In response to specific stresses, the budding yeast Saccharomyces cerevisiae undergoes a morphogenetic program wherein cells elongate and interconnect, forming pseudohyphal filaments. This filamentous growth transition has been studied extensively as a model signaling system with relevance to fungal pathogenicity. Classic studies have identified core pseudohyphal growth signaling modules in yeast; however, the scope of regulatory networks that control yeast filamentation is broad and incompletely defined. In this work, we address the genetic basis of yeast pseudohyphal growth by implementing a systematic analysis of 4909 genes for overexpression phenotypes in a filamentous strain of S. cerevisiae. Our results identify 551 genes conferring exaggerated invasive growth upon overexpression under normal vegetative growth conditions. In particular, overexpression screening suggests that nuclear export of the osmoresponsive MAPK Hog1p may enhance pseudohyphal growth. The function of nuclear Hog1p is unclear from previous studies, but our analysis using a nuclear-depleted form of Hog1p is consistent with a role for nuclear Hog1p in repressing pseudohyphal growth. In a second study, we interrogate the kinase signaling network regulating filamentous growth using a quantitative phosphoproteomic approach. The filamentous growth transition is controlled by at least three kinase signaling pathways; however, the global scope of filamentous growth kinase signaling networks is not presently understood. We engineered kinase-dead mutations in a core set of eight regulatory protein kinases and identified differentially phosphorylated proteins relative to wild type by SILAC-based mass spectrometry.Our analysis reveals 752 significantly differentially phosphorylated phosphopeptides, including many that are previously unsurveyed in any yeast strain.From this set of significantly differentially abundant phosphopeptides, we identify novel functional regulatory phosphorylation events crucial for proper filamentation.This collective phosphoproteomic data also reveals novel contributions of two cellular processes during filamentous growth: First, genetic analysis suggests that the components of a translationally repressive mRNA decay complex regulate MAPK signaling downstream of the MAPKKK Stellp. Second, null mutants of the inositol kinase pathway genes indicate an unexpected regulatory role for soluble inositol polyphosphates in the regulation of filamentous growth.In sum, this collective work more completely defines the genomic complement and kinase signaling networks contributing to this model stress response.
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