Genetic evidence for the functional redundancy of the calcineurin- and Mpk1-mediated pathways in the regulation of cellular events important for growth in Saccharomyces cerevisiae

Clausmarin A, Potential Immunosuppressant Revealed by Yeast-Based Assay and Interleukin-2 Production Assay in Jurkat T Cells

Small-molecule inhibitors of Ca2+-signaling pathways are of medicinal importance, as exemplified by the immunosuppressants FK506 and cyclosporin A. Using a yeast-based assay devised for the specific detection of Ca2+-signaling inhibitors, clausmarin A, a previously reported terpenoid coumarin, was identified as an active substance. Here, we investigated the likely mechanism of clausmarin A action in yeast and Jurkat T-cells. In the presence of 100 mM CaCl2 in the growth medium of Ca2+-sensitive Δzds1 strain yeast, clausmarin A exhibited a dose-dependent alleviation of various defects due to hyperactivation of Ca2+ signaling, such as growth inhibition, polarized bud growth and G2 phase cell-cycle arrest. Furthermore, clausmarin A inhibited the growth of Δmpk1 (lacking the Mpk1 MAP kinase pathway) but not Δcnb1 (lacking the calcineurin pathway) strain, suggesting that clausmarin A inhibited the calcineurin pathway as presumed from the synthetic lethality of these pathways. Furthermore, clausmarin A alleviated the serious defects of a strain expressing a constitutively active form of calcineurin. In the human Jurkat T-cell line, clausmarin A exhibited a dose-dependent inhibition of IL-2 production and IL-2 gene transcription, as well as an inhibition of NFAT dephosphorylation. The effects of clausmarin A observed in both yeast and Jurkat cells are basically similar to those of FK506. Our study revealed that clausmarin A is an inhibitor of the calcineurin pathway, and that this is probably mediated via inhibition of calcineurin phosphatase activity. As such, clausmarin A is a potential immunosuppressant.

Methylglyoxal activates the target of rapamycin complex 2-protein kinase C signaling pathway in Saccharomyces cerevisiae

Methylglyoxal is a typical 2-oxoaldehyde derived from glycolysis. We show here that methylglyoxal activates the Pkc1-Mpk1 mitogen-activated protein (MAP) kinase cascade in a target of rapamycin complex 2 (TORC2)-dependent manner in the budding yeast Saccharomyces cerevisiae. We demonstrate that TORC2 phosphorylates Pkc1 at Thr(1125) and Ser(1143). Methylglyoxal enhanced the phosphorylation of Pkc1 at Ser(1143), which transmitted the signal to the downstream Mpk1 MAP kinase cascade. We found that the phosphorylation status of Pkc1(T1125) affected the phosphorylation of Pkc1 at Ser(1143), in addition to its protein levels. Methylglyoxal activated mammalian TORC2 signaling, which, in turn, phosphorylated Akt at Ser(473). Our results suggest that methylglyoxal is a conserved initiator of TORC2 signaling among eukaryotes.

Genome-wide analysis of sterol-lipid storage and trafficking in Saccharomyces cerevisiae

The pandemic of lipid-related disease necessitates a determination of how cholesterol and other lipids are transported and stored within cells. The first step in this determination is the identification of the genes involved in these transport and storage processes. Using genome-wide screens, we identified 56 yeast (Saccharomyces cerevisiae) genes involved in sterol-lipid biosynthesis, intracellular trafficking, and/or neutral-lipid storage. Direct biochemical and cytological examination of mutant cells revealed an unanticipated link between secretory protein glycosylation and triacylglycerol (TAG)/steryl ester (SE) synthesis for the storage of lipids. Together with the analysis of other deletion mutants, these results suggested at least two distinct events for the biogenesis of lipid storage particles: a step affecting neutral-lipid synthesis, generating the lipid core of storage particles, and another step for particle assembly. In addition to the lipid storage mutants, we identified mutations that affect the localization of unesterified sterols, which are normally concentrated in the plasma membrane. These findings implicated phospholipase C and the protein phosphatase Ptc1p in the regulation of sterol distribution within cells. This study identified novel sterol-related genes that define several distinct processes maintaining sterol homeostasis.

