The product of the geneGEF1ofSaccharomyces cerevisiaetransports Cl−across the plasma membrane

Chloride Channel Family in the Euhalophyte Suaeda altissima (L.) Pall: Cloning of Novel Members SaCLCa2 and SaCLCc2, General Characterization of the Family

CLC family genes, comprising anion channels and anion/H+ antiporters, are widely represented in nearly all prokaryotes and eukaryotes. CLC proteins carry out a plethora of functions at the cellular level. Here the coding sequences of the SaCLCa2 and SaCLCc2 genes, homologous to Arabidopsis thaliana CLCa and CLCc, were cloned from the euhalophyte Suaeda altissima (L.) Pall. Both the genes cloned belong to the CLC family as supported by the presence of the key conserved motifs and glutamates inherent for CLC proteins. SaCLCa2 and SaCLCc2 were heterologously expressed in Saccharomyces cerevisiae GEF1 disrupted strain, Δgef1, where GEF1 encodes the only CLC family protein, the Cl− transporter Gef1p, in undisrupted strains of yeast. The Δgef1 strain is characterized by inability to grow on YPD yeast medium containing Mn2+ ions. Expression of SaCLCa2 in Δgef1 cells growing on this medium did not rescue the growth defect phenotype of the mutant. However, a partial growth restoration occurred when the Δgef1 strain was transformed by SaCLCa2(C544T), the gene encoding protein in which proline, specific for nitrate, was replaced with serine, specific for chloride, in the selectivity filter. Unlike SaCLCa2, expression of SaCLCc2 in Δgef1 resulted in a partial growth restoration under these conditions. Analysis of SaCLCa2 and SaCLCc2 expression in the euhalophyte Suaeda altissima (L.) Pall by quantitative real-time PCR (qRT-PCR) under different growth conditions demonstrated stimulation of SaCLCa2 expression by nitrate and stimulation of SaCLCc2 expression by chloride. The results of yeast complementation assay, the presence of both the “gating” and “proton” glutamates in aa sequences of both the proteins, as well results of the gene expression in euhalophyte Suaeda altissima (L.) Pall suggest that SaCLCa2 and SaCLCc2 function as anion/H+ antiporters with nitrate and chloride specificities, respectively. The general bioinformatic overview of seven CLC genes cloned from euhalophyte Suaeda altissima is given, together with results on their expression in roots and leaves under different levels of salinity.

Molecular Cloning and Characterization of SaCLCd, SaCLCf, and SaCLCg, Novel Proteins of the Chloride Channel Family (CLC) from the Halophyte Suaeda altissima (L.) Pall

Coding sequences of the CLC family genes SaCLCd, SaCLCf, and SaCLCg, the putative orthologs of Arabidopsis thaliana AtCLCd, AtCLCf, and AtCLCg genes, were cloned from the euhalophyte Suaeda altissima (L.) Pall. The key conserved motifs and glutamates inherent in proteins of the CLC family were identified in SaCLCd, SaCLCf, and SaCLCg amino acid sequences. SaCLCd and SaCLCg were characterized by higher homology to eukaryotic (human) CLCs, while SaCLCf was closer to prokaryotic CLCs. Ion specificities of the SaCLC proteins were studied in complementation assays by heterologous expression of the SaCLC genes in the Saccharomyces cerevisiae GEF1 disrupted strain Δgef1. GEF1 encoded the only CLC family protein, the Cl^- transporter Gef1p, in undisrupted strains of this organism. Expression of SaCLCd in Δgef1 cells restored their ability to grow on selective media. The complementation test and the presence of both the "gating" and "proton" conservative glutamates in SaCLCd amino acid sequence and serine specific for Cl^- in its selectivity filter suggest that this protein operates as a Cl^-/H⁺ antiporter. By contrast, expression of SaCLCf and SaCLCg did not complement the growth defect phenotype of Δgef1 cells. The selectivity filters of SaCLCf and SaCLCg also contained serine. However, SaCLCf included only the "gating" glutamate, while SaCLCg contained the "proton" glutamate, suggesting that SaCLCf and SaCLCg proteins act as Cl^- channels. The SaCLCd, SaCLCf, and SaCLCg genes were shown to be expressed in the roots and leaves of S. altissima. In response to addition of NaCl to the growth medium, the relative transcript abundances of all three genes of S. altissima increased in the leaves but did not change significantly in the roots. The increase in expression of SaCLCd, SaCLCf, and SaCLCg in the leaves in response to increasing salinity was in line with Cl^- accumulation in the leaf cells, indicating the possible participation of SaCLCd, SaCLCf, and SaCLCg proteins in Cl^- sequestration in cell organelles. Generally, these results suggest the involvement of SaCLC proteins in the response of S. altissima plants to increasing salinity and possible participation in mechanisms underlying salt tolerance.

