Escherichia coli as an expression system for K(+) transport systems from plants

Dissecting structure and function of the monovalent cation/H+ antiporters Mdm38 and Ylh47 in Saccharomyces cerevisiae

Saccharomyces cerevisiae Mdm38 and Ylh47 are homologs of the Ca2+/H+ antiporter Letm1, a candidate gene for seizures associated with Wolf-Hirschhorn syndrome in humans. Mdm38 is important for K+/H+ exchange across the inner mitochondrial membrane and contributes to membrane potential formation and mitochondrial protein translation. Ylh47 also localizes to the inner mitochondrial membrane. However, knowledge of the structures and detailed transport activities of Mdm38 and Ylh47 is limited. In this study, we conducted characterization of the ion transport activities and related structural properties of Mdm38 and Ylh47. Growth tests using Na+/H+ antiporter-deficient Escherichia coli strain TO114 showed that Mdm38 and Ylh47 had Na+ efflux activity. Measurement of transport activity across E. coli-inverted membranes showed that Mdm38 and Ylh47 had K+/H+, Na+/H+, and Li+/H+ antiport activity, but unlike Letm1, they lacked Ca2+/H+ antiport activity. Deletion of the ribosome-binding domain resulted in decreased Na+ efflux activity in Mdm38. Structural models of Mdm38 and Ylh47 identified a highly conserved glutamic acid in the pore-forming membrane-spanning region. Replacement of this glutamic acid with alanine, a non-polar amino acid, significantly impaired the ability of Mdm38 and Ylh47 to complement the salt sensitivity of E. coli TO114. These findings not only provide important insights into the structure and function of the Letm1-Mdm38-Ylh47 antiporter family but by revealing their distinctive properties also shed light on the physiological roles of these transporters in yeast and animals.

IMPORTANCE

The inner membrane of mitochondria contains numerous ion transporters, including those facilitating H+ transport by the electron transport chain and ATP synthase to maintain membrane potential. Letm1 in the inner membrane of mitochondria in animals functions as a Ca2+/H+ antiporter. However, this study reveals that homologous antiporters in mitochondria of yeast, Mdm38 and Ylh47, do not transport Ca2+ but instead are selective for K+ and Na+. Additionally, the identification of conserved amino acids crucial for antiporter activity further expanded our understanding of the structure and function of the Letm1-Mdm38-Ylh47 antiporter family.

Potassium transporter KUP9 participates in K+ distribution in roots and leaves under low K+ stress

Potassium (K) is a major essential element in plant cells, and KUP/HAK/KT-type K+ transporters participate in the absorption of K+ into roots and in the long-distance transport to above-ground parts. In Arabidopsis thaliana, KUP9 is involved in the transport of K+ and Cs+ in roots. In this study, we investigated KUP9 function in relation to the K+ status of the plant. The expression of KUP9 was upregulated in older leaves on K+-depleted medium, compared to the expression of the other 12 KUP genes in the KUP/HAK/KT family in Arabidopsis. When grown on low K+ medium, the kup9 mutant had reduced chlorophyll content in seedlings and chlorosis in older rosette leaves. Tissue-specific expression of KUP9 determined by KUP9 promoter:GUS assay depended on the K+ status of the plants: In K+ sufficient medium, KUP9 was expressed in the leaf blade towards the leaf tip, whereas in K+ depleted medium expression was mainly found in the petioles. In accordance with this, K+ accumulated in the roots of kup9 plants. The short-term 43K+ tracer measurement showed that 43K was transferred at a lower rate in roots and shoots of kup9, compared to the wild type. These data show that KUP9 participates in the distribution of K+ in leaves and K+ absorption in roots under low K+ conditions.

