An Unexpected Link Between Fig4 and the Cell Wall Integrity Pathway

Fig4 is a phosphoinositide lipid phosphatase that converts its lipid substrate PI3,5P2 to PI3P in response to osmotic shock. Fig4 is also responsible for regulating levels of its own substrate by activating its opposing kinase Fab1. Fig4 performs these functions through association with the Fig4-Fab1-Vac14 complex through direct binding to the scaffolding protein Vac14. Specific mutations in human Fig4 are associated with neurodegenerative diseases that have been attributed to a dysregulation of PI3,5P2 due to impaired function of the Fig4-Fab1- Vac14 complex. We have shown that Fig4 disease related mutations that are impaired in association with the Fig4-Fab1-Vac14 complex confer tolerance to rapamycin at elevated temperatures in Saccharomyces cerevisiae, independent of Fig4 catalytic activity. This phenotype is conferred not just by mutant Fig4 but also by the addition of multiple copies of wild type Fig4. We hypothesize that Fig4 that is dissociated from Fab1 and Vac14 confers this phenotype through context-inappropriate association with a protein factor due to its increased availability outside of the complex. We previously identified the p21-activated kinase Ste20 as a candidate for this protein factor mediating tolerance to rapamycin as it was shown to be necessary for this phenotype at 37℃. Surprisingly, Ste20 is not required for rapamycin tolerance all temperatures, suggesting that it is not essential for this mechanism but is required to observe it in some contexts. This thesis shows that Ste20 kinase activity supports this novel Fig4 function as a critical activator for a non-canonical mitogen activated protein kinase pathway target, the cell wall integrity pathway, at high temperatures.