The yeast endosomal Na+K+/H+ exchanger Nhx1 regulates cellular pH to control vesicle trafficking

Membrane hyperpolarization drives cation influx and fungicidal activity of amiodarone

Cationic amphipathic drugs, such as amiodarone, interact preferentially with lipid membranes to exert their biological effect. In the yeast Saccharomyces cerevisiae, toxic levels of amiodarone trigger a rapid influx of Ca(2+) that can overwhelm cellular homeostasis and lead to cell death. To better understand the mechanistic basis of antifungal activity, we assessed the effect of the drug on membrane potential. We show that low concentrations of amiodarone (0.1-2 microm) elicit an immediate, dose-dependent hyperpolarization of the membrane. At higher doses (>3 microm), hyperpolarization is transient and is followed by depolarization, coincident with influx of Ca(2+) and H(+) and loss in cell viability. Proton and alkali metal cation transporters play reciprocal roles in membrane polarization, depending on the availability of glucose. Diminishment of membrane potential by glucose removal or addition of salts or in pma1, tok1Delta, ena1-4Delta, or nha1Delta mutants protected against drug toxicity, suggesting that initial hyperpolarization was important in the mechanism of antifungal activity. Furthermore, we show that the link between membrane hyperpolarization and drug toxicity is pH-dependent. We propose the existence of pH- and hyperpolarization-activated Ca(2+) channels in yeast, similar to those described in plant root hair and pollen tubes that are critical for cell elongation and growth. Our findings illustrate how membrane-active compounds can be effective microbicidals and may pave the way to developing membrane-selective agents.

Cardiolipin mediates cross-talk between mitochondria and the vacuole

Cardiolipin (CL) is an anionic phospholipid with a dimeric structure predominantly localized in the mitochondrial inner membrane, where it is closely associated with mitochondrial function, biogenesis, and genome stability (Daum, 1985; Janitor and Subik, 1993; Jiang et al., 2000; Schlame et al., 2000; Zhong et al., 2004). Previous studies have shown that yeast mutant cells lacking CL due to a disruption in CRD1, the structural gene encoding CL synthase, exhibit defective colony formation at elevated temperature even on glucose medium (Jiang et al., 1999; Zhong et al., 2004), suggesting a role for CL in cellular processes apart from mitochondrial bioenergetics. In the current study, we present evidence that the crd1Delta mutant exhibits severe vacuolar defects, including swollen vacuole morphology and loss of vacuolar acidification, at 37 degrees C. Moreover, vacuoles from crd1Delta show decreased vacuolar H(+)-ATPase activity and proton pumping, which may contribute to loss of vacuolar acidification. Deletion mutants in RTG2 and NHX1, which mediate vacuolar pH and ion homeostasis, rescue the defective colony formation phenotype of crd1Delta, strongly suggesting that the temperature sensitivity of crd1Delta is a consequence of the vacuolar defects. Our results demonstrate the existence of a novel mitochondria-vacuole signaling pathway mediated by CL synthesis.

The yeast lysosome-like vacuole: endpoint and crossroads

Fungal vacuoles are acidic organelles with degradative and storage capabilities that have many similarities to mammalian lysosomes and plant vacuoles. In the past several years, well-developed genetic, genomic, biochemical and cell biological tools in S. cerevisiae have provided fresh insights into vacuolar protein sorting, organelle acidification, ion homeostasis, autophagy, and stress-related functions of the vacuole, and these insights have often found parallels in mammalian lysosomes. This review provides a broad overview of the defining features and functions of S. cerevisiae vacuoles and compares these features to mammalian lysosomes. Recent research challenges the traditional view of vacuoles and lysosomes as simply the terminal compartment of biosynthetic and endocytic pathways (i.e. the "garbage dump" of the cell), and suggests instead that these compartments are unexpectedly dynamic and highly regulated.

Overexpression of the tomato K+/H+ antiporter LeNHX2 confers salt tolerance by improving potassium compartmentalization

Here, the function of the tomato (Solanum lycopersicon) K+/H+ antiporter LeNHX2 was studied using 35S-driven gene overexpression of a histagged LeNHX2 protein in Arabidopsis thaliana and LeNHX2 gene silencing in tomato. Transgenic A. thaliana plants expressed the histagged LeNHX2 both in shoots and in roots, as assayed by western blotting. Transitory expression of a green fluorescent protein (GFP) tagged protein showed that the antiporter is present in small vesicles. Internal membrane vesicles from transgenic plants displayed enhanced K+/H+ exchange activity, confirming the K+/H+ antiporter function of this enzyme. Transgenic A. thaliana plants overexpressing the histagged tomato antiporter LeNHX2 exhibited inhibited growth in the absence of K+ in the growth medium, but were more tolerant to high concentrations of Na+ than untransformed controls. When grown in the presence of NaCl, transgenic plants contained lower concentrations of intracellular Na+, but more K+, as compared with untransformed controls. Silencing of LeNHX2 in S. lycopersicon plants produced significant inhibition of plant growth and fruit and seed production as well as increased sensitivity to NaCl. The data indicate that regulation of K+ homeostasis by LeNHX2 is essential for normal plant growth and development, and plays an important role in the response to salt stress by improving K+ accumulation.

