Relationship of the Camp-Dependent Protein Kinase Pathway to the Snf1 Protein Kinase and Invertase Expression in Saccharomyces Cerevisiae

The SNF1 protein kinase and the associated SNF4 protein are required for release of glucose repression in Saccharomyces cerevisiae. To identify functionally related proteins, we selected genes that in multicopy suppress the raffinose growth defect of snf4 mutants. Among the nine genes recovered were two genes from the cAMP-dependent protein kinase (cAPK) pathway, MSI1 and PDE2. Increased dosage of these genes partially compensates for defects in nutrient utilization and sporulation in snf1 and snf4 null mutants, but does not restore invertase expression. These results suggest that SNF1 and cAPK affect some of the same cellular responses to nutrients. To examine the role of the cAPK pathway in regulation of invertase, we assayed mutants in which the cAPK is not modulated by cAMP. Expression of invertase was regulated in response to glucose and was dependent on SNF1 function. Thus, a cAMP-responsive cAPK is dispensable for regulation of invertase.     

Calcineurin inhibits VCX1-dependent H+/Ca2+ exchange and induces Ca2+ ATPases in Saccharomyces cerevisiae.

The PMC1 gene in Saccharomyces cerevisiae encodes a vacuolar Ca2+ ATPase required for growth in high-Ca2+ conditions. Previous work showed that Ca2+ tolerance can be restored to pmc1 mutants by inactivation of calcineurin, a Ca2+/calmodulin-dependent protein phosphatase sensitive to the immunosuppressive drug FK506. We now report that calcineurin decreases Ca2+ tolerance of pmc1 mutants by inhibiting the function of VCX1, which encodes a vacuolar H+/Ca2+ exchanger related to vertebrate Na+/Ca2+ exchangers. The contribution of VCX1 in Ca2+ tolerance is low in strains with a functional calcineurin and is high in strains which lack calcineurin activity. In contrast, the contribution of PMC1 to Ca2+ tolerance is augmented by calcineurin activation. Consistent with these positive and negative roles of calcineurin, expression of a vcx1::lacZ reporter was slightly diminished and a pmc1::lacZ reporter was induced up to 500-fold by processes dependent on calcineurin, calmodulin, and Ca2+. It is likely that calcineurin inhibits VCX1 function mainly by posttranslational mechanisms. Activities of VCX1 and PMC1 help to control cytosolic free Ca2+ concentrations because their function can decrease pmc1::lacZ induction by calcineurin. Additional studies with reporter genes and mutants indicate that PMR1 and PMR2A, encoding P-type ion pumps required for Mn2+ and Na+ tolerance, may also be induced physiologically in response to high-Mn2+ and -Na+ conditions through calcineurin-dependent mechanisms. In these situations, inhibition of VCX1 function may be important for the production of Ca2+ signals. We propose that elevated cytosolic free Ca2+ concentrations, calmodulin, and calcineurin regulate at least four ion transporters in S. cerevisiae in response to several environmental conditions.     

Calcineurin, the Ca2+/calmodulin-dependent protein phosphatase, is essential in yeast mutants with cell integrity defects and in mutants that lack a functional vacuolar H(+)-ATPase.

Calcineurin is a conserved Ca2+/calmodulin-dependent protein phosphatase that plays a critical role in Ca(2+)-mediated signaling in many cells. Yeast cells lacking functional calcineurin (cna1 cna2 or cnb1 mutants) display growth defects under specific environmental conditions, for example, in the presence of high concentrations of Na+, Li+, Mn2+, or OH- but are indistinguishable from wild-type cells under standard culture conditions. To characterize regulatory pathways that may overlap with calcineurin, we performed a synthetic lethal screen to identify mutants that require calcineurin on standard growth media. The characterization of one such mutant, cnd1-8, is presented. The CND1 gene was cloned, and sequence analysis predicts that it encodes a novel protein 1,876 amino acids in length with multiple membrane-spanning domains. CND1 is identical to the gene identified previously as FKS1, ETG1, and CWH53, cnd1 mutants are sensitive to FK506 and cyclosporin A and exhibit slow growth that is improved by the addition of osmotic stabilizing agents. This osmotic agent-remedial growth defect and microscopic evidence of spontaneous cell lysis in cnd1 cultures suggest that cell integrity is compromised in these mutants. Mutations in the genes for yeast protein kinase C (pkc1) and a MAP kinase (mpk1/slt2) disrupt a Ca(2+)-dependent signaling pathway required to maintain a normal cell wall and cell integrity. We show that pkc1 and mpk1/slt2 growth defects are more severe in the absence of calcineurin function and less severe in the presence of a constitutively active form of calcineurin. These observations suggest that calcineurin and protein kinase C perform independent but physiologically related functions in yeast cells. We show that several mutants that lack a functional vacuolar H(+)-ATPase (vma) require calcineurin for vegetative growth. We discuss possible roles for calcineurin in regulating intracellular ion homeostasis and in maintaining cell integrity.     

