Functional role of polar amino acid residues in Na+/H+ exchangers

Plant nitrogen uptake and assimilation: regulation of cellular pH homeostasis

The enzymatic controlled metabolic processes in cells occur at their optimized pH ranges, therefore cellular pH homeostasis is fundamental for life. In plants, the nitrogen (N) source for uptake and assimilation, mainly in the forms of nitrate (NO3-) and ammonium (NH4+) quantitatively dominates the anion and cation equilibrium and the pH balance in cells. Here we review ionic and pH homeostasis in plant cells and regulation by N source from the rhizosphere to extra- and intracellular pH regulation for short- and long-distance N distribution and during N assimilation. In the process of N transport across membranes for uptake and compartmentation, both proton pumps and proton-coupled N transporters are essential, and their proton-binding sites may sense changes of apoplastic or intracellular pH. In addition, during N assimilation, carbon skeletons are required to synthesize amino acids, thus the combination of NO3- or NH4+ transport and assimilation results in different net charge and numbers of protons in plant cells. Efficient maintenance of N-controlled cellular pH homeostasis may improve N uptake and use efficiency, as well as enhance the resistance to abiotic stresses.

Structural dynamics and regulation of the mammalian SLC9A family of Na⁺/H⁺ exchangers

Mammalian Na⁺/H⁺ exchangers of the SLC9A family are widely expressed and involved in numerous essential physiological processes. Their primary function is to mediate the 1:1 exchange of Na⁺ for H⁺ across the membrane in which they reside, and they play central roles in regulation of body, cellular, and organellar pH. Their function is tightly regulated through mechanisms involving interactions with multiple protein and lipid-binding partners, phosphorylations, and other posttranslational modifications. Biochemical and mutational analyses indicate that the SLC9As have a short intracellular N-terminus, 12 transmembrane (TM) helices necessary and sufficient for ion transport, and a C-terminal cytoplasmic tail region with essential regulatory roles. No high-resolution structures of the SLC9As exist; however, models based on crystal structures of the bacterial NhaAs support the 12 TM organization and suggest that TMIV and XI may form a central part of the ion-translocation pathway, whereas pH sensing may involve TMII, TMIX, and several intracellular loops. Similar to most ion transporters studied, SLC9As likely exist as coupled dimers in the membrane, and this appears to be important for the well-studied cooperativity of H⁺ binding. The aim of this work is to summarize and critically discuss the currently available evidence on the structural dynamics, regulation, and binding partner interactions of SLC9As, focusing in particular on the most widely studied isoform, SLC9A1/NHE1. Further, novel bioinformatic and structural analyses are provided that to some extent challenge the existing paradigm on how ions are transported by mammalian SLC9As.

Functional role and analysis of cysteine residues of the salt tolerance protein Sod2

Sod2 is the major salt tolerance plasma membrane protein of Schizosaccharomyces pombe. It functions to remove excess intracellular sodium (or lithium) in exchange for protons. We investigated the role of cysteine residues and created a cysteine-free Sod2 protein. Each cysteine residue of the ten present was individually mutated to serine and the different proteins expressed and characterized in S. pombe. Western blotting revealed that all the individual mutant proteins were expressed. We examined the ability of the mutant proteins to confer salt tolerance to S. pombe with the endogenous Sod2 protein deleted. Only proteins with C26S and C374S mutations were partially reduced in their ability to confer salt tolerance. Additionally, they showed a change in conformation in comparison to the wild-type protein, indicated by differential sensitivity to trypsin. Deletion of all the cysteine residues of Sod2 resulted in a functional protein that was expressed in S. pombe at levels similar to the wild type and also conferred salt tolerance. The conformation of the cysteine-free Sod2 protein was not altered relative to the wild-type protein. We examined the accessibility of amino acids of the cysteineless protein present on putative extracellular loop 2. A cysteine placed at position Ala119 was accessible to externally applied [2-(trimethylammonium)ethyl] methane thiosulfonate bromide. The results demonstrate that cysteines in the Sod2 protein can be changed to serine residues resulting in an expressed, functional protein. The utility of the cysteine-free Sod2 protein for determination of topology and amino acid accessibility is demonstrated.