Vacuolar and plasma membrane proton pumps collaborate to achieve cytosolic pH homeostasis in yeast

Vacuolar proton-translocating ATPases (V-ATPases) play a central role in organelle acidification in all eukaryotic cells. To address the role of the yeast V-ATPase in vacuolar and cytosolic pH homeostasis, ratiometric pH-sensitive fluorophores specific for the vacuole or cytosol were introduced into wild-type cells and vma mutants, which lack V-ATPase subunits. Transiently glucose-deprived wild-type cells respond to glucose addition with vacuolar acidification and cytosolic alkalinization, and subsequent addition of K(+) ion increases the pH of both the vacuole and cytosol. In contrast, glucose addition results in an increase in vacuolar pH in both vma mutants and wild-type cells treated with the V-ATPase inhibitor concanamycin A. Cytosolic pH homeostasis is also significantly perturbed in the vma mutants. Even at extracellular pH 5, conditions optimal for their growth, cytosolic pH was much lower, and response to glucose was smaller in the mutants. In plasma membrane fractions from the vma mutants, activity of the plasma membrane proton pump, Pma1p, was 65-75% lower than in fractions from wild-type cells. Immunofluorescence microscopy confirmed decreased levels of plasma membrane Pma1p and increased Pma1p at the vacuole and other compartments in the mutants. Pma1p was not mislocalized in concanamycin-treated cells, but a significant reduction in cytosolic pH under all conditions was still observed. We propose that short-term, V-ATPase activity is essential for both vacuolar acidification in response to glucose metabolism and for efficient cytosolic pH homeostasis, and long-term, V-ATPases are important for stable localization of Pma1p at the plasma membrane.

Functional comparison of plasma-membrane Na+/H+ antiporters from two pathogenic Candida species

BACKGROUND

The virulence of Candida species depends on many environmental conditions. Extracellular pH and concentration of alkali metal cations belong among important factors. Nevertheless, the contribution of transporters mediating the exchange of alkali metal cations for protons across the plasma membrane to the cell salt tolerance and other physiological properties of various Candida species has not been studied so far.

RESULTS

The tolerance/sensitivity of four pathogenic Candida species to alkali metal cations was tested and the role of one of the cation transporters in that tolerance (presumed to be the plasma-membrane Na+/H+ antiporter) was studied. The genes encoding these antiporters in the most and least salt sensitive species, C. dubliniensis and C. parapsilosis respectively, were identified, cloned and functionally expressed in the plasma membranes of Saccharomyces cerevisiae cells lacking their own cation exporters. Both CpCnh1 and CdCnh1 antiporters had broad substrate specificity and transported Na+, K+, Li+, and Rb+. Their activity in S. cerevisiae cells differed; CpCnh1p provided cells with a much higher salt tolerance than the CdCnh1 antiporter. The observed difference in activity was confirmed by direct measurements of sodium and potassium efflux mediated by these antiporters.

CONCLUSION

We have cloned two genes encoding putative Na+/H+ antiporters in C. parapsilosis and C. dubliniensis, and characterized the transport properties of encoded proteins. Our results show that the activity of plasma-membrane Na+/H+ antiporters is one of the factors determining the tolerance of pathogenic Candida species to high external concentrations of alkali metal cations.

Structure and regulation of the vacuolar ATPases

The vacuolar (H(+))-ATPases (V-ATPases) are ATP-dependent proton pumps responsible for both acidification of intracellular compartments and, for certain cell types, proton transport across the plasma membrane. Intracellular V-ATPases function in both endocytic and intracellular membrane traffic, processing and degradation of macromolecules in secretory and digestive compartments, coupled transport of small molecules such as neurotransmitters and ATP and in the entry of pathogenic agents, including envelope viruses and bacterial toxins. V-ATPases are present in the plasma membrane of renal cells, osteoclasts, macrophages, epididymal cells and certain tumor cells where they are important for urinary acidification, bone resorption, pH homeostasis, sperm maturation and tumor cell invasion, respectively. The V-ATPases are composed of a peripheral domain (V(1)) that carries out ATP hydrolysis and an integral domain (V(0)) responsible for proton transport. V(1) contains eight subunits (A-H) while V(0) contains six subunits (a, c, c', c'', d and e). V-ATPases operate by a rotary mechanism in which ATP hydrolysis within V(1) drives rotation of a central rotary domain, that includes a ring of proteolipid subunits (c, c' and c''), relative to the remainder of the complex. Rotation of the proteolipid ring relative to subunit a within V(0) drives active transport of protons across the membrane. Two important mechanisms of regulating V-ATPase activity in vivo are reversible dissociation of the V(1) and V(0) domains and changes in coupling efficiency of proton transport and ATP hydrolysis. This review focuses on recent advances in our lab in understanding the structure and regulation of the V-ATPases.

Characterization and expression of a vacuolar Na(+)/H(+) antiporter gene from the monocot halophyte Aeluropus littoralis

Plant vacuolar Na(+)/H(+) antiporter plays an important role in salt tolerance. In order to understand the molecular basis of vacuolar Na(+)/H(+) antiporter responded to salinity and reveal a possible role of salt tolerance in monocots, a vacuolar Na(+)/H(+) antiporter gene (AlNHX) was isolated by reverse transcription-PCR and RNA ligase mediated rapid amplification of cDNA ends (RLM-RACE) based on the homology from Aeluropus littoralis (Gouan) Parl, a graminaceous halophyte. The AlNHX sequence contained 2706bp with an open read frame of 1623bp and the deduced transcripts encoding 540 amino acids shared a high homology with those putative vacuolar Na(+)/H(+) antiporters of higher plants. AlNHX was predicted containing ten putative hydrophobic regions, which was different with AtNHX1 and OsNHX1. DNA gel blot analysis indicated that there were two or three copies of AlNHX in the A. littoralis genome. The increased transcript levels of AlNHX were much higher in roots than that in shoots under salt stress. In addition, overexpression of AlNHX in tobacco conferred high salt tolerance to the transgenic plants. The analysis of ion contents indicated that under high salt stress for one month, the transgenic plants compartmentalized more Na(+) in the roots and kept a relative high K(+)/Na(+) ratio in the leaves compared with wild-type plants.