Molecular cloning and characterisation of SaCLCa1, a novel protein of the chloride channel (CLC) family from the halophyte Suaeda altissima (L.) Pall

The SaCLCa1 gene, a putative orthologue of AtCLCa, the Arabidopsis thaliana gene encoding a NO₃-/H⁺ antiporter, was cloned from the halophyte Suaeda altissima. It belonged to the CLC family, comprising anionic channels and anion/H⁺ antiporters. SaCLCa1 ion specificity was studied by heterologous expression of this gene in Saccharomyces cerevisiae GEF1 disrupted strain, Δgef1, where GEF1 encoded the only CLC family protein, the Cl- transporter Gef1p, in undisrupted strains of this organism. For comparison, the function of another recently identified S. altissima CLC family gene, SaCLCc1, was also characterised. Expression of SaCLCc1 in Δgef1 cells restored their ability to grow on selective media. This supported the chloride specificity of this transporter. By contrast, expression of SaCLCa1 did not complement the growth defect phenotype of Δgef1 cells. However, growth of the Δgef1 mutant on the selective media was partially restored when it was transformed with SaCLCa1(C562 T), encoding the modified protein SaCLCa1(P188S), in which proline responsible for NO₃- selectivity in selective filter was replaced by serine providing chloride selectivity. Quantitative real-time polymerase chain reactions (qRT-PCR) showed that significant induction of SaCLCa1 occurred in the roots of S. altissima when plants were grown on nitrate-deficient medium, while SaCLCc1 activation was observed in S. altissima leaves of plants grown in increasing Cl- concentrations of nutrient solution. These results suggested that SaCLCa1 and SaCLCc1 function as anionic transporters with nitrate and chloride specificities, respectively.

Vhc1, a novel transporter belonging to the family of electroneutral cation-Cl(-) cotransporters, participates in the regulation of cation content and morphology of Saccharomyces cerevisiae vacuoles

Cation-Cl(-) cotransporters (CCCs) are integral membrane proteins which catalyze the coordinated symport of Cl(-) with Na(+) and/or K(+) ions in plant and mammalian cells. Here we describe the first Saccharomyces cerevisiae CCC protein, encoded by the YBR235w open reading frame. Subcellular localization studies showed that this yeast CCC is targeted to the vacuolar membrane. Deletion of the YBR235w gene in a salt-sensitive strain (lacking the plasma-membrane cation exporters) resulted in an increased sensitivity to high KCl, altered vacuolar morphology control and decreased survival upon hyperosmotic shock. In addition, deletion of the YBR235w gene in a mutant strain deficient in K(+) uptake produced a significant growth advantage over the parental strain under K(+)-limiting conditions, and a hypersensitivity to the exogenous K(+)/H(+) exchanger nigericin. These results strongly suggest that we have identified a novel yeast vacuolar ion transporter mediating a K(+)-Cl(-) cotransport and playing a role in vacuolar osmoregulation. Considering its identified function, we propose to refer to the yeast YBR235w gene as VHC1 (vacuolar protein homologous to CCC family 1).

The metal-coordinated Casiopeína IIIEa induces the petite-like phenotype in Saccharomyces cerevisiae

The Casiopeínas® are mixed chelate copper (II) complexes and promising antineoplastics agents against cancer cells and tumors in vitro and in vivo. However, the action mode of these compounds is poorly characterized. In this work the effect of the antineoplastic Casiopeína IIIEa on the metabolism and ultrastructure of the yeast Saccharomyces cerevisiae was investigated. Exposure of cells growing in rich or in low-iron medium to 5 μM of the compound decreased duplication time and reduced oxygen consumption. Those cells formed smaller colonies when growing in a non-fermentable carbon source and low-iron medium, and under the light microscope, multiple folds were observed along the plasma membrane accompanied with a reduction in the diameter of the yeast. These observations were confirmed under the electron microscope, which also revealed a slight reduction of the mitochondrial size. A correlation was found with smaller colonies exhibiting lower rates of oxygen consumption, and yeast labelled with fluorescent MitoTracker(TM) consistently exhibited reduced mitochondrial activity. It appears that Casiopeína IIIEa gives rise to smaller yeast and petite-like colonies by reducing the mitochondrial respiratory activity without significantly affecting the mitochondrial structure.

