Yeast putative transcription factors involved in salt tolerance

The Sko1p repressor and Gcn4p activator antagonistically modulate stress-regulated transcription in Saccharomyces cerevisiae

In the transcriptional response of Saccharomyces cerevisiae to stress, both activators and repressors are implicated. Here we demonstrate that the ion homeostasis determinant, HAL1, is regulated by two antagonistically operating bZIP transcription factors, the Sko1p repressor and the Gcn4p activator. A single CRE-like sequence (CRE(HAL1)) at position -222 to -215 with the palindromic core sequence TTACGTAA is essential for stress-induced expression of HAL1. Down-regulation of HAL1 under normal growth conditions requires specific binding of Sko1p to CRE(HAL1) and the corepressor gene SSN6. Release from this repression depends on the function of the high-osmolarity glycerol pathway. The Gcn4p transcriptional activator binds in vitro to the same CRE(HAL1) and is necessary for up-regulated HAL1 expression in vivo, indicating a dual control mechanism by a repressor-activator pair occupying the same promoter target sequence. gcn4 mutants display a strong sensitivity to elevated K(+) or Na(+) concentrations in the growth medium. In addition to reduced HAL1 expression, this sensitivity is explained by the fact that amino acid uptake is drastically impaired by high Na(+) and K(+) concentrations in wild-type yeast cells. The reduced amino acid biosynthesis of gcn4 mutants would result in amino acid deprivation. Together with the induction of HAL1 by amino acid starvation, these results suggest that salt stress and amino acid availability are physiologically interconnected.

A conserved family of calcineurin regulators

The protein phosphatase calcineurin mediates many cellular responses to calcium signals. Using a genetic screen in yeast, we identified a new family of proteins conserved in fungi and animals that inhibit calcineurin function when overexpressed. Overexpression of the yeast protein Rcn1p or the human homologs DSCR1 or ZAKI-4 inhibited two independent functions of calcineurin in yeast: The activation of the transcription factor Tcn1p and the inhibition of the H+/Ca2+ exchanger Vcx1p. Purified recombinant Rcn1p and DSCR1 bound calcineurin in vitro and inhibited its protein phosphatase activity. Signaling via calmodulin, calcineurin, and Tcn1p induced Rcn1p expression, suggesting that Rcn1p operates as an endogenous feedback inhibitor of calcineurin. Surprisingly, rcn1 null mutants exhibited phenotypes similar to those of Rcn1p-overexpressing cells. This effect may be due to lower expression of calcineurin in rcn1 mutants during signaling conditions. Thus, Rcn1p levels may fine-tune calcineurin signaling in yeast. The structural and functional conservation between Rcn1p and DSCR1 suggests that the mammalian Rcn1p-related proteins, termed calcipressins, will modulate calcineurin signaling in humans and potentially contribute to disorders such as Down Syndrome.

The transcriptional response of yeast to saline stress

Adaptation to changes in extracellular salinity is a critical event for cell survival. Genome-wide DNA chip analysis has been used to analyze the transcriptional response of yeast cells to saline stress. About 7% of the genes encoded in the yeast genome are induced more than 5-fold after a mild and brief saline shock (0.4 m NaCl, 10 min). Interestingly, most responsive genes showed a very transient expression pattern, as mRNA levels dramatically declined after 20 min in the presence of stress. A quite similar set of genes increased expression in cells subjected to higher saline concentrations (0.8 m NaCl), although in this case the response was delayed. Therefore, our data show that cells respond to saline stress by inducing the expression of a very large number of genes and suggest that stress adaptation requires regulation of many cellular aspects. The transcriptional induction of most genes that are strongly responsive to salt stress was highly or fully dependent on the presence of the stress-activated mitogen-activated protein kinase Hog1, indicating that the Hog1-mediated signaling pathway plays a key role in global gene regulation under saline stress conditions.