Attenuation of the activity of caspofungin at high concentrations against candida albicans: possible role of cell wall integrity and calcineurin pathways

Caspofungin had diminished activity in vitro against Candida albicans at concentrations of 8 to 32 microg/ml. This phenomenon was markedly attenuated in a deltamkc1/deltamkc1 deletion mutant and by the addition of cyclosporine to the wild type. Short exposure to these caspofungin concentrations resulted in MKC1 up-regulation, suggesting roles of cell wall integrity and calcineurin pathways.

Phospholipid-binding protein Cts1 controls septation and functions coordinately with calcineurin in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that causes life-threatening meningoencephalitis in immunocompromised patients. The Ca(2+)-calmodulin-activated protein phosphatase calcineurin is necessary for virulence of C. neoformans. Mutants lacking the calcineurin catalytic (Cna1) or regulatory (Cnb1) subunit fail to grow at elevated temperature and are defective in virulence and hyphal elongation. Here we isolated a multicopy suppressor gene, CTS1, which restores growth of a calcineurin mutant strain at 37 degrees C. The CTS1 gene (for calcineurin temperature suppressor 1) encodes a protein containing a C2 domain and a leucine zipper motif that may function as an effector of calcineurin. The CTS1 gene was disrupted by homologous recombination, and cts1 mutants were viable but exhibited defects in cell separation, growth, mating, and haploid fruiting. In addition, cts1 mutants were inviable when calcineurin was mutated or inhibited. Taken together, these findings suggest that calcineurin and Cts1 function in parallel pathways that regulate growth, cell separation, and hyphal elongation.

Calmodulin controls organization of the actin cytoskeleton via regulation of phosphatidylinositol (4,5)-bisphosphate synthesis in Saccharomyces cerevisiae

Phosphoinositides regulate a wide range of cellular processes, including proliferation, survival, cytoskeleton remodelling and membrane trafficking, yet the mechanisms controlling the kinases, phosphatases and lipases that modulate phosphoinositide levels are poorly understood. In the present study, we describe a mechanism controlling MSS4, the sole phosphatidylinositol (4)-phosphate 5-kinase in Saccharomyces cerevisiae. Mutations in MSS4 and CMD1, encoding the small Ca(2+)-binding protein calmodulin, confer similar phenotypes, including loss of viability and defects in endocytosis and in organization of the actin cytoskeleton. Overexpression of MSS4 suppresses the growth and actin defects of cmd1-226, a temperature-sensitive calmodulin mutant which is defective in the organization of the actin cytoskeleton. Finally, the cmd1-226 mutant exhibits reduced levels of phosphatidylinositol (4,5)-bisphosphate. These findings suggest that calmodulin positively controls MSS4 activity and thereby the actin cytoskeleton.

GSK-3 kinase Mck1 and calcineurin coordinately mediate Hsl1 down-regulation by Ca2+ in budding yeast

The Ca2+-activated pathways of Saccharomyces cerevisiae induce a delay in the onset of mitosis through the activation of Swe1, a negative regulatory kinase that inhibits the Cdc28-Clb complex. Calcineurin and Mpk1 activate Swe1 at the transcriptional and post-translational level, respectively, and both pathways are essential for the cell cycle delay. Our genetic screening identified the MCK1 gene, which encodes a glycogen synthetase kinase-3 family protein kinase, as a component of the Ca2+ signaling pathway. Genetic analyses indicated that Mck1 functions downstream of the Mpk1 pathway and down-regulates Hsl1, an inhibitory kinase of Swe1. In medium with a high concentration of Ca2+, Hsl1 was delocalized from the bud neck and destabilized in a manner dependent on both calcineurin and Mck1. Calcineurin was required for the dephosphorylation of autophosphorylated Hsl1. The E3 ubiquitin ligase complex SCF(Cdc4), but not the anaphase-promoting complex (APC), was essential for Hsl1 destabilization. The Ca2+-activated pathway may play a role in the rapid inactivation of Hsl1 at the cell cycle stage(s) when APC activity is low.