The Arabidopsis cax3 mutants display altered salt tolerance, pH sensitivity and reduced plasma membrane H+-ATPase activity

Perturbing CAX1, an Arabidopsis vacuolar H+/Ca2+ antiporter, and the related vacuolar transporter CAX3, has been previously shown to cause severe growth defects; however, the specific function of CAX3 has remained elusive. Here, we describe plant phenotypes that are shared among cax1 and cax3 including an increased sensitivity to both abscisic acid (ABA) and sugar during germination, and an increased tolerance to ethylene during early seedling development. We have also identified phenotypes unique to cax3, namely salt, lithium and low pH sensitivity. We used biochemical measurements to ascribe these cax3 sensitivities to a reduction in vacuolar H+/Ca2+ transport during salt stress and decreased plasma membrane H+-ATPase activity. These findings catalog an array of CAX phenotypes and assign a specific role for CAX3 in response to salt tolerance.

Cell surface levels of organellar Na+/H+ exchanger isoform 6 are regulated by interaction with RACK1

In mammalian cells, four Na(+)/H(+) exchangers (NHE6 - NHE9) are localized to intracellular compartments. NHE6 and NHE9 are predominantly localized to sorting and recycling endosomes, NHE7 to the trans-Golgi network, and NHE8 to the mid-trans-Golgi stacks. The unique localization of NHEs may contribute to establishing organelle-specific pH values and ion homeostasis in cells. Mechanisms underlying the regulation and targeting of organellar NHEs are largely unknown. We identified an interaction between NHE9 and RACK1 (receptor for activated C kinase 1), a cytoplasmic scaffold protein, by yeast two-hybrid screening using the NHE9 C terminus as bait. The NHE9 C terminus is exposed to the cytoplasm, verifying that the interaction is topologically possible. The binding region was further delineated to the central region of the NHE9 C terminus. RACK1 also bound NHE6 and NHE7, but not NHE8, in vitro. Endogenous association between NHE6 and RACK1 was confirmed by co-immunoprecipitation and co-localization in HeLa cells. The luminal pH of the recycling endosome was elevated in RACK1 knockdown cells, accompanied by a decrease in the amount of NHE6 on the cell surface, although the total level of NHE6 was not significantly altered. These results indicate that RACK1 plays a role in regulating the distribution of NHE6 between endosomes and the plasma membrane and contributes to maintaining luminal pH of the endocytic recycling compartments.

Plasma-membrane Cnh1 Na+/H+ antiporter regulates potassium homeostasis in Candida albicans

The physiological role of Candida albicans Cnh1, a member of the Na+/H+ antiporter family, was characterized. Though CaCnh1p had broad substrate specificity and mediated efflux of at least four alkali metal cations upon heterologous expression in Saccharomyces cerevisiae, its presence in C. albicans cells was important especially for potassium homeostasis. In C. albicans, CaCnh1p tagged with GFP was localized in the plasma membrane of cells growing as both yeasts and hyphae. Deletion of CNH1 alleles did not affect tolerance to NaCl, LiCl or CsCl, but resulted in increased sensitivity to high external concentrations of KCl and RbCl. The potassium and rubidium tolerance of a cnh1 homozygous mutant was fully restored by reintegration of CNH1 into the genome. The higher sensitivity of the cnh1/cnh1 mutant to external KCl was caused by a lower K+ efflux from these cells. Together, the functional characterization of the CaCnh1 antiporter in C. albicans revealed that this antiporter plays a significant role in C. albicans physiology. It ensures potassium and rubidium tolerance and participates in the regulation of intracellular potassium content of C. albicans cells.

A human Na+/H+ antiporter sharing evolutionary origins with bacterial NhaA may be a candidate gene for essential hypertension

Phylogenetic analysis of the cation/proton antiporter superfamily has uncovered a previously unknown clade of genes in metazoan genomes, including two previously uncharacterized human isoforms, NHA1 and NHA2, found in tandem on human chromosome 4. The NHA (sodium hydrogen antiporter) family members share significant sequence similarity with Escherichia coli NhaA, including a conserved double aspartate motif in predicted transmembrane 5. We show that HsNHA2 (Homo sapiens NHA2) resides on the plasma membrane and, in polarized MDCK cells, localizes to the apical domain. Analysis of mouse tissues indicates that NHA2 is ubiquitous. When expressed in the yeast Saccharomyces cerevisiae lacking endogenous cation/proton antiporters and pumps, HsNHA2 can confer tolerance to Li(+) and Na(+) ions but not to K(+). HsNHA2 transformants accumulated less Li(+) than the salt-sensitive host; however, mutagenic replacement of the conserved aspartates abolished all observed phenotypes. Functional complementation by HsNHA2 was insensitive to amiloride, a characteristic inhibitor of plasma membrane sodium hydrogen exchanger isoforms, but was inhibited by phloretin. These are hallmarks of sodium-lithium countertransport activity, a highly heritable trait correlating with hypertension. Our findings raise the possibility that NHA genes may contribute to sodium-lithium countertransport activity and salt homeostasis in humans.

