour comprendre ces mecanismes et leurs interactions, nous avons choisi un organisme modele, la levure Saccharomyces cerevisiae, et etudie sa reponse a des agents perturbant la synthese de la paroi (populacandine B, rouge Congo, cafeine et Calcofluor White), ainsi qu'a des inhibiteurs de la biosynthese de l'ergosterol (lovastatine et acide zaragozic). Nous avons decouvert que la cafeine inhibe Tor1p et provoque une baisse rapide et transitoire du niveau intracellulaire en AMPc. Nous avons ainsi montre que Rom2p, un element de la voie du maintien de l’integrite parietale, agit sous l'effet de la cafeine comme une proteine "pivot" en transmettant le signal a la kinase Mpk1, mais egalement a la voie de signalisation de l'AMPc. La cafeine a induit un "renforcement" de la paroi mais, contrairement aux autres drogues testees, cet effet ne s'exerce pas a travers une interaction directe. En effet, ce remodelage ne se produit pas a travers la voie habituelle Rlm1p, mais necessite d'autres facteurs de transcription comme Crz1p, Swi4p et Msn2/4p. Ces resultats nous ont permis d'identifier de nouvelles relations entre les voies de maintien de l'integrite parietale, TOR, et la cascade de signalisation de l'AMPc, qui exercent un controle synergique sur la croissance des champignons en reponse aux stress. ___________________________________________________________________This work is focused on the fungal response to the chemical compounds, which similarly to antifungal drugs, impair the cell wall or ergosterol biosynthesis. Both examined pathways are essential for survival of fungi. The cell wall protects them from the harmful environment whereas ergosterol ensures correct functioning of the plasma membrane. For investigation of those pathways, we chose the genetically tractable fungus, the yeast Saccharomyces cerevisiae, and studied its response to the cell wall damaging agents, i.e. Papulacandin B, Congo red, Calcofluor white and caffeine, and to the inhibitors of the ergosterol biosynthesis like lovastatin and zaragozic acid. A novel function of caffeine was discovered to inhibit Tor1p and to cause a rapid but transient decrease in the intracellular level of cAMP. We have identified Rom2p, a component of the cell wall integrity pathway, as a pivotal protein mediating caffeine-derived signaling to Mpk1 kinase as well as to the cAMP signaling pathway. Caffeine induced strengthening of the cell wall but, in contrast to other tested drugs, this effect was not exerted through a direct interaction with the cell wall structure. Moreover, the caffeine-induced remodeling of the cell wall did not occur through the usual Rlm1p-mediated way but it required some other transcription factors like Crz1p, Swi4p and Msn2/4p. Altogether, we report here a number of results that bear on the relationship between the cell wall integrity, the TOR pathway and the cAMP signaling cascade that together control the growth of the fungal cell in response to stressing conditions.