Aca1 and Aca2, ATF/CREB activators in Saccharomyces cerevisiae, are important for carbon source utilization but not the response to stress

In Saccharomyces cerevisiae, the family of ATF/CREB transcriptional regulators consists of a repressor, Acr1 (Sko1), and two activators, Aca1 and Aca2. The AP-1 factor Gen4 does not activate transcription through ATF/CREB sites in vivo even though it binds these sites in vitro. Unlike ATF/CREB activators in other species, Aca1- and Aca2-dependent transcription is not affected by protein kinase A or by stress, and Aca1 and Aca2 are not required for Hog1-dependent salt induction of transcription through an optimal ATF/CREB site. Aca2 is important for a variety of biological functions including growth on nonoptimal carbon sources, and Aca2-dependent activation is modestly regulated by carbon source. Strains lacking Aca1 are phenotypically normal, but overexpression of Aca1 suppresses some defects associated with the loss of Aca2, indicating a functional overlap between Aca1 and Aca2. Acr1 represses transcription both by recruiting the Cyc8-Tup1 corepressor and by directly competing with Aca1 and Aca2 for target sites. Acr1 does not fully account for osmotic regulation through ATF/CREB sites, and a novel Hog1-dependent activator(s) that is not a bZIP protein is required for ATF/CREB site activation in response to high salt. In addition, Acr1 does not affect a number of phenotypes that arise from loss of Aca2. Thus, members of the S. cerevisiae ATF/CREB family have overlapping, but distinct, biological functions and target genes.

Novel roles for elongin C in yeast

Mammalian Elongin C is a 112-amino acid protein that binds to the von Hippel-Lindau (VHL) tumor suppressor and to Elongin A, the transcriptionally active subunit of the RNA polymerase II elongation factor, SIII. It is conserved in eukaryotic cells, as homologs have been identified in Saccharomyces cerevisiae, Drosophila melanogaster and Caenorhabditis elegans. The mammalian protein is thought to function as part of a ubiquitin targeting E3 ligase, yet the function in yeast has not been determined. In this report we examine the role of Elongin C in yeast and establish that yeast Elongin C may function in a mode distinct from its role as an E3 ligase. The RNA is expressed ubiquitously, albeit at low levels. Two hybrid analyses demonstrate that Elongin C in yeast interacts with a specific set of proteins that are involved in the stress response. This suggests a novel role for Elongin C and provides insights into additional potential functions in mammalian cells.

Biochemical and genetic analyses of the role of yeast casein kinase 2 in salt tolerance

Saccharomyces cerevisiae cells lacking the regulatory subunit of casein kinase 2 (CK-2), encoded by the gene CKB1, display a phenotype of hypersensitivity to Na(+) and Li(+) cations. The sensitivity of a strain lacking ckb1 is higher than that of a calcineurin mutant and similar to that of a strain lacking HAL3, the regulatory subunit of the Ppz1 protein phosphatase. Genetic analysis indicated that Ckb1 participates in regulatory pathways different from that of Ppz1 or calcineurin. Deletion of CKB1 increased the salt sensitivity of a strain lacking Ena1 ATPase, the major determinant for sodium efflux, suggesting that the function of the kinase is not mediated by Ena1. Consistently, ckb1 mutants did not show an altered cation efflux. The function of Ckb1 was independent of the TRK system, which is responsible for discrimination of potassium and sodium entry, and in the absence of the kinase regulatory subunit, the influx of sodium was essentially normal. Therefore, the salt sensitivity of a ckb1 mutant cannot be attributed to defects in the fluxes of sodium. In fact, in these cells, both the intracellular content and the cytoplasm/vacuole ratio for sodium were similar to those features of wild-type cells. The possible causes for the salt sensitivity phenotype of casein kinase mutants are discussed in the light of these findings.