Regulation of vacuolar pH and its modulation by some microbial species

To survive within the host, pathogens such as Mycobacterium tuberculosis and Helicobacter pylori need to evade the immune response and find a protected niche where they are not exposed to microbicidal effectors. The pH of the microenvironment surrounding the pathogen plays a critical role in dictating the organism's fate. Specifically, the acidic pH of the endocytic organelles and phagosomes not only can affect bacterial growth directly but also promotes a variety of host microbicidal responses. The development of mechanisms to avoid or resist the acidic environment generated by host cells is therefore crucial to the survival of many pathogens. Here we review the processes that underlie the generation of organellar acidification and discuss strategies employed by pathogens to circumvent it, using M. tuberculosis and H. pylori as examples.

Applications of a microplate reader in yeast physiology research

Microplate readers have been useful assistants of researchers for several decades. This work is focused on the applications of a simple absorbance microplate reader in yeast physiology research, and its advantages and limitations in comparison with alternative methods are discussed. The two main procedures involved are measuring growth curves and monitoring the pH changes of medium using two different pH indicators. We suggest mathematical formulas for converting absorbance data into pH values. With a microplate reader, as many as 96 samples can be simultaneously analyzed, while medium consumption is minimized to 100 µL per sample. The results can be observed in 24–48 h (for growth curves) or in 1–3 h (for pH changes) with minimal hands-on time required.

Characterization ofDhKHA1, a gene coding for a putative Na+transporter fromDebaryomyces hansenii

The KHA1 gene from Debaryomyces hansenii has been identified and characterized by heterologous expression in Saccharomyces cerevisiae. The gene is orthologous to ScKHA1, previously reported in S. cerevisiae, and on the basis of the deduced amino acid sequence, DhKha1p can be classified as an Na(+)/H(+) transporter. Reverse transcriptase (RT)-PCR experiments indicated that the expression level of DhKHA1 was not dependent on high pH or on the presence of a high salt level in the growth medium. Overexpression of DhKHA1 in a salt-sensitive S. cerevisiae mutant (ena1-4 nha1 kha1) rendered cells specifically more tolerant to Na(+). In addition, internal K(+) and Na(+) measurements and experiments performed with green fluorescence protein (GFP)-tagged DhKha1p indicated the intracellular localization of this protein when expressed in S. cerevisiae.

pH-dependent binding of the Epsin ENTH domain and the AP180 ANTH domain to PI(4,5)P2-containing bilayers

Epsin and AP180 are essential components of the endocytotic machinery, which controls internalization of protein receptors and other macromolecules at the cell surface. Epsin and AP180 are recruited to the plasma membrane by their structurally and functionally related N-terminal ENTH and ANTH domains that specifically recognize PtdIns(4,5)P2. Here, we show that membrane anchoring of the ENTH and ANTH domains is regulated by the acidic environment. Lowering the pH enhances PtdIns(4,5)P2 affinity of the ENTH and ANTH domains reinforcing their association with lipid vesicles and monolayers. The pH dependency is due to the conserved histidine residues of the ENTH and ANTH domains, protonation of which is necessary for the strong PtdIns(4,5)P2 recognition, as revealed by liposome binding, surface plasmon resonance, NMR, monolayer surface tension and mutagenesis experiments. The pH sensitivity of the ENTH and ANTH domains is reminiscent to the pH dependency of the FYVE domain suggesting a common regulatory mechanism of membrane anchoring by a subset of the PI-binding domains.

Identification and Characterization of Vnx1p, a Novel Type of Vacuolar Monovalent Cation/H+ Antiporter of Saccharomyces cerevisiae

We identified and characterized Vnx1p, a novel vacuolar monovalent cation/H+ antiporter encoded by the open reading frame YNL321w from Saccharomyces cerevisiae. Despite the homology of Vnx1p with other members of the CAX (calcium exchanger) family of transporters, Vnx1p is unable to mediate Ca2+ transport but is a low affinity Na+/H+ and K+/H+ anti-porter with a Km of 22.4 and 82.2 mm for Na+ and K+, respectively. Sequence analyses of Vnx1p revealed the absence of key amino acids shown to be essential for Ca2+/H+ exchange. vnx1Delta cells displayed growth inhibition when grown in the presence of hygromycin B or NaCl. Vnx1p activity was found in the vacuoles and shown to be dependent on the electrochemical potential gradient of H+ generated by the action of the V-type H+-ATPase. The presence of Vnx1p at the vacuolar membrane was further confirmed with cells expressing a VNX1::GFP chimeric gene. Similar to Nhx1p, the prevacuolar compartment-bound Na+/H+ antiporter, the vacuole-bound Vnx1p appears to play roles in the regulation of ion homeostasis and cellular pH.

Emerging roles of alkali cation/proton exchangers in organellar homeostasis

The regulated movement of monovalent cations such as H(+), Li(+), Na(+) and K(+) across biological membranes influences a myriad of cellular processes and is fundamental to all living organisms. This is accomplished by a multiplicity of ion channels, pumps and transporters. Our insight into their molecular, cellular and physiological diversity has increased greatly in the past few years with the advent of genome sequencing, genetic manipulation and sophisticated imaging techniques. One of the revelations from these studies is the emergence of novel alkali cation/protons exchangers that are present in endomembranes, where they function to regulate not only intraorganellar pH but also vesicular biogenesis, trafficking and other aspects of cellular homeostasis.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Na+ transport in plants

The ability of plants to grow in high NaCl concentrations is associated with the ability of the plants to transport, compartmentalize, extrude, and mobilize Na(+) ions. While the influx and efflux at the roots establish the steady state rate of entry of Na(+) into the plant, the compartmentation of Na(+) into the cell vacuoles and the radial transport of Na(+) to the stele and its loading into the xylem establish the homeostatic control of Na(+) in the cytosol of the root cells. Removal of Na(+) from the transpirational stream, its distribution within the plant and its progressive accumulation in the leaf vacuoles, will determine the ability to deal with the toxic effects of Na(+). The aim of this review is to highlight and discuss the recent progress in understanding of Na(+) transport in plants.