Analysis of Arabidopsis TPK2 and KCO3 reveals structural properties required for K+ channel function

Arabidopsis thaliana contains five tandem-pore domain potassium channels, TPK1-TPK5 and the related one-pore domain potassium channel, KCO3. Although KCO3 is unlikely to be an active channel, it still has a physiological role in plant cells. TPK2 is most similar to KCO3 and both are localized to the tonoplast. However, their function remains poorly understood. Here, taking advantage of the similarities between TPK2 and KCO3, we evaluated Ca²⁺ binding to the EF hands in TPK2, and the elements of KCO3 required for K⁺ channel activity. Presence of both EF-hand motifs in TPK2 resulted in Ca²⁺ binding, but EF1 or EF2 alone failed to interact with Ca²⁺. The EF hands were not required for K⁺ transport activity. EF1 contains two cysteines separated by two amino acids. Replacement of both cysteines with serines in TPK2 increased Ca²⁺ binding. We generated a two-pore domain chimeric K⁺ channel by replacing the missing pore region in KCO3 with a pore domain of TPK2. Alternatively, we generated two versions of simple one-pore domain K⁺ channels by removal of an extra region from KCO3. The chimera and one of the simple one-pore variants were functional channels. This strongly suggests that KCO3 is not a pseudogene and KCO3 retains components required for the formation of a functional K⁺ channel and oligomerization. Our results contribute to our understanding of the structural properties required for K⁺ channel activity.

Evidence for potassium transport activity of Arabidopsis KEA1-KEA6

Arabidopsis thaliana contains the putative K+ efflux transporters KEA1-KEA6, similar to KefB and KefC of Escherichia coli. KEA1-KEA3 are involved in the regulation of photosynthetic electron transport and chloroplast development. KEA4-KEA6 mediate pH regulation of the endomembrane network during salinity stress. However, the ion transport activities of KEA1-KEA6 have not been directly characterized. In this study, we used an E. coli expression system to examine KEA activity. KEA1-KEA3 and KEA5 showed bi-directional K+ transport activity, whereas KEA4 and KEA6 functioned as a K+ uptake system. The thylakoid membrane-localized Na+/H+ antiporter NhaS3 from the model cyanobacterium Synechocystis is the closest homolog of KEA3. Changing the putative Na+/H+ selective site of KEA3 (Gln-Asp) to that of NhaS3 (Asp-Asp) did not alter the ion selectivity without loss of K+ transport activity. The first residue in the conserved motif was not a determinant for K+ or Na+ selectivity. Deletion of the possible nucleotide-binding KTN domain from KEA3 lowered K+ transport activity, indicating that the KTN domain was important for this function. The KEA3-G422R mutation discovered in the Arabidopsis dpgr mutant increased K+ transport activity, consistent with the mutant phenotype. These results indicate that Arabidopsis KEA1-KEA6 act as K+ transport systems, and support the interpretation that KEA3 promotes dissipation of ΔpH in the thylakoid membrane.

Characterization of rice KT/HAK/KUP potassium transporters and K+ uptake by HAK1 from Oryza sativa

Plant high-affinity K⁺ (HAK) transporters are divided into four major clusters. Cluster I transporters, in particular, are thought to have high-affinity for K⁺. Of the 27 HAK genes in rice, eight HAK transporters belong to cluster I. In this study, we investigated the temporal expression patterns during K⁺ deficiency and K⁺ transport activity of these eight HAK transporters. The expression of seven HAK genes except OsHAK20 was detected. Expression of OsHAK1, OsHAK5 and OsHAK21 was induced in response to K⁺ deficiency; however, that of other genes was not. Six of the eight HAK transporters-OsHAK1, OsHAK5, OsHAK19, OsHAK20, OsHAK21, and OsHAK27-complemented the K⁺-transporter-deficient yeast or bacterial strain. Further, the yeast cells expressing OsHAK1 were more sensitive to Na⁺ than those expressing OsHAK5. Mutant analysis showed that the high-affinity K⁺ uptake activity was almost undetectable in oshak1 mutants in a low-K⁺ medium (0.02 mM). In addition, the high-affinity K⁺ uptake activity of wild-type plants was inhibited by mild salt stress (20 mM NaCl); however, Na⁺ permeability of OsHAK1 was not detected in Escherichia coli cells. The high-affinity K⁺ uptake activity by leaf blades was detected in wild-type plants, while it was not detected in oshak1 mutants. Our results suggest that OsHAK1 and OsHAK5 are the two important components of cluster I corresponding to low-K⁺ conditions, and that the transport activity of OsHAK1, unlike that of OsHAK5, is sensitive to Na⁺. Further, OsHAK1 is suggested to involve in foliar K⁺ uptake.