Tcn1p/Crz1p, a calcineurin-dependent transcription factor that differentially regulates gene expression in Saccharomyces cerevisiae

Ca2+ signals regulate gene expression in animal and yeast cells through mechanisms involving calcineurin, a protein phosphatase activated by binding Ca2+ and calmodulin. Tcn1p, also named Crz1p, was identified as a transcription factor in yeast required for the calcineurin-dependent induction of PMC1, PMR1, PMR2A, and FKS2 which confer tolerance to high Ca2+, Mn2+, Na+, and cell wall damage, respectively. Tcn1p was not required for other calcineurin-dependent processes, such as inhibition of a vacuolar H+/Ca2+ exchanger and inhibition of a pheromone-stimulated Ca2+ uptake system, suggesting that Tcn1p functions downstream of calcineurin on a branch of the calcium signaling pathway leading to gene expression. Tcn1p contains three zinc finger motifs at its carboxyl terminus resembling the DNA-binding domains of Zif268, Swi5p, and other transcription factors. When fused to the transcription activation domain of Gal4p, the carboxy terminal domain of Tcn1p directed strong calcineurin-independent expression of PMC1-lacZ and other target genes. The amino-terminal domain of Tcn1p was found to function as a calcineurin-dependent transcription activation domain when fused to the DNA-binding domain of Gal4p. This amino-terminal domain also formed Ca2+-dependent and FK506-sensitive interactions with calcineurin in the yeast two-hybrid assay. These findings suggest that Tcn1p functions as a calcineurin-dependent transcription factor. Interestingly, induction of Tcn1p-dependent genes was found to be differentially controlled in response to physiological Ca2+ signals generated by treatment with mating pheromone and high salt. We propose that different promoters are sensitive to variations in the strength of Ca2+ signals generated by these stimuli and to effects of other signaling pathways.

Characterization of a serum response factor-like protein in Saccharomyces cerevisiae, Rlm1, which has transcriptional activity regulated by the Mpk1 (Slt2) mitogen-activated protein kinase pathway

The Mpk1 (Slt2) mitogen-activated protein (MAP) kinase has been implicated in several biological processes in Saccharomyces cerevisiae. The Rlm1 protein, a member of the MADS box family of transcription factors, functions downstream of Mpk1 in the pathway. To characterize the role of Rlm1 in mediating the transcriptional activation by the Mpk1 pathway, we constructed a LexA-Rlm1 deltaN chimera in which sequences, including the MADS box domain of the Rlm1 protein, were replaced by the LexA DNA binding domain and tested the ability of this chimera to activate a LexA operator-controlled reporter gene. In this assay, the Rlm1 protein was found to activate transcription in a manner regulated by the Mpk1 pathway. The Mpk1 protein kinase phosphorylated Rlm1 deltaN in vitro and the LexA-Rlm1 deltaN chimera protein was phosphorylated in vivo in a Mpk1-dependent manner. These results suggest that Mpk1 regulates the transcriptional activity of Rlm1 by directly phosphorylating it. We identified a Mpk1-like protein kinase, Mlp1, as an Rlm1-associated protein by using the yeast two-hybrid system. Overexpression of MLP1 suppresses the caffeine-sensitive phenotype of the bck1 delta mutation. The additivity of the mlp1 delta defect with the Mpk1 delta defect with regard to the caffeine sensitivity, combined with the results of genetic epistasis experiments, suggested that the activity of Rlm1 is regulated independently by Mpk1 MAP kinase and the Mlp1 MAP kinase-like kinase.