Structural and functional analysis of transmembrane segment IV of the salt tolerance protein Sod2

Sod2 is the plasma membrane Na(+)/H(+) exchanger of the fission yeast Schizosaccharomyces pombe. It provides salt tolerance by removing excess intracellular sodium (or lithium) in exchange for protons. We examined the role of amino acid residues of transmembrane segment IV (TM IV) ((126)FPQINFLGSLLIAGCITSTDPVLSALI(152)) in activity by using alanine scanning mutagenesis and examining salt tolerance in sod2-deficient S. pombe. Two amino acids were critical for function. Mutations T144A and V147A resulted in defective proteins that did not confer salt tolerance when reintroduced into S. pombe. Sod2 protein with other alanine mutations in TM IV had little or no effect. T144D and T144K mutant proteins were inactive; however, a T144S protein was functional and provided lithium, but not sodium, tolerance and transport. Analysis of sensitivity to trypsin indicated that the mutations caused a conformational change in the Sod2 protein. We expressed and purified TM IV (amino acids 125-154). NMR analysis yielded a model with two helical regions (amino acids 128-142 and 147-154) separated by an unwound region (amino acids 143-146). Molecular modeling of the entire Sod2 protein suggested that TM IV has a structure similar to that deduced by NMR analysis and an overall structure similar to that of Escherichia coli NhaA. TM IV of Sod2 has similarities to TM V of the Zygosaccharomyces rouxii Na(+)/H(+) exchanger and TM VI of isoform 1 of mammalian Na(+)/H(+) exchanger. TM IV of Sod2 is critical to transport and may be involved in cation binding or conformational changes of the protein.

Salt overly sensitive pathway members are influenced by diurnal rhythm in rice

The diurnal rhythm controls many aspects of plant physiology such as flowering, photosynthesis and growth. Rice is one of the staple foods for world's population. Abiotic stresses such as salinity, drought, heat and cold severely affect rice production. Under salinity stress, maintenance of ion homeostasis is a major challenge, which also defines the tolerance level of a given genotype. Salt overly sensitive (SOS) pathway is well documented to play a key role in maintaining the Na(+) homeostasis in plant cell. However, it is not reported yet whether the transcriptional regulation of genes of this pathway are influenced by diurnal rhythm. In the present work, we have studied the diurnal pattern of transcript abundance of SOS pathway genes in rice at seedling stage.To rule out the effect of temperature fluctuations on the expression patterns of these genes, the seedlings were grown under constant temperature. We found that OsSOS3 and OsSOS2 exhibited a rhythmic and diurnal expression pattern, while OsSOS1did not have any specific pattern of expression. This analysis establishes a cross-link between diurnal rhythm and SOS pathway and suggests that SOS pathway is influenced by diurnal rhythm in rice.

Molecular characterization of a novel Na⁺/H⁺ antiporter cDNA from Eucalyptus globulus

Environmental stress factors such as salt, drought and heat are known to affect plant productivity. However, high salinity is spreading throughout the world, currently affecting more than 45 millionha. One of the mechanisms that allow plants to withstand salt stress consists on vacuolar sequestration of Na(+), through a Na(+)/H(+) antiporter. We isolated a new vacuolar Na(+)/H(+) antiporter from Eucalyptus globulus from a cDNA library. The cDNA had a 1626 bp open reading frame encoding a predicted protein of 542 amino acids with a deduced molecular weight of 59.1 KDa. Phylogenetic and bioinformatic analyses indicated that EgNHX1 localized in the vacuole. To assess its role in Na(+) exchange, we performed complementation studies using the Na(+) sensitive yeast mutant strain Δnhx1. The results showed that EgNHX1 partially restored the salt sensitive phenotype of the yeast Δnhx1 strain. However, its overexpression in transgenic Arabidopsis confers tolerance in the presence of increasing NaCl concentrations while the wild type plants exhibited growth retardation. Expression profiles of Eucalyptus seedlings subjected to salt, drought, heat and ABA treatment were established. The results revealed that Egnhx1 was induced significantly only by drought. Together, these results suggest that the product of Egnhx1 from E. globulus is a functional vacuolar Na(+)/H(+) antiporter.