Protein architecture and core residues in unwound α-helices provide insights to the transport function of plant AtCHX17

Using Arabidopsis thaliana AtCHX17 as an example, we combine structural modeling and mutagenesis to provide insights on its protein architecture and transport function which is poorly characterized. This approach is based on the observation that protein structures are significantly more conserved in evolution than linear sequences, and mechanistic similarities among diverse transporters are emerging. Two homology models of AtCHX17 were obtained that show a protein fold similar to known structures of bacterial Na(+)/H(+) antiporters, EcNhaA and TtNapA. The distinct secondary and tertiary structure models highlighted residues at positions potentially important for CHX17 activity. Mutagenesis showed that asparagine-N200 and aspartate-D201 inside transmembrane5 (TM5), and lysine-K355 inside TM10 are critical for AtCHX17 activity. We reveal previously unrecognized threonine-T170 and lysine-K383 as key residues at unwound regions in the middle of TM4 and TM11 α-helices, respectively. Mutation of glutamate-E111 located near the membrane surface inhibited AtCHX17 activity, suggesting a role in pH sensing. The long carboxylic tail of unknown purpose has an alternating β-sheet and α-helix secondary structure that is conserved in prokaryote universal stress proteins. These results support the overall architecture of AtCHX17 and identify D201, N200 and novel residues T170 and K383 at the functional core which likely participates in ion recognition, coordination and/or translocation, similar to characterized cation/H(+) exchangers. The core of AtCHX17 models according to EcNhaA and TtNapA templates faces inward and outward, respectively, which may reflect two conformational states of the alternating access transport mode for proteins belonging to the plant CHX family.

Negative and positive temperature dependence of potassium leak in MscS mutants: Implications for understanding thermosensitive channels

Bacterial mechanosensitive channel of small conductance (MscS) is a protein, whose activity is modulated by membrane tension, voltage and cytoplasmic crowding. MscS is a homoheptamer and each monomer consists of three transmembrane helices (TM1-3). Hydrophobic pore of the channel is made of TM3s surrounded by peripheral TM1/2s. MscS gating is a complex process, which involves opening and inactivation in response to the increase of membrane tension. A number of MscS mutants were isolated. Among them mutants affecting gating have been found including gain-of-function (GOF) and loss-of-function (LOF) that open at lower or at higher thresholds, respectively. Previously, using an in vivo screen we isolated multiple MscS mutants that leak potassium and some of them were GOF or LOF. Here we show that for a subset of these mutants K+ leak is negatively (NTD) or positively (PTD) temperature dependent. We show that temperature reliance of these mutants does not depend on how MS gating is affected by a particular mutation. Instead, we argue that NTD or PTD leak is due to the opposite allosteric coupling of the structures that determine the temperature dependence to the channel gate. In PTD mutants an increased hydration of the pore vestibule is directly coupled to the increase in the channel conductance. In NTD mutants, at higher temperatures an increased hydration of peripheral structures leads to complete separation of TM3 and a pore collapse.

The application of the Escherichia coli giant spheroplast for drug screening with automated planar patch clamp system

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Human Kv2.1 was expressed in the inner membrane of E. coli using prokaryotic codon.

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Bacterial spheroplasts large enough for the automated patch clamp were prepared by microfluidic chips.

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Kv2.1 current was recorded from the giant spheroplast by the automated patch clamp.

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E. coli spheroplasts were used for dose–response assay of potassium channel inhibitors.

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Our system will become the simple and sensitive drug assay method anyone can use.