Impact of AtNHX1, a vacuolar Na+/H+ antiporter, upon gene expression during short- and long-term salt stress in Arabidopsis thaliana

BACKGROUND

AtNHX1, the most abundant vacuolar Na+/H+ antiporter in Arabidopsis thaliana, mediates the transport of Na+ and K+ into the vacuole, influencing plant development and contributing to salt tolerance. In this report, microarray expression profiles of wild type plants, a T-DNA insertion knockout mutant of AtNHX1 (nhx1), and a 'rescued' line (NHX1::nhx1) were exposed to both short (12 h and 48 h) and long (one and two weeks) durations of a non-lethal salt stress to identify key gene transcripts associated with the salt response that are influenced by AtNHX1.

RESULTS

147 transcripts showed both salt responsiveness and a significant influence of AtNHX1. Fifty-seven of these genes showed an influence of the antiporter across all salt treatments, while the remaining genes were influenced as a result of a particular duration of salt stress. Most (69%) of the genes were up-regulated in the absence of AtNHX1, with the exception of transcripts encoding proteins involved with metabolic and energy processes that were mostly down-regulated.

CONCLUSION

While part of the AtNHX1-influenced transcripts were unclassified, other transcripts with known or putative roles showed the importance of AtNHX1 to key cellular processes that were not necessarily limited to the salt stress response; namely calcium signaling, sulfur metabolism, cell structure and cell growth, as well as vesicular trafficking and protein processing. Only a small number of other salt-responsive membrane transporter transcripts appeared significantly influenced by AtNHX1.

Intracellular Na+ and K+ distribution in Debaryomyces hansenii. Cloning and expression in Saccharomyces cerevisiae of DhNHX1

Debaryomyces hansenii is a salt-tolerant yeast that contains high amounts of internal Na(+). Debaryomyces hansenii kept more sodium than Saccharomyces cerevisiae in both the cytoplasm and vacuole when grown under a variety of NaCl concentrations. These results indicate a higher tolerance of Debaryomyces to high internal Na(+), and, in addition, suggest the existence of a transporter driving Na(+) into the vacuole. Moreover, a gene encoding a Na(+) (K(+))/H(+) antiporter from D. hansenii was cloned and sequenced. The gene, designated DhNHX1, exhibited significant homology with genes of the NHE/NHX family. DhNHX1 expression was induced neither at low pH nor by extracellular NaCl. A mutant of S. cerevisiae lacking its own Na(+) transporters (ena1-4Delta nha1 Delta nhx1 Delta), when transformed with DhNHX1, partially recovered cation tolerance as well as the ability to accumulate Na(+) and K(+) into the vacuole. Our analysis provides evidence that DhNhx1p transports Na(+) (and K(+)) into the vacuole and that it can play an important role in ion homeostasis and salt tolerance.

Genes required for osmoregulation and apical secretion in Caenorhabditis elegans

Few studies have investigated whether or not there is an interdependence between osmoregulation and vesicular trafficking. We previously showed that in Caenorhabditis elegans che-14 mutations affect osmoregulation, cuticle secretion, and sensory organ development. We report the identification of seven lethal mutations displaying che-14-like phenotypes, which define four new genes, rdy-1-rdy-4 (rod-like larval lethality and dye-filling defective). rdy-1, rdy-2, and rdy-4 mutations affect excretory canal function and cuticle formation. Moreover, rdy-1 and rdy-2 mutations reduce the amount of matrix material normally secreted by sheath cells in the amphid channel. In contrast, rdy-3 mutants have short cystic excretory canals, suggesting that it acts in a different process. rdy-1 encodes the vacuolar H+-ATPase a-subunit VHA-5, whereas rdy-2 encodes a new tetraspan protein. We suggest that RDY-1/VHA-5 acts upstream of RDY-2 and CHE-14 in some tissues, since it is required for their delivery to the epidermal, but not the amphid sheath, apical plasma membrane. Hence, the RDY-1/VHA-5 trafficking function appears essential in some cells and its proton pump function essential in others. Finally, we show that RDY-1/VHA-5 distribution changes prior to molting in parallel with that of actin microfilaments and propose a model for molting whereby actin provides a spatial cue for secretion.

Vestibular hair bundles control pH with (Na+, K+)/H+ exchangers NHE6 and NHE9

In hair cells of the inner ear, robust Ca2+/H+ exchange mediated by plasma-membrane Ca2+-ATPase would rapidly acidify mechanically sensitive hair bundles without efficient removal of H+. We found that, whereas the basolateral membrane of vestibular hair cells from the frog saccule extrudes H+ via an Na+-dependent mechanism, bundles rapidly remove H+ in the absence of Na+ and HCO3(-), even when the soma is acidified. K+ was fully effective and sufficient for H+ removal; in contrast, Rb+ failed to support pH recovery. Na+/H+-exchanger isoform 1 (NHE1) was present on hair-cell soma membranes and was likely responsible for Na+-dependent H+ extrusion. NHE6 and NHE9 are organellar isoforms that can appear transiently on plasma membranes and have been proposed to mediate K+/H+ exchange. We identified NHE6 in a subset of hair bundles; NHE9 was present in all bundles. Heterologous expression of these isoforms in yeast strains lacking endogenous exchangers conferred pH-dependent tolerance to high levels of KCl and NaCl. NHE9 preferred cations in the order K+, Na+ >> Rb+, consistent with the relative efficacies of these ions in promoting pH recovery in hair bundles. Electroneutral K+/H+ exchange, which we propose is performed by NHE9 in hair bundles, exploits the high-K+ endolymph, responds only to pH imbalance across the bundle membrane, is unaffected by the +80 mV endocochlear potential, and uses mechanisms already present in the ear for K+ recycling. This mechanism allows the hair cell to remove H+ generated by Ca2+ pumping without ATP hydrolysis in the cell.

