COORDINATED ACTIVATION OF THE MAPK NETWORK BY MATING PHEROMONE

RODRIGO BALTANÁS; DAVID LEVIN; STEFAN HOHMANN; ALEJANDRO COLMAN-LERNER

Cold Spring Harbor Laboratory, New York, USA

Congreso; Yeast Cell Biology Meeting; 2009

Cold Spring Harbor Laboratory

In this work we found that the coordinated activation of the mating pheromone and the cell wall integrity MAPK cascades modulates a third MAPK cascade, the “High Osmolarity Glycerol” response in S. cerevisiae. We studied these pathways at the single cell level using quantitative cytometry of transcriptional- and relocalization-based fluorescent reporters; and, at the population level, using quantitative western blots. The HOG pathway has always been thought of as a transient, homeostatic response to osmoshock. Here we found that in yeast adapted to grow in high osmolarity (e.g, 1M sorbitol or 0.5M NaCl), activation of the cell wall integrity pathway (CWIP) MAPK Mpk1/Slt2 leads to a persistent activation HOG pathway. We observed this behavior activating the CWIP with several different stimuli, including mating pheromone, butanol induced filamentation and high temperature shock. The activation of the HOG pathway is seen at the level of phosphorylation of Hog1, nuclear translocation of Hog1-YFP, and also at a transcriptional level using PSTL1-YFP. When stimulating with pheromone, Fus3, Kss1, Mpk1 and Hog1 MAPKs are activated, and the output of the HOG response follows mating response dynamics, which is stable over time. This suggests that HOG activation is part of an integrated mating response and not a transient response to stress. Pheromone-induced activation of HOG employs the CWI pathway sensor Mtl1, and essential components of this cascade, Pkc1, Bck1 and the MAPK Slt2/Mpk1, but not the transcription factor Rlm1. Our data suggests a model in which the CWIP activates the HOG pathway indirectly, by opening the glyceroporin Fps1, leading to glycerol loss, and subsequent reduction of turgor pressure, leading to activation of the Sln1 branch of the HOG pathway. This hypothesis is supported by a) the observation that cells adapted to 1M glycerol (instead of 1M sorbitol) do not activate the HOG pathway after α factor treatment; and b) Δfps1 and Δssk1 cells fail to activate the HOG pathway. Finally, we found that the correct and coordinated activation of the MAPK network is biologically significant, since its disruption leads to reduced mating efficiency in cells adapted to grow in high osmolarity.