PvAMT1;1, a highly selective ammonium transporter that functions as H+/NH4(+) symporter

One of the main forms of nitrogen assimilated by microorganisms and plants is ammonium, despite its toxicity at low millimolar concentrations. Ammonium absorption has been demonstrated to be carried out by highly selective plasma membrane-located transporters of the AMT/MEP/Rh family and characterized by the presence of a well conserved hydrophobic pore through which ammonia is proposed to move. However, uncertainties exist regarding the exact chemical species transported by these membrane proteins, which can be in the form of either hydrophobic ammonia or charged ammonium. Here, we present the characterization of PvAMT1;1 from the common bean and demonstrate that it mediates the high affinity (micromolar), rapidly saturating (1 mM) electrogenic transport of ammonium. Activity of the transporter is enhanced by low extracellular pH, and associated with this acidic pH stimulation are changes in the reversal potential and cytoplasm acidification, indicating that PvAMT1;1 functions as an H(+)/NH(4)(+) symporter. Mutation analysis of a unique histidine present in PvAMT1;1 (H125R) leads to the stimulation of ammonium transport by decreasing the K(m) value by half and by increasing the V(max) 3-fold, without affecting the pH dependence of the symporter. In contrast, mutation of the first conserved histidine within the channel modifies the properties of PvAMT1;1, increasing its K(m) and V(max) values and transforming it into a pH-independent mechanism.

Evidence that highly conserved residues of transmembrane segment 6 of Escherichia coli MntH are important for transport activity

Nramp (natural resistance-associated macrophage protein) family members have been characterized in mammals, yeast, and bacteria as divalent metal ion/H(+) symporters. In previous work, a bioinformatic approach was used for the identification of residues that are conserved within the Nramp family [Haemig, H. A., and Brooker, R. J. (2004) J. Membr. Biol. 201 (2), 97-107]. On the basis of site-directed mutagenesis of highly conserved negatively charged residues, a model was proposed for the metal binding site of the Escherichia coli homologue, MntH. In this study, we have focused on the highly conserved residues, including two histidines, of transmembrane segment 6 (TMS-6). Multiple mutants were made at the eight conserved sites (i.e., Gly-205, Ala-206, Met-209, Pro-210, His-211, Leu-215, His-216, and Ser-217) in TMS-6 of E. coli MntH. Double mutants involving His-211 and His-216 were also created. The results indicate the side chain volume of these residues is critically important for function. In most cases, only substitutions that are closest in side chain volume still permit transport. In addition, the K(m) for metal binding is largely unaffected by mutations in TMS-6, whereas V(max) values were decreased in all mutants characterized kinetically. Thus, these residues do not appear to play a role in metal binding. Instead, they may comprise an important face on TMS-6 that is critical for protein conformational changes during transport. Also, in contrast to other studies, our data do not strongly indicate that the conserved histidine residues play a role in the pH regulation of metal transport.

A novel plant vacuolar Na+/H+ antiporter gene evolved by DNA shuffling confers improved salt tolerance in yeast

Plant vacuolar Na(+)/H(+) antiporters play important roles in maintaining cellular ion homeostasis and mediating the transport of Na(+) out of the cytosol and into the vacuole. Vacuolar antiporters have been shown to play significant roles in salt tolerance; however the relatively low V(max) of the Na(+)/H(+) exchange of the Na(+)/H(+) antiporters identified could limit its application in the molecular breeding of salt tolerant crops. In this study, we applied DNA shuffling methodology to generate and recombine the mutations of Arabidopsis thaliana vacuolar Na(+)/H(+) antiporter gene AtNHX1. Screening using a large scale yeast complementation system identified AtNHXS1, a novel Na(+)/H(+) antiporter. Expression of AtNHXS1 in yeast showed that the antiporter localized to the vacuolar membrane and that its expression improved the tolerance of yeast to NaCl, KCl, LiCl, and hygromycin B. Measurements of the ion transport activity across the intact yeast vacuole demonstrated that the AtNHXS1 protein showed higher Na(+)/H(+) exchange activity and a slightly improved K(+)/H(+) exchange activity.