Identification and characterization of a highly conserved calcineurin binding protein, CBP1/calcipressin, in Cryptococcus neoformans

Calcineurin is the conserved target of the immunosuppressants cyclosporin A and FK506. Using the yeast two-hybrid system, we identified a novel calcineurin binding protein, CBP1, from the pathogenic fungus Cryptococcus neoformans. We show that CBP1 binds to calcineurin in vitro and in vivo, and FKBP12-FK506 inhibits CBP1 binding to calcineurin. Cryptococcus neoformans cbp1 mutant strains exhibit modest defects in growth under stress conditions and virulence, similar to but less severe than the phenotypes of calcineurin mutants. Saccharomyces cerevisiae mutants lacking the CBP1 homolog RCN1 are, like calcineurin mutants, sensitive to lithium cation stress. CBP1 shares a central peptide sequence motif, SPPxSPP, with related proteins in S.CEREVISIAE:, Schizosaccharomyces pombe, Drosophila melanogaster, Caenorhabditis elegans and humans, and peptides containing this motif altered calcineurin activity in vitro. Interestingly, the human CBP1 homolog DSCR1 is encoded by the Down's syndrome candidate region interval on chromosome 21, is highly expressed in the heart and central nervous system, and may play a role in calcineurin functions in heart development, neurite extension and memory.     

Unique and redundant roles for HOG MAPK pathway components as revealed by whole-genome expression analysis

The Saccharomyces cerevisiae high osmolarity glycerol (HOG) mitogen-activated protein kinase pathway is required for osmoadaptation and contains two branches that activate a mitogen-activated protein kinase (Hog1) via a mitogen-activated protein kinase kinase (Pbs2). We have characterized the roles of common pathway components (Hog1 and Pbs2) and components in the two upstream branches (Ste11, Sho1, and Ssk1) in response to elevated osmolarity by using whole-genome expression profiling. Several new features of the HOG pathway were revealed. First, Hog1 functions during gene induction and repression, cross talk inhibition, and in governing the regulatory period. Second, the phenotypes of pbs2 and hog1 mutants are identical, indicating that the sole role of Pbs2 is to activate Hog1. Third, the existence of genes whose induction is dependent on Hog1 and Pbs2 but not on Ste11 and Ssk1 suggests that there are additional inputs into Pbs2 under our inducing conditions. Fourth, the two upstream pathway branches are not redundant: the Sln1-Ssk1 branch has a much more prominent role than the Sho1-Ste11 branch for activation of Pbs2 by modest osmolarity. Finally, the general stress response pathway and both branches of the HOG pathway all function at high osmolarity. These studies demonstrate that cells respond to increased osmolarity by using different signal transduction machinery under different conditions.     

ARL1 participates with ATC1/LIC4 to regulate responses of yeast cells to ions

ATC1/LIC4, previously identified as a suppressor of the Li(+)-sensitive phenotype of calcineurin mutants, was also identified as a suppressor of the hygromycin B-sensitive phenotype of strains lacking the G protein gene, ARL1. Although loss of ARL1 confers several phenotypes, including sensitivity to hygromycin B and Li(+), reduced influx of K(+), and increased secretion of carboxypeptidase Y (CPY), loss of ATC1 was without effect by these and other measures. However, loss of ATC1 in an arl1 background exacerbated ion sensitivities, although not the CPY phenotype. Moreover, overexpression of ATC1 in an arl1 background partially suppressed ion sensitivities, but not the CPY phenotype. Additionally, expression of ENA1, the Na(+)/Li(+) efflux ATPase, and activated calcineurin, but not normal calcineurin, suppressed the Li(+)-sensitive phenotype of the arl1 atc1 double mutant. These results show ARL1 and ATC1 interact to control intracellular ion levels, but ATC1 has little influence on other functions of ARL1.     

Deletion of Snf1 Affects the Nutrient Response of Yeast and Resembles Mutations Which Activate the Adenylate Cyclase Pathway

We have isolated a snf1/ccr1 mutant of Saccharomyces cerevisiae which loses viability upon starvation and fails to accumulate glycogen in response to abrupt depletion of phosphate or glucose. A snf1 null mutant is sensitive to heat stress and starvation and fails to accumulate glycogen during growth in rich medium. The phenotypes of the snf1 mutants are those commonly associated with an overactivation of the adenylate cyclase pathway. Mutations in adenylate cyclase or RAS2 which decrease the level of cAMP in the cell moderate the snf1 phenotype. In contrast, a mutation in RAS2 (RAS2val19) which increases the level of cAMP or a mutation in the regulatory subunit (BCY1) of cAMP-dependent protein kinase which results in unregulated cAMP-dependent protein kinase activity accentuates the snf1 phenotype. However, the action of SNF1 in the stress response appears at least partly independent of cAMP-dependent protein kinase because a snf1 phenotype is observed in a strain that lacks all three of the genes that encode the catalytic subunits of cAMP-dependent protein kinase. SNF1 therefore acts at least in part through a cAMP-independent pathway.