Kv2.1, the voltage-gated ion channel, is ubiquitously expressed in variety of tissues and dysfunction of this ion channel is responsible for multiple diseases. Electrophysiological properties of ion channels are so far characterized with eukaryotic cells using the manual patch clamp which requires skilful operators and expensive equipments. In this research, we created a simple and sensitive drug screen method using bacterial giant spheroplasts and the automated patch clamp which does not require special skills. We expressed a eukaryotic voltage-gated ion channel Kv2.1 in Escherichia coli using prokaryotic codon, and prepared giant spheroplasts large enough for the patch clamp. Human Kv2.1 currents were successfully recorded from giant spheroplasts with the automated system, and Kv2.1-expressed E. coli spheroplasts could steadily reacted to the dose–response assay with TEA and 4-AP. Collectively, our results indicate for the first time that the bacterial giant spheroplast can be applied for practical pharmaceutical assay using the automated patch clamp.

Organelle-localized potassium transport systems in plants

Some intracellular organelles found in eukaryotes such as plants have arisen through the endocytotic engulfment of prokaryotic cells. This accounts for the presence of plant membrane intrinsic proteins that have homologs in prokaryotic cells. Other organelles, such as those of the endomembrane system, are thought to have evolved through infolding of the plasma membrane. Acquisition of intracellular components (organelles) in the cells supplied additional functions for survival in various natural environments. The organelles are surrounded by biological membranes, which contain membrane-embedded K(+) transport systems allowing K(+) to move across the membrane. K(+) transport systems in plant organelles act coordinately with the plasma membrane intrinsic K(+) transport systems to maintain cytosolic K(+) concentrations. Since it is sometimes difficult to perform direct studies of organellar membrane proteins in plant cells, heterologous expression in yeast and Escherichia coli has been used to elucidate the function of plant vacuole K(+) channels and other membrane transporters. The vacuole is the largest organelle in plant cells; it has an important task in the K(+) homeostasis of the cytoplasm. The initial electrophysiological measurements of K(+) transport have categorized three classes of plant vacuolar cation channels, and since then molecular cloning approaches have led to the isolation of genes for a number of K(+) transport systems. Plants contain chloroplasts, derived from photoautotrophic cyanobacteria. A novel K(+) transport system has been isolated from cyanobacteria, which may add to our understanding of K(+) flux across the thylakoid membrane and the inner membrane of the chloroplast. This chapter will provide an overview of recent findings regarding plant organellar K(+) transport proteins.

Using heterologous expression systems to characterize potassium and sodium transport activities

The expression of plant transporters in simple well-characterized cell systems is an irreplaceable technique for gaining insights into the kinetic and energetic features of plant transporters. Among all the available expression systems, yeast cells offer the highest simplicity and have the capacity to mimic the in vivo properties of plant transporters. Here, we describe the use of yeast mutants to express K(+) and Na(+) plant transporters and discuss some experimental problems that can produce misleading results.

Plant-specific cation/H+ exchanger 17 and its homologs are endomembrane K+ transporters with roles in protein sorting

The complexity of intracellular compartments in eukaryotic cells evolved to provide distinct environments to regulate processes necessary for cell proliferation and survival. A large family of predicted cation/proton exchangers (CHX), represented by 28 genes in Arabidopsis thaliana, are associated with diverse endomembrane compartments and tissues in plants, although their roles are poorly understood. We expressed a phylogenetically related cluster of CHX genes, encoded by CHX15-CHX20, in yeast and bacterial cells engineered to lack multiple cation-handling mechanisms. Of these, CHX16-CHX20 were implicated in pH homeostasis because their expression rescued the alkaline pH-sensitive growth phenotype of the host yeast strain. A smaller subset, CHX17-CHX19, also conferred tolerance to hygromycin B. Further differences were observed in K(+)- and low pH-dependent growth phenotypes. Although CHX17 did not alter cytoplasmic or vacuolar pH in yeast, CHX20 elicited