Genomic screening in vivo reveals the role played by vacuolar H+ ATPase and cytosolic acidification in sensitivity to DNA-damaging agents such as cisplatin

Screening the Saccharomyces cerevisiae homozygous diploid deletion library against a sublethal concentration of cisplatin revealed 76 strains sensitive to the drug. As expected, the largest category of deletions, representing 40% of the sensitive strains, was composed of strains lacking genes involved in DNA replication and damage repair. Deletions lacking function of the highly conserved vacuolar H+ translocating ATPase (V-ATPase) composed the category representing the second largest number of sensitive strains. The effect on cell death exhibited by V-ATPase mutants was found to be a general response to various DNA damaging agents as opposed to being specific to cisplatin, as evidenced by sensitivity of the mutants to hydroxyurea (a DNA-alkylating agent) and UV irradiation. Loss of V-ATPase does not affect DNA repair, because double mutants defective for V-ATPase function and DNA repair pathways were more sensitive to cisplatin than the single mutants. V-ATPase mutants are more prone to DNA damage than wild-type cells, indicated by enhanced activation of the DNA damage checkpoint. Vacuole function per se is not cisplatin-sensitive, because vacuolar morphology and vacuolar acidification were unaffected by cisplatin in wild-type cells. V-ATPase also controls cytoplasmic pH, so the enhanced sensitivity to DNA damage may be associated with the drop in pHi associated with V-ATPase mutants. The increased loss in cell viability induced by cisplatin at lower pH in V-ATPase mutants supports this hypothesis. The loss in viability seen in wild-type cells under the same conditions was far less dramatic.

Measurements of plasma membrane potential changes inSaccharomyces cerevisiaecells reveal the importance of the Tok1 channel in membrane potential maintenance

K+ is one of the cations (besides protons) whose transport across the plasma membrane is believed to contribute to the maintenance of membrane potential. To ensure K+ transport, Saccharomyces cerevisiae cells possess several types of active and passive transporters mediating the K+ influx and efflux, respectively. A diS-C3(3) assay was used to compare the contributions of various potassium transporters to the membrane potential changes of S. cerevisiae cells in the exponential growth phase. Altogether, the contributions of six K+ transporters to the maintenance of a stable membrane potential were tested. As confirmed by the observed hyperpolarization of trk1 trk2 deletion strains, the diS-C3(3) assay is a suitable method for comparative studies of the membrane potential of yeast strains differing in the presence/absence of one or more cation transporters. We have shown that the presence of the Tok1 channel strongly influences membrane potential: deletion of the TOK1 gene results in significant plasma membrane depolarization, whereas strains overexpressing the TOK1 gene are hyperpolarized. We have also proved that plasma membrane potential is not the only parameter determining the hygromycin B sensitivity of yeast cells, and that the role of intracellular transporters in protecting against its toxic effects must also be considered.

Exploration of yeast alkali metal cation/H+ antiporters: Sequence and structure comparison

The Saccharomyces cerevisiae genome contains three genes encoding alkali metal cation/H+ antiporters (Nha1p, Nhx1p, Kha1p) that differ in cell localization, substrate specificity and physiological function. Systematic genome sequencing of other yeast species revealed highly conserved homologous ORFs in all of them. We compared the yeast sequences both at DNA and protein levels. The subfamily of yeast endosomal/prevacuolar Nhx1 antiporters is closely related to mammalian plasma membrane NHE proteins and to both plasma membrane and vacuolar plant antiporters. The high sequence conservation within this subfamily of yeast antiporters suggests that Nhx1p is of great importance in cell physiology. Yeast Kha1 proteins probably belong to the same subfamily as bacterial antiporters, whereas Nhal proteins form a distinct subfamily.

Mutational analysis of the intramembranous H10 loop of yeast Nhx1 reveals a critical role in ion homoeostasis and vesicle trafficking

Yeast Nhx1 [Na+(K+)/H+ exchanger 1] is an intracellular Na+(K+)/H+ exchanger, localizing to the late endosome where it is important for ion homoeostasis and vesicle trafficking. Phylogenetic analysis of NHE (Na+/H+ exchanger) sequences has identified orthologous proteins, including HsNHE6 (human NHE6), HsNHE7 and HsNHE9 of unknown physiological role. These appear distinct from well-studied mammalian plasma membrane isoforms (NHE1-NHE5). To explore the differences between plasma membrane and intracellular NHEs and understand the link between ion homoeostasis and vesicle trafficking, we examined the consequence of replacing residues in the intramembranous H10 loop of Nhx1 between transmembrane segments 9 and 10. The critical role for the carboxy group of Glu355 in ion transport is consistent with the invariance of this residue in all NHEs. Surprisingly, residues specifically conserved in the intracellular isoforms (such as Phe357 and Tyr361) could not be replaced with closely similar residues (leucine and phenylalanine) found in the plasma membrane isoforms without loss of function, revealing unexpected side chain specificity. The trafficking phenotypes of all Nhx1 mutants, including hygromycin-sensitivity and missorting of carboxypeptidase Y, were found to directly correlate with pH homoeostasis defects and could be proportionately corrected by titration with weak base. The present study demonstrates the importance of the H10 loop of the NHE family, highlights the differences between plasma membrane and intracellular isoforms and shows that trafficking defects are tightly coupled with pH homoeostasis.