Gex1 is a yeast glutathione exchanger that interferes with pH and redox homeostasis

In the yeast Saccharomyces cerevisiae, glutathione plays a major role in heavy metal detoxification and protection of cells against oxidative stress. We show that Gex1 is a new glutathione exchanger. Gex1 and its paralogue Gex2 belong to the major facilitator superfamily of transporters and display similarities to the Aft1-regulon family of siderophore transporters. Gex1 was found mostly at the vacuolar membrane and, to a lesser extent, at the plasma membrane. Gex1 expression was induced under conditions of iron depletion and was principally dependent on the iron-responsive transcription factor Aft2. However, a gex1Δ gex2Δ strain displayed no defect in known siderophore uptake. The deletion mutant accumulated intracellular glutathione, and cells overproducing Gex1 had low intracellular glutathione contents, with glutathione excreted into the extracellular medium. Furthermore, the strain overproducing Gex1 induced acidification of the cytosol, confirming the involvement of Gex1 in proton transport as a probable glutathione/proton antiporter. Finally, the imbalance of pH and glutathione homeostasis in the gex1Δ gex2Δ and Gex1-overproducing strains led to modulations of the cAMP/protein kinase A and protein kinase C1 mitogen-activated protein kinase signaling pathways.

The proline 160 in the selectivity filter of the Arabidopsis NO(3)(-)/H(+) exchanger AtCLCa is essential for nitrate accumulation in planta

Nitrate, the major nitrogen source for plants, can be accumulated in the vacuole. Its transport across the vacuolar membrane is mediated by AtCLCa, an antiporter of the chloride channel (CLC) protein family. In contrast to other CLC family members, AtCLCa transports nitrate coupled to protons. Recently, the different behaviour towards nitrate of CLC proteins has been linked to the presence of a serine or proline in the selectivity filter motif GXGIP. By monitoring AtCLCa activity in its native environment, we show that if proline 160 in AtCLCa is changed to a serine (AtCLCa(P160S) ), the transporter loses its nitrate selectivity, but the anion proton exchange mechanism is unaffected. We also performed in vivo analyses in yeast and Arabidopsis. In contrast to native AtCLCa, expression of AtCLCa(P160S) does not complement either the ΔScCLC yeast mutant grown on nitrate or the nitrate under-accumulation phenotype of clca knockout plants. Our results confirm the significance of this amino acid in the conserved selectivity filter of CLC proteins and highlight the importance of the proline in AtCLCa for nitrate metabolism in Arabidopsis.

Oxidative stress and alterations in actin cytoskeleton trigger glutathione efflux in Saccharomyces cerevisiae

A marked deficiency in glutathione (GSH), the most abundant antioxidant in living systems, plays a major role in aging and the pathogenesis of diseases ranging from neurological disorders to early atherosclerosis and the impairment of various immunological functions. In an attempt to shed light on GSH homeostasis, we carried out the space experiment SCORE (Saccharomyces cerevisiae oxidative stress response evaluation) during the FOTON-M3 mission. Microgravity and hyperoxic conditions induced an enormous extracellular release of GSH from S. cerevisiae cells (≈40% w/dw), changed the distribution of the buds, and activated the high osmolarity glycerol (HOG) and cell integrity/PKC pathways, as well as protein carbonylation. The results from the single spaceflight experiment were validated by a complete set of experiments under conditions of simulated microgravity and indicate that cytoskeletal alterations are mainly responsible for the observed effects. The results of ground experiments in which we induced cytoskeletal modifications by means of treatment with dihydrocytochalasin B (DHCB), a potent inhibitor of actin polymerisation, or (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclohexanecarboxamide dihydrochloride monohydrate (Y-27632), a selective ROCK (Rho-associated coiled-coil forming protein serine/threonine kinase) inhibitor, confirmed the role of actin in GSH efflux. We also found that the GSH release can be inhibited using the potent chloride channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB).