Shewanella spp. genomic evolution for a cold marine lifestyle and in-situ explosive biodegradation

Shewanella halifaxensis and Shewanella sediminis were among a few aquatic gamma-proteobacteria that were psychrophiles and the first anaerobic bacteria that degraded hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Although many mesophilic or psychrophilic strains of Shewanella and gamma-proteobacteria were sequenced for their genomes, the genomic evolution pathways for temperature adaptation were poorly understood. On the other hand, the genes responsible for anaerobic RDX mineralization pathways remain unknown. To determine the unique genomic properties of bacteria responsible for both cold-adaptation and RDX degradation, the genomes of S. halifaxensis and S. sediminis were sequenced and compared with 108 other gamma-proteobacteria including Shewanella that differ in temperature and Na+ requirements, as well as RDX degradation capability. Results showed that for coping with marine environments their genomes had extensively exchanged with deep sea bacterial genomes. Many genes for Na+-dependent nutrient transporters were recruited to use the high Na+ content as an energy source. For coping with low temperatures, these two strains as well as other psychrophilic strains of Shewanella and gamma-proteobacteria were found to decrease their genome G+C content and proteome alanine, proline and arginine content (p-value <0.01) to increase protein structural flexibility. Compared to poorer RDX-degrading strains, S. halifaxensis and S. sediminis have more number of genes for cytochromes and other enzymes related to RDX metabolic pathways. Experimentally, one cytochrome was found induced in S. halifaxensis by RDX when the chemical was the sole terminal electron acceptor. The isolated protein degraded RDX by mono-denitration and was identified as a multiheme 52 kDa cytochrome using a proteomic approach. The present analyses provided the first insight into divergent genomic evolution of bacterial strains for adaptation to the specific cold marine conditions and to the degradation of the pollutant RDX. The present study also provided the first evidence for the involvement of a specific c-type cytochrome in anaerobic RDX metabolism.

The role of annexin 1 in drought stress in Arabidopsis

Annexins act as targets of calcium signals in eukaryotic cells, and recent results suggest that they play an important role in plant stress responses. We found that in Arabidopsis (Arabidopsis thaliana), AnnAt1 (for annexin 1) mRNA levels were up-regulated in leaves by most of the stress treatments applied. Plants overexpressing AnnAt1 protein were more drought tolerant and knockout plants were more drought sensitive than ecotype Columbia plants. We also observed that hydrogen peroxide accumulation in guard cells was reduced in overexpressing plants and increased in knockout plants both before and after treatment with abscisic acid. Oxidative protection resulting from AnnAt1 overexpression could be due to the low level of intrinsic peroxidase activity exhibited by this protein in vitro, previously linked to a conserved histidine residue found in a peroxidase-like motif. However, analyses of a mutant H40A AnnAt1 protein in a bacterial complementation test and in peroxidase activity assays indicate that this residue is not critical to the ability of AnnAt1 to confer oxidative protection. To further examine the mechanism(s) linking AnnAt1 expression to stress resistance, we analyzed the reactive S3 cluster to determine if it plays a role in AnnAt1 oligomerization and/or is the site for posttranslational modification. We found that the two cysteine residues in this cluster do not form intramolecular or intermolecular bonds but are highly susceptible to oxidation-driven S-glutathionylation, which decreases the Ca(2+) affinity of AnnAt1 in vitro. Moreover, S-glutathionylation of AnnAt1 occurs in planta after abscisic acid treatment, which suggests that this modification could be important in regulating the cellular function of AnnAt1 during stress responses.

Functional importance of charged residues within the putative intracellular loops in pH regulation by Na+/ H+ exchanger NHE1