The Na+,K+/H+-antiporter Nha1 influences the plasma membrane potential ofSaccharomyces cerevisiae

There are three different sodium transport systems (Ena1-4p, Nha1p, Nhx1p) in Saccharomyces cerevisiae. The effect of their absence on the tolerance to alkali-metal cations and on the membrane potential was studied. All three sodium transporters were found to participate in the maintenance of Na+, Li+, K+ and Cs+ homeostasis. Measurements of the distribution of a fluorescent potentiometric probe (diS-C3(3) assay) in cell suspensions showed that the lack of all three transporters depolarizes the plasma membrane. The overexpression of the Na+,K+/H+ antiporter Nha1 resulted in the hyperpolarization of the plasma membrane and consequently increased the sensitivity to Cs+, Tl+ and hygromycin B. This is the first evidence that the activity of a Na+,K+/H+ antiporter could play a role in the homeostatic regulation of the plasma membrane potential in yeast cells.

Endosomal and non-endosomal functions of ESCRT proteins

The three endosomal sorting complexes required for transport (ESCRTs) are integral to the degradation of endocytosed membrane proteins and multivesicular body (MVB) biogenesis. Here, we review evidence that ESCRTs have evolved as a specialized machinery for the degradative sorting of ubiquitinated membrane proteins and we highlight recent studies that have shed light on the mechanisms by which these complexes mediate protein sorting, MVB biogenesis, tumour suppression and viral budding. We also discuss evidence that some ESCRT subunits have evolved additional functions that are unrelated to membrane trafficking.

Saccharomyces cerevisiae Lacking Btn1p Modulate Vacuolar ATPase Activity to Regulate pH Imbalance in the Vacuole

The vacuolar H(+)-ATPase (V-ATPase) along with ion channels and transporters maintains vacuolar pH. V-ATPase ATP hydrolysis is coupled with proton transport and establishes an electrochemical gradient between the cytosol and vacuolar lumen for coupled transport of metabolites. Btn1p, the yeast homolog to human CLN3 that is defective in Batten disease, localizes to the vacuole. We previously reported that Btn1p is required for vacuolar pH maintenance and ATP-dependent vacuolar arginine transport. We report that extracellular pH alters both V-ATPase activity and proton transport into the vacuole of wild-type Saccharomyces cerevisiae. V-ATPase activity is modulated through the assembly and disassembly of the V(0) and V(1) V-ATPase subunits located in the vacuolar membrane and on the cytosolic side of the vacuolar membrane, respectively. V-ATPase assembly is increased in yeast cells grown in high extracellular pH. In addition, at elevated extracellular pH, S. cerevisiae lacking BTN1 (btn1-Delta), have decreased V-ATPase activity while proton transport into the vacuole remains similar to that for wild type. Thus, coupling of V-ATPase activity and proton transport in btn1-Delta is altered. We show that down-regulation of V-ATPase activity compensates the vacuolar pH imbalance for btn1-Delta at early growth phases. We therefore propose that Btn1p is required for tight regulation of vacuolar pH to maintain the vacuolar luminal content and optimal activity of this organelle and that disruption in Btn1p function leads to a modulation of V-ATPase activity to maintain cellular pH homeostasis and vacuolar luminal content.

Does the proteome encode organellar pH?

Inherent to the proteome itself, may be information that enables proteins to buffer pH at a level that promotes their own function within a specialized compartment. We observe that the distribution of computed isoelectric points in the yeast proteome matches experimentally derived organellar pH estimates across distinct subcellular compartments. This raises an interesting evolutionary question: did the pI of proteins and the pH of organelles co-evolve to optimize function?

Alkali cation exchangers: roles in cellular homeostasis and stress tolerance

Uptake and translocation of cations play essential roles in plant nutrition, signal transduction, growth, and development. Among them, potassium (K+) and sodium (Na+) have been the focus of numerous physiological studies because K+ is an essential macronutrient and the most abundant inorganic cation in plant cells, whereas Na+ toxicity is a principal component of the deleterious effects associated with salinity stress. Although the homeostasis of these two ions was long surmised to be fine tuned and under complex regulation, the myriad of candidate membrane transporters mediating their uptake, intracellular distribution, and long-distance transport is nevertheless perplexing. Recent advances have shown that, in addition to their function in vacuolar accumulation of Na+, proteins of the NHX family are endosomal transporters that also play critical roles in K+ homeostasis, luminal pH control, and vesicle trafficking. The plasma membrane SOS1 protein from Arabidopsis thaliana, a highly specific Na+/H+ exchanger that catalyses Na+ efflux and that regulates its root/shoot distribution, has also revealed surprising interactions with K+ uptake mechanisms by roots. Finally, the function of individual members of the large CHX family remains largely unknown but two CHX isoforms, AtCHX17 and AtCH23, have been shown to affect K+ homeostasis and the control of chloroplast pH, respectively. Recent advances on the understanding of the physiological processes that are governed by these three families of cation exchangers are reviewed and discussed.

