Abstract #9

Crosstalk between HOG1 and SLT2 MAPK pathways is necessary for Saccharomyces cerevisiae cell survival under cell wall stress. C. Bermejo, E. Rodríguez, R. García, P. Arias, A. B. Sanz, N. Blanco, S. Díez, J. M. Rodríguez-Peña, C. Nombela, J. Arroyo. Dpto. Microbiología II. Fac. Farmacia. Universidad Complutense de Madrid 28040. Spain.
   The fungal cell wall is a highly adaptable organelle with an essential role in growth, development and interaction of the yeast with their environment. Under challenging conditions that alter this structure, activation of a specific transcriptional response, that includes CRH1, is observed. This response is mainly regulated by the MAPK Slt2p/Mpk1p. However, we found that the HOG1 pathway also plays an important role in the control of cell integrity. Both, Congo Red, that binds to chitin, and Zymolyase which degrades the b-1,3 glucan network, lead to hyper-phosphorylation of the MAPK Slt2p, that activates the transcription factor Rlm1p, inducing CRH1 expression. However, while transcriptional activation of CRH1 by Congo Red was not dependent on the MAPK Hog1p, the induction by Zymolyase was dependent on this MAPK. The effect of Hog1p activation on CRH1 transcription was not mediated by any of the Hog1p regulated transcription factors tested. However, Slt2p phosphorylation in response to Zymolyase required the presence of a functional Hog1p. In addition, a whole functional Sho1p branch of the HOG1 pathway and the presence of Bck1p and Mkk1p/Mkk2p were also necessary for Slt2p activation. Interestingly, mutants affected in different elements of the Sho1 branch (hog1, pbs2, ste11, ste50, sho1), as well as in different elements of the cell integrity pathway (slt2, mkkk1 mkk2, bck1)were hypersensitive to Zymolyase. Hyper-phosphorylation of Slt2p by Zymolyase and transcriptional activation of CRH1 in these conditions was not dependent on the cell integrity pathway sensors Mid2p, Wsc1p, Wsc2p, Wsc3p, Wsc4p and Mtl1p, but was completely abrogated in sho1 deletant strains. Furthermore, microarray experiments revealed that the whole yeast transcriptional response to Zymolyase was mediated by Sho1p. All these results revealed a new level of crosstalk between the high osmolarity response pathway and the cell integrity pathway in the control of cell wall construction.

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