The plasma membrane Na+/H+ exchanger 1 is activated in response to various extrinsic factors, and this process is regulated by an intracellular pH-sensing mechanism. To identify the candidate residues responsible for intracellular pH regulation, we analyzed the functional properties of engineered Na+/H+ exchanger 1 mutants with charge-reversal mutations of charged residues located in the intracellular loops. Na+/H+ exchanger 1 mutants with mutations at 11 positions were well expressed in the plasma membrane, but that with E247R was not, suggesting that Glu247 is important for the functional expression of Na+/H+ exchanger 1. Charge-reversal mutations of Glu131 (E131R, E131K) and Arg327 (R327E) resulted in a shift in the intracellular pH dependence of the exchange activity measured by 22Na+ uptake to the acidic side, and it abolished the response to growth factors and a hyperosmotic medium; however, mutations of Asp448 (D448R) and Arg500 (R500E) slightly shifted it to the alkaline side. In E131R, in addition to the change in intracellular pH dependence, the affinities for extracellular Na+, Li+ and the inhibitor 5-(N-ethyl-N-isopropyl)amiloride significantly increased. Furthermore, charge-conserved mutation of E131 (E131D) was found to have no effect, whereas charge neutralization (E131Q) resulted in a slight acidic shift of exchange. These results support the view that the multiple charged residues identified in this study, along with several basic residues reported previously, participate in the regulation of the intracellular pH sensing of Na+/H+ exchanger 1. In addition, Glu131 may also be important for cation transport.

Structural and functional analysis of the Na+/H+ exchanger

The mammalian NHE (Na+/H+ exchanger) is a ubiquitously expressed integral membrane protein that regulates intracellular pH by removing a proton in exchange for an extracellular sodium ion. Of the nine known isoforms of the mammalian NHEs, the first isoform discovered (NHE1) is the most thoroughly characterized. NHE1 is involved in numerous physiological processes in mammals, including regulation of intracellular pH, cell-volume control, cytoskeletal organization, heart disease and cancer. NHE comprises two domains: an N-terminal membrane domain that functions to transport ions, and a C-terminal cytoplasmic regulatory domain that regulates the activity and mediates cytoskeletal interactions. Although the exact mechanism of transport by NHE1 remains elusive, recent studies have identified amino acid residues that are important for NHE function. In addition, progress has been made regarding the elucidation of the structure of NHEs. Specifically, the structure of a single TM (transmembrane) segment from NHE1 has been solved, and the high-resolution structure of the bacterial Na+/H+ antiporter NhaA has recently been elucidated. In this review we discuss what is known about both functional and structural aspects of NHE1. We relate the known structural data for NHE1 to the NhaA structure, where TM IV of NHE1 shows surprising structural similarity with TM IV of NhaA, despite little primary sequence similarity. Further experiments that will be required to fully understand the mechanism of transport and regulation of the NHE1 protein are discussed.

Acid DNases and their interest among apoptotic endonucleases

Apoptosis is characterized by cell shrinkage, nuclear condensation and internucleosomal DNA cleavage. Besides the central role of caspases and other proteases, cell death triggers DNA degradation so that DNases have an active role in apoptotic cell death. The best-characterized apoptotic DNase is CAD, a neutral Mg-dependent endonuclease. Its activity is regulated by its inhibitor, ICAD, which is cleaved by caspases. Other neutral DNases have been shown to cleave nuclear DNA in apoptotic conditions: endonuclease G, GADD. In cells, the cytosolic pH is maintained to 7.2, mostly due to the activity of the Na(+)/H(+) exchanger. In many apoptotic conditions, a decrease of the intracellular pH has been shown. This decrease may activate different acid DNases, mostly when pH decreases below 6.5. Three acidic DNases II are so far known: DNase II alpha, DNase II beta and L-DNase II, a DNase II, derived from the serpin LEI (Leukocyte Elastase Inhibitor). Their activation during cell death is discussed in this review.

Evidence for allosteric regulation of pH-sensitive System A (SNAT2) and System N (SNAT5) amino acid transporter activity involving a conserved histidine residue