The Yeast Arr4p ATPase Binds the Chloride Transporter Gef1p When Copper Is Available in the Cytosol

Cellular ion homeostasis involves communication between the cytosol and the luminal compartment of organelles. This is particularly critical for metal ions because of their toxic potential. We have identified the yeast homologue of the prokaryotic ArsA protein, the homodimeric ATPase Arr4p, as a protein that binds to the yeast intracellular CLC chloride-transport protein, Gef1p. We show that binding of Arr4p to the C terminus of Gef1p requires the presence of yeast cytosol and is sensitive to a highly specific copper chelator in vitro and in vivo. Copper alone can substitute for cytosol to support the interaction of Arr4p with the C terminus of Gef1p. The migration behavior of Arr4p in nonreducing gel electrophoresis correlates with cellular copper deficiency, repletion, or stress. Our homology model of Arr4p shows that the antimony (arsenic) metal binding site of ArsA is not conserved in Arr4p. The model suggests that a pair of cysteines, Cys285 and Cys288, is located in the interface of the Arr4p dimer. These residues are required for Arr4p homodimerization and for binding to the C terminus of Gef1p. Whereas both proteins are required for normal growth under iron-limiting conditions, they play opposite roles when copper and heat stress are combined in an alkaline environment. Under these conditions, deltagef1 cells grow much better than wild type yeast, whereas deltaarr4 cells are unable to grow. Comparison of the deltaarr4 with the deltaarr4deltagef1 strain suggests that Arr4p antagonizes the function of Gef1p.

Targeting of the FYVE domain to endosomal membranes is regulated by a histidine switch

Specific recognition of phosphatidylinositol 3-phosphate [PtdIns3P] by the FYVE domain targets cytosolic proteins to endosomal membranes during key signaling and trafficking events within eukaryotic cells. Here, we show that this membrane targeting is regulated by the acidic cellular environment. Lowering the cytosolic pH enhances PtdIns3P affinity of the FYVE domain, reinforcing the anchoring of early endosome antigen 1 (EEA1) to endosomal membranes. Reversibly, increasing the pH disrupts phosphoinositide binding and leads to cytoplasmic redistribution of EEA1. pH dependency is due to a pair of conserved His residues, the successive protonation of which is required for PtdIns3P head group recognition as revealed by NMR. Substitution of the His residues abolishes PtdIns3P binding by the FYVE domain in vitro and in vivo. Another PtdIns3P-binding module, the PX domain of Vam7 and p40phox is shown to be pH-independent. This provides the fundamental functional distinction between the two phosphoinositide-recognizing domains. The presented mode of FYVE regulation establishes the unique function of FYVE proteins as low pH sensors of PtdIns3P and reveals the critical role of the histidine switch in targeting of these proteins to endosomal membranes.

Identification of Conserved Prolyl Residue Important for Transport Activity and the Substrate Specificity Range of Yeast Plasma Membrane Na+/H+ Antiporters

Yeast plasma membrane Na+/H+ antiporters are divided according to their substrate specificity in two distinct subfamilies. To identify amino acid residues responsible for substrate specificity determination (recognition of K+), the Zygosaccharomyces rouxii Sod2-22 antiporter (non-transporting K+) was mutagenized and a collection of ZrSod2-22 mutants that improved the KCl tolerance of a salt-sensitive Saccharomyces cerevisiae strain was isolated. Several independent ZrSod2-22 mutated alleles contained the replacement of a highly conserved proline 145 with a residue containing a hydroxyl group (Ser, Thr). Site-directed mutagenesis of Pro145 proved that an amino acid with a hydroxyl group at this position is enough to enable ZrSod2-22p to transport K+. Simultaneously, the P145(S/T) mutation decreased the antiporter transport activity for both Na+ and Li+. Replacement of Pro145 with glycine resulted in a ZrSod2-22p with extremely low activity only for Na+, and the exchange of a charged residue (Asp, Lys) for Pro145 completely stopped the activity. Mutagenesis of the corresponding proline in the S. cerevisiae Nha1 antiporter (Pro146) confirmed that this proline of the fifth transmembrane domain is a critical residue for antiporter function. This is the first evidence that a non-polar amino acid residue is important for the substrate specificity and activity of yeast Nha antiporters.

Protein transport from the late Golgi to the vacuole in the yeast Saccharomyces cerevisiae

The late Golgi compartment is a major protein sorting station in the cell. Secreted proteins, cell surface proteins, and proteins destined for endosomes or lysosomes must be sorted from one another at this compartment and targeted to their correct destinations. The molecular details of protein trafficking pathways from the late Golgi to the endosomal system are becoming increasingly well understood due in part to information obtained by genetic analysis of yeast. It is now clear that proteins identified in yeast have functional homologues (orthologues) in higher organisms. We will review the molecular mechanisms of protein targeting from the late Golgi to endosomes and to the vacuole (the equivalent of the mammalian lysosome) of the budding yeast Saccharomyces cerevisiae.

Evidence of differential pH regulation of the Arabidopsis vacuolar Ca2+/H+ antiporters CAX1 and CAX2

The Arabidopsis Ca(2+)/H(+) antiporters cation exchanger (CAX) 1 and 2 utilise an electrochemical gradient to transport Ca(2+) into the vacuole to help mediate Ca(2+) homeostasis. Previous whole plant studies indicate that activity of Ca(2+)/H(+) antiporters is regulated by pH. However, the pH regulation of individual Ca(2+)/H(+) antiporters has not been examined. To determine whether CAX1 and CAX2 activity is affected by pH, Ca(2+)/H(+) antiport activity was measured in vacuolar membrane vesicles isolated from yeast heterologously expressing either transporter. Ca(2+) transport by CAX1 and CAX2 was regulated by cytosolic pH and each transporter had a distinct cytosolic pH profile. Screening of CAX1/CAX2 chimeras identified an amino acid domain within CAX2 that altered the pH-dependent Ca(2+) transport profile so that it was almost identical to the pH profile of CAX1. Results from mutagenesis of a specific His residue within this domain suggests a role for this residue in pH regulation.