The Ras/cAMP Pathway and the CDK-Like Kinase Ime2 Regulate the MAPK Smk1 and Spore Morphogenesis in Saccharomyces cerevisiae

Abstract

Meiotic development (sporulation) in the yeast Saccharomyces cerevisiae is induced by nutritional deprivation. Smk1 is a meiosis-specific MAP kinase homolog that controls spore morphogenesis after the meiotic divisions have taken place. In this study, recessive mutants that suppress the sporulation defect of a smk1-2 temperature-sensitive hypomorph were isolated. The suppressors are partial function alleles of CDC25 and CYR1, which encode the Ras GDP/GTP exchange factor and adenyl cyclase, respectively, and MDS3, which encodes a kelch-domain protein previously implicated in Ras/cAMP signaling. Deletion of PMD1, which encodes a Mds3 paralog, also suppressed the smk1-2 phenotype, and a mds3-D pmd1-D double mutant was a more potent suppressor than either single mutant. The mds3-D, pmd1-D, and mds3-D pmd1-D mutants also exhibited mitotic Ras/cAMP phenotypes in the same rank order. The effect of Ras/cAMP pathway mutations on the smk1-2 phenotype required the presence of low levels of glucose. Ime2 is a meiosis-specific CDK-like kinase that is inhibited by low levels of glucose via its carboxy-terminal regulatory domain. IME2-DC241, which removes the carboxy-terminal domain of Ime2, exacerbated the smk1-2 spore formation phenotype and prevented cyr1 mutations from suppressing smk1-2. Inhibition of Ime2 in meiotic cells shortly after Smk1 is expressed revealed that Ime2 promotes phosphorylation of Smk1's activation loop. These findings demonstrate that nutrients can negatively regulate Smk1 through the Ras/cAMP pathway and that Ime2 is a key activator of Smk1 signaling.

Acetate Regulation of Spore Formation Is under the Control of the Ras/Cyclic AMP/Protein Kinase A Pathway and Carbon Dioxide in Saccharomyces cerevisiae

“…Before commitment, a high level of PKA activity leads to the abandonment of meiosis and return to growth (53). However, measurements of cAMP levels during sporulation and mutational analyses of Ras/cAMP/PKA pathway components (33,61,62,64) have suggested that PKA is active during a longer period after meiotic entry. Moreover, a negative-feedback loop relying on the expression of BCY1 prevents very high levels of activity of PKA after commitment to meiosis (14).…”

“…Indeed, the expression of glucose-dependent PKA targets is not upregulated under these conditions (14). The last step in spore biogenesis, the synthesis of mature spore walls, has been found to be influenced by PKA activity via the activation of the meiosis-specific mitogen-activated protein (MAP) kinase homolog Smk1 (33).…”

Evaluation of uridine 5’-eicosylphosphate as a stimulant of cyclic AMP-dependent cellular function

“…The cellular cAMP level is regulated by phosphodiesterase specific for its degradation in addition to adenylyl cyclase specific for its production (Sunahara and Taussig 2002;Houslay and Adams 2003). This cyclic nucleotide also participates in the control of cell cycle progression in the yeast Saccharomyces cerevisiae, as judged by its inhibitory activity on ascospore formation in response to nutrient starvation (McDonald et al 2009). It seems possible to find a substance that can elevate the intracellular cAMP level by applying simple microscopic observation of the yeast cell differentiation process.…”

Activation of the Smk1 Mitogen-Activated Protein Kinase by Developmentally Regulated Autophosphorylation

“…Although there are many possible explanations for these negative data, they do raise the possibility that additional regulatory features that are present only in meiotic cells promote the SSP2-dependent phosphorylation of Smk1. AMA1 and IME2 have been shown to promote Smk1 activation (20,37), and the role of these meiosis-specific genes in controlling the SSP2-dependent activation of Smk1 requires further investigation.…”

“…AMA1, which encodes a meiosis-specific activator of the anaphase-promoting complex E3 ubiquitin-ligase (32)(33)(34)(35)(36), was also identified in the deletion mutant screen. In addition, Ime2 has been shown to increase the fraction of Smk1 that is phosphorylated (37).…”