System A and N amino acid transporters are key effectors of movement of amino acids across the plasma membrane of mammalian cells. These Na+-dependent transporters of the SLC38 gene family are highly sensitive to changes in pH within the physiological range, with transport markedly depressed at pH 7.0. We have investigated the possible role of histidine residues in the transporter proteins in determining this pH-sensitivity. The histidine-modifying agent DEPC (diethyl pyrocarbonate) markedly reduces the pH-sensitivity of SNAT2 and SNAT5 transporters (representative isoforms of System A and N respectively, overexpressed in Xenopus oocytes) in a concentration-dependent manner but does not completely inactivate transport activity. These effects of DEPC were reversed by hydroxylamine and partially blocked in the presence of excess amino acid substrate. DEPC treatment also blocked a reduction in apparent affinity for Na+ (K0.5Na+) of the SNAT2 transporter at low external pH. Mutation of the highly conserved C-terminal histidine residue to alanine in either SNAT2 (H504A) or SNAT5 (H471A) produced a transport phenotype exhibiting reduced, DEPC-resistant pH-sensitivity with no change in K0.5Na+ at low external pH. We suggest that the pH-sensitivity of these structurally related transporters results at least partly from a common allosteric mechanism influencing Na+ binding, which involves an H+-modifier site associated with C-terminal histidine residues.

Diverse functions and molecular properties emerging for CAX cation/H+ exchangers in plants

Steep concentration gradients of many ions are actively maintained, with lower concentrations typically located in the cytosol, and higher concentrations in organelles and outside the cell. The vacuole is an important storage organelle for many ions. The concentration gradient of cations is established across the plant tonoplast, in part, by high-capacity cation/H+ (CAX) exchange activity. While plants may not be green yeast, analysis of CAX regulation and substrate specificity has been greatly aided by utilizing yeast as an experimental tool. The basic CAX biology in ARABIDOPSIS has immediate relevance toward understanding the functional interplay between diverse transport processes. The long-range applied goals are to identify novel transporters and express them in crop plants in order to "mine" nutrients out of the soil and into plants. In doing so, this could boost the levels of essential nutrients in plants.

Identification of a pH sensor in the furin propeptide that regulates enzyme activation

The folding and activation of furin occur through two pH- and compartment-specific autoproteolytic steps. In the endoplasmic reticulum (ER), profurin folds under the guidance of its prodomain and undergoes an autoproteolytic excision at the consensus furin site Arg-Thr-Lys-Arg107/ generating an enzymatically masked furin-propeptide complex competent for transport to late secretory compartments. In the mildly acidic environment of the trans-Golgi network/endosomal system, the bound propeptide is cleaved at the internal site 69HRGVTKR75/, unmasking active furin capable of cleaving substrates in trans. Here, by using cellular, biochemical, and modeling studies, we demonstrate that the conserved His69 is a pH sensor that regulates the compartment-specific cleavages of the propeptide. In the ER, unprotonated His69 stabilizes a solvent-accessible hydrophobic pocket necessary for autoproteolytic excision at Arg107. Profurin molecules unable to form the hydrophobic pocket, and hence, the furin-propeptide complex, are restricted to the ER by a PACS-2- and COPI-dependent mechanism. Once exposed to the acidic pH of the late secretory pathway, protonated His69 disrupts the hydrophobic pocket, resulting in exposure and cleavage of the internal cleavage site at Arg75 to unmask the enzyme. Together, our data explain the pH-regulated activation of furin and how this His-dependent regulatory mechanism is a model for other proteins.

The conserved histidine 295 does not contribute to proton cotransport by the glutamate transporter EAAC1

Transmembrane glutamate transport by the excitatory amino acid carrier (EAAC1) is coupled to the cotransport of three Na(+) ions and one proton. Previously, we suggested that the mechanism of H(+) cotransport involves protonation of the conserved glutamate residue E373. However, it was also speculated that the cotransported proton is shared in a H(+)-binding network, possibly involving the conserved histidine 295 in the sixth transmembrane domain of EAAC1. Here, we used site-directed mutagenesis together with pre-steady-state electrophysiological analysis of the mutant transporters to test the protonation state of H295 and to determine its involvement in proton transport by EAAC1. Our results show that replacement of H295 with glutamine, an amino acid residue that cannot be protonated, generates a fully functional transporter with transport kinetics that are close to those of the wild-type EAAC1. In contrast, replacement with lysine results in a transporter in which substrate binding and translocation are dramatically inhibited. Furthermore, it is demonstrated that the effect of the histidine 295 to lysine mutation on the glutamate affinity is caused by its positive charge, since wild-type-like affinity can be restored by changing the extracellular pH to 10.0, thus partially deprotonating H295K. Together, these results suggest that histidine 295 is not protonated in EAAC1 at physiological pH and, thus, does not contribute to H(+) cotransport. This conclusion is supported by data from H295C-E373C double mutant transporters which demonstrate that these residues cannot be linked by oxidation, indicating that H295 and E373 are not close in space and do not form a proton binding network. A kinetic scheme is used to quantify the results, which includes binding of the cotransported proton to E373 and binding of a modulatory, nontransported proton to the amino acid side chain in position 295.