A novel mechanism of ion homeostasis and salt tolerance in yeast: the Hal4 and Hal5 protein kinases modulate the Trk1-Trk2 potassium transporter

The regulation of intracellular ion concentrations is a fundamental property of living cells. Although many ion transporters have been identified, the systems that modulate their activity remain largely unknown. We have characterized two partially redundant genes from Saccharomyces cerevisiae, HAL4/SAT4 and HAL5, that encode homologous protein kinases implicated in the regulation of cation uptake. Overexpression of these genes increases the tolerance of yeast cells to sodium and lithium, whereas gene disruptions result in greater cation sensitivity. These phenotypic effects of the mutations correlate with changes in cation uptake and are dependent on a functional Trk1-Trk2 potassium transport system. In addition, hal4 hal5 and trk1 trk2 mutants exhibit similar phenotypes: (i) they are deficient in potassium uptake; (ii) their growth is sensitive to a variety of toxic cations, including lithium, sodium, calcium, tetramethylammonium, hygromycin B, and low pH; and (iii) they exhibit increased uptake of methylammonium, an indicator of membrane potential. These results suggest that the Hal4 and Hal5 protein kinases activate the Trk1-Trk2 potassium transporter, increasing the influx of potassium and decreasing the membrane potential. The resulting loss in electrical driving force reduces the uptake of toxic cations and improves salt tolerance. Our data support a role for regulation of membrane potential in adaptation to salt stress that is mediated by the Hal4 and Hal5 kinases.

AP-1 transcription factors in yeast

In the past two years, the completion of the Saccharomyces cerevisiae genome project and molecular analysis of other fungal species has resulted in the identification of a growing number of yeast AP-1 transcription factors. Characterisation of these factors indicates that, like their mammalian counterparts, they activate gene expression in response to a variety of extracellular stimuli. In particular, these factors are required for the response to oxidative stress and for surviving exposure to a variety of cytotoxic agents. Much progress has also been made in understanding how members of this family of proteins are regulated. These studies promise to further our awareness of eukaryotic stress responses and are likely to have implications for the study of mammalian AP-1.

Repressors and upstream repressing sequences of the stress-regulated ENA1 gene in Saccharomyces cerevisiae: bZIP protein Sko1p confers HOG-dependent osmotic regulation

The yeast ENA1/PMR2A gene encodes a cation extrusion ATPase in Saccharomyces cerevisiae which is essential for survival under salt stress conditions. One important mechanism of ENA1 transcriptional regulation is based on repression under normal growth conditions, which is relieved by either osmotic induction or glucose starvation. Analysis of the ENA1 promoter revealed a Mig1p-binding motif (-533 to -544) which was characterized as an upstream repressing sequence (URSMIG-ENA1) regulated by carbon source. Its function was abolished in a mig1 mig2 double-deletion strain as well as in either ssn6 or tup1 single mutants. A second URS at -502 to -513 is responsible for transcriptional repression regulated by osmotic stress and is similar to mammalian cyclic AMP response elements (CREs) that are recognized by CREB proteins. This URSCRE-ENA1 element requires for its repression function the yeast CREB homolog Sko1p (Acr1p) as well as the integrity of the Ssn6p-Tup1p corepressor complex. When targeted to the GAL1 promoter by fusing with the Gal4p DNA-binding domain, Sko1p acts as an Ssn6/Tup1p-dependent repressor regulated by osmotic stress. A glutathione S-transferase-Sko1 fusion protein binds specifically to the URSCRE-ENA1 element. Furthermore, a hog1 mitogen-activated protein kinase deletion strain could not counteract repression on URSCRE-ENA1 during osmotic shock. The loss of SKO1 completely restored ENA1 expression in a hog1 mutant and partially suppressed the osmotic stress sensitivity, qualifying Sko1p as a downstream effector of the HOG pathway. Our results indicate that different signalling pathways (HOG osmotic pathway and glucose repression pathway) use distinct promoter elements of ENA1 (URSCRE-ENA1 and URSMIG-ENA1) via specific transcriptional repressors (Sko1p and Mig1/2p) and via the general Ssn6p-Tup1p complex. The physiological importance of the relief from repression during salt stress was also demonstrated by the increased tolerance of sko1 or ssn6 mutants to Na+ or Li+ stress.