Outwardly rectifying anionic channel from the plasma membrane of the fungus Phycomyces blakesleeanus

In the present report, by using a patch clamp technique, we provide, to our knowledge, the first detailed description of an anionic channel from filamentous fungi. The characterized channel, an outwardly rectifying anionic channel (ORAC), is the most prominent feature of the cell membrane of the fungus Phycomyces blakesleeanus in the absence of energizing substrates. The unitary conductance of the channel is 11.3 +/- 0.4 pS. It is characterized by a strong voltage dependence of the open-channel probability (zdelta; the gating charge is 2.1 +/- 0.1), and the channel is activated by depolarization. The values of the time constants for voltage-induced activation and deactivation of 28 +/- 3 ms for tau(a) and 39 +/- 9 ms for tau(d) show that the ORAC is characterized by fast activation/deactivation kinetics. The ORAC shows strong selectivity for anions over cations and weak selectivity among anions, with a selectivity sequence of I(-) >or= NO(3)(-) > Br(-) > Cl(-) > SO(4)(2-) = 4.8 > 4.4 > 2.2 > 1 > 0.55, which corresponds to Eisenman series 1. The channel is characterized by two open and two closed states, with dominant long open (tau(o2) = 35.0 +/- 3.9 ms) and long closed (tau(c2) = 166 +/- 28 ms) states occupying 63% +/- 8% and 79% +/- 3% of total open and closed times, respectively. The ORAC is insensitive to anthracene-9-carboxylic acid (<200 microM), but 2 mM malate reversibly inhibits 59% +/- 12% of the channel activity. Based on the electrophysiological properties of the channel, we propose that the ORAC plays a role in anion accumulation and in membrane potential regulation through local membrane depolarization.

Ion channels in microbes

Studies of ion channels have for long been dominated by the animalcentric, if not anthropocentric, view of physiology. The structures and activities of ion channels had, however, evolved long before the appearance of complex multicellular organisms on earth. The diversity of ion channels existing in cellular membranes of prokaryotes is a good example. Although at first it may appear as a paradox that most of what we know about the structure of eukaryotic ion channels is based on the structure of bacterial channels, this should not be surprising given the evolutionary relatedness of all living organisms and suitability of microbial cells for structural studies of biological macromolecules in a laboratory environment. Genome sequences of the human as well as various microbial, plant, and animal organisms unambiguously established the evolutionary links, whereas crystallographic studies of the structures of major types of ion channels published over the last decade clearly demonstrated the advantage of using microbes as experimental organisms. The purpose of this review is not only to provide an account of acquired knowledge on microbial ion channels but also to show that the study of microbes and their ion channels may also hold a key to solving unresolved molecular mysteries in the future.

Chloride homeostasis in Saccharomyces cerevisiae: high affinity influx, V-ATPase-dependent sequestration, and identification of a candidate Cl- sensor

Chloride homeostasis in Saccharomyces cerevisiae has been characterized with the goal of identifying new Cl⁻ transport and regulatory pathways. Steady-state cellular Cl⁻ contents (∼0.2 mEq/liter cell water) differ by less than threefold in yeast grown in media containing 0.003–5 mM Cl⁻. Therefore, yeast have a potent mechanism for maintaining constant cellular Cl⁻ over a wide range of extracellular Cl⁻. The cell water:medium [Cl⁻] ratio is >20 in media containing 0.01 mM Cl⁻ and results in part from sequestration of Cl⁻ in organelles, as shown by the effect of deleting genes involved in vacuolar acidification. Organellar sequestration cannot account entirely for the Cl⁻ accumulation, however, because the cell water:medium [Cl⁻] ratio in low Cl⁻ medium is ∼10 at extracellular pH 4.0 even in vma1 yeast, which lack the vacuolar H⁺-ATPase. Cellular Cl⁻ accumulation is ATP dependent in both wild type and vma1 strains. The initial ³⁶Cl⁻ influx is a saturable function of extracellular [³⁶Cl⁻] with K_1/2 of 0.02 mM at pH 4.0 and >0.2 mM at pH 7, indicating the presence of a high affinity Cl⁻ transporter in the plasma membrane. The transporter can exchange ³⁶Cl⁻ for either Cl⁻ or Br⁻ far more rapidly than SO₄⁼, phosphate, formate, HCO₃⁻, or NO₃⁻. High affinity Cl⁻ influx is not affected by deletion of any of several genes for possible Cl⁻ transporters. The high affinity Cl⁻ transporter is activated over a period of ∼45 min after shifting cells from high-Cl⁻ to low-Cl⁻ media. Deletion of ORF YHL008c (formate-nitrite transporter family) strongly reduces the rate of activation of the flux. Therefore, Yhl008cp may be part of a Cl⁻-sensing mechanism that activates the high affinity transporter in a low Cl⁻ medium. This is the first example of a biological system that can regulate cellular Cl⁻ at concentrations far below 1 mM.