Metabolic interdependence of obligate intracellular bacteria and their insect hosts

Mutualistic associations of obligate intracellular bacteria and insects have attracted much interest in the past few years due to the evolutionary consequences for their genome structure. However, much less attention has been paid to the metabolic ramifications for these endosymbiotic microorganisms, which have to compete with but also to adapt to another metabolism--that of the host cell. This review attempts to provide insights into the complex physiological interactions and the evolution of metabolic pathways of several mutualistic bacteria of aphids, ants, and tsetse flies and their insect hosts.

Identification and disruption of a rhoptry-localized homologue of sodium hydrogen exchangers in Toxoplasma gondii

Na+/H+ exchangers (NHEs) are ubiquitous membrane proteins that catalyze the exchange of Na+ for H+ and are critical in pH and cell volume regulation, as well as osmotolerance. In this study, we identify and characterize a novel NHE, TgNHE2, in Toxoplasma gondii. Immunofluorescence studies show that TgNHE2 is localized to the rhoptries, secretory organelles involved in invasion. TgNHE2 is the first intracellular NHE to be characterized in a protozoan parasite and its localization suggests possible roles for the rhoptries in osmotolerance and/or as secretory lysosomes-like granules.

Molecular mechanisms of electrogenic sodium bicarbonate cotransport: structural and equilibrium thermodynamic considerations

The electrogenic Na(+)-HCO(3)(-) cotransporters play an essential role in regulating intracellular pH and extracellular acid-base homeostasis. Of the known members of the bicarbonate transporter superfamily (BTS), NBC1 and NBC4 proteins have been shown to be electrogenic. The electrogenic nature of these transporters results from the unequal coupling of anionic and cationic fluxes during each transport cycle. This unique property distinguishes NBC1 and NBC4 proteins from other sodium bicarbonate cotransporters and members of the bicarbonate transporter superfamily that are known to be electroneutral. Structure-function studies have played an essential role in revealing the basis for the modulation of the coupling ratio of NBC1 proteins. In addition, the recent transmembrane topographic analysis of pNBC1 has shed light on the potential structural determinants that are responsible for ion permeation through the cotransporter. The experimentally difficult problem of determining the nature of anionic species being transported by these proteins (HCO(3)(-) versus CO(3)(2-)) is analyzed using a theoretical equilibrium thermodynamics approach. Finally, our current understanding of the molecular mechanisms responsible for the regulation of ion coupling and flux through electrogenic sodium bicarbonate cotransporters is reviewed in detail.

Functional analysis of amino acids of the Na+/H+ exchanger that are important for proton translocation

The Na+/H+ exchanger is an integral membrane protein found in the plasma membrane of eukaryotic and prokaryotic cells. In eukaryotes it functions to exchange one proton for a sodium ion. In mammals it removes intracellular protons while in plants and fungal cells the plasma membrane form removes intracellular sodium in exchange for extracellular protons. In this study we used the Na+/H+ exchanger of Schizosaccharomyces pombe (Sod2) as a model system to study amino acids critical for activity of the protein. Twelve mutant forms of the Na+/H+ exchanger were examined for their ability to translocate protons as assessed by a Cytosensor microphysiometer. Mutation of the amino acid Histidine 367 resulted in defective proton translocation. The acidic residues Asp145, Asp178, Asp266 and Asp267 were important in the proton translocation activity of the Na+/H+ exchanger. Mutation of amino acids His98, His233 and Asp241 did not significantly impair proton translocation by the Na+/H+ exchanger. These results confirm that polar amino acids are important in proton flux activity of Na+/H+ exchangers.

