Systematic Epistasis Analysis of the Contributions of Protein Kinase A- and Mitogen-Activated Protein Kinase-Dependent Signaling to Nutrient Limitation-Evoked Responses in the Yeast Saccharomyces cerevisiae

Abstract

Cellular responses to environmental stimuli require conserved signal transduction pathways. In budding yeast (Saccharomyces cerevisiae), nutrient limitation induces morphological changes that depend on the protein kinase A (PKA) pathway and the Kss1 mitogen-activated protein kinase (MAPK) pathway. It was unclear to what extent and at what level there is synergy between these two distinct signaling modalities. We took a systematic genetic approach to clarify the relationship between these inputs. We performed comprehensive epistasis analysis of mutants lacking different combinations of all relevant pathway components. We found that these two pathways contribute additively to nutrient limitationinduced haploid invasive growth. Moreover, full derepression of either pathway rendered it individually sufficient for invasive growth and thus, normally, both are required only because neither is maximally active. Furthermore, in haploids, the MAPK pathway contributes more strongly than the PKA pathway to cell elongation and adhesion, whereas nutrient limitation-induced unipolar budding is independent of both pathways. In contrast, in diploids, upon nutrient limitation the MAPK pathway regulates cell elongation, the PKA pathway regulates unipolar budding, and both regulate cell adhesion. Thus, although there are similarities between haploids and diploids, cell type-specific differences clearly alter the balance of the signaling inputs required to elicit the various nutrient limitation-evoked cellular behaviors.

The Regulation of Filamentous Growth in Yeast

“…It was initially reported that haploid cells undergo invasive growth better than diploid cells (Roberts and Fink 1994), although we found the opposite to be true (Cullen and Sprague 2002). Nevertheless, the expression of filamentation target genes is regulated by different stimuli in haploid compared to diploid cells (Lo and Dranginis 1998), and regulatory pathways Ras2/PKA and MAPK (discussed below) have different roles in regulating the response in haploid and diploid cells (Chen and Thorner 2010).…”

“…Rupp and colleagues showed that the MAPK-dependent transcription factors Ste12 and Tec1, and the RAS/cAMP-PKAdependent transcription factor Flo8 each binds to the FLO11 promoter (Rupp et al 1999). Chen and Thorner (2010) followed up on this study by showing that the two pathways contribute additively to the filamentation response. When maximally activated, either pathway can fully induce filamentous growth, which suggests that, normally, both pathways are required because neither pathway is maximally active.…”

Global Regulation of a Differentiation MAPK Pathway in Yeast

“…Plates were photographed (top) and washed in a stream of water to reveal invaded cells (bottom). Examples response (Chen and Thorner 2010). To clarify this issue, the budding pattern of filamentous cells was examined by two approaches.…”

“…The ste12D mutant showed a 15% reduction in distal-unipolar budding, and the ste20D mutant showed a 20% reduction ( Figure 2A). Many cells retained distal-pole budding (60%), which can account for the conclusion that the filamentous growth MAPK pathway is dispensable (Chen and Thorner 2010). Therefore, the filamentous growth MAPK pathway regulates the change in polarity during filamentous growth.…”

Cdk8 Regulates Stability of the Transcription Factor Phd1 To Control Pseudohyphal Differentiation of Saccharomyces cerevisiae

“…These differentiation processes, known as pseudohyphal and haploid invasive growth, or collectively as filamentous growth, have similar genetic requirements, and the presence of different colony morphologies between haploids and diploids is primarily related to their distinct budding patterns (36). Differentiation is regulated by a set of sequence-specific DNA binding factors, including Ste12, Tec1, Flo8, Phd1, Mga1, and Sok2 (3,29), among others, whose activities are responsive to signals transmitted by the pheromone response Kss1-mitogen-activated protein kinase (MAPK) (2, 9), RAS-cyclic AMP (cAMP)-protein kinase A (PKA) (5,37), and Snf1-AMP-dependent protein kinase (AMPK) (24) signaling pathways for activation of genes required to drive differentiation to a filamentous morphology, known as filamentous response genes. Global genomic localization indicates a complex network of interactions among these factors that include multiple autofeedback and cross-regulatory circuits.…”