Ionophore-resistant mutant of Toxoplasma gondii reveals involvement of a sodium/hydrogen exchanger in calcium regulation

Calcium is a critical mediator of many intracellular processes in eukaryotic cells. In the obligate intracellular parasite Toxoplasma gondii, for example, a rise in [Ca²⁺] is associated with significant morphological changes and rapid egress from host cells. To understand the mechanisms behind such dramatic effects, we isolated a mutant that is altered in its responses to the Ca²⁺ ionophore A23187 and found the affected gene encodes a homologue of Na⁺/H⁺ exchangers (NHEs) located on the parasite's plasma membrane. We show that in the absence of TgNHE1, Toxoplasma is resistant to ionophore-induced egress and extracellular death and amiloride-induced proton efflux inhibition. In addition, the mutant has increased levels of intracellular Ca²⁺, which explains its decreased sensitivity to A23187. These results provide direct genetic evidence of a role for NHE1 in Ca²⁺ homeostasis and important insight into how this ubiquitous pathogen senses and responds to changes in its environment.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

Na+/H+ exchange activity in the plasma membrane of Arabidopsis

In plants, Na+/H+ exchangers in the plasma membrane are critical for growth in high levels of salt, removing toxic Na+ from the cytoplasm by transport out of the cell. The molecular identity of a plasma membrane Na+/H+ exchanger in Arabidopsis (SOS1) has recently been determined. In this study, immunological analysis provided evidence that SOS1 localizes to the plasma membrane of leaves and roots. To characterize the transport activity of this protein, purified plasma membrane vesicles were isolated from leaves of Arabidopsis. Na+/H+ exchange activity, monitored as the ability of Na to dissipate an established pH gradient, was absent in plants grown without salt. However, exchange activity was induced when plants were grown in 250 mm NaCl and increased with prolonged salt exposure up to 8 d. H+-coupled exchange was specific for Na, because chloride salts of other monovalent cations did not dissipate the pH gradient. Na+/H+ exchange activity was dependent on Na (substrate) concentration, and kinetic analysis indicated that the affinity (apparent Km) of the transporter for Na+ is 22.8 mm. Data from two experimental approaches supports electroneutral exchange (one Na+ exchanged for one proton): (a) no change in membrane potential was measured during the exchange reaction, and (b) Na+/H+ exchange was unaffected by the presence or absence of a membrane potential. Results from this research provide a framework for future studies into the regulation of the plant plasma membrane Na+/H+ exchanger and its relative contribution to the maintenance of cellular Na+ homeostasis during plant growth in salt.

Regulation of SOS1, a plasma membrane Na+/H+ exchanger in Arabidopsis thaliana, by SOS2 and SOS3

Maintaining low levels of sodium ions in the cell cytosol is critical for plant growth and development. Biochemical studies suggest that Na(+)/H(+) exchangers in the plasma membrane of plant cells contribute to cellular sodium homeostasis by transporting sodium ions out of the cell; however, these exchangers have not been identified at the molecular level. Genetic analysis has linked components of the salt overly sensitive pathway (SOS1-3) to salt tolerance in Arabidopsis thaliana. The predicted SOS1 protein sequence and comparisons of sodium ion accumulation in wild-type and sos1 plants suggest that SOS1 is involved directly in the transport of sodium ions across the plasma membrane. To demonstrate the transport capability of SOS1, we studied Na(+)/H(+)-exchange activity in wild-type and sos plants using highly purified plasma membrane vesicles. The results showed that plasma membrane Na(+)/H(+)-exchange activity was present in wild-type plants treated with 250 mM NaCl, but this transport activity was reduced by 80% in similarly treated sos1 plants. In vitro addition of activated SOS2 protein (a protein kinase) increased Na(+)/H(+)-exchange activity in salt-treated wild-type plants 2-fold relative to transport without added protein. However, the addition of activated SOS2 did not have any stimulatory effect on the exchange activity in sos1 plants. Although vesicles of sos2 and sos3 plants had reduced plasma membrane Na(+)/H(+)-exchange activity, transport activity in both increased with the addition of activated SOS2 protein. These results demonstrate that SOS1 contributes to plasma membrane Na(+)/H(+) exchange and that SOS2 and SOS3 regulate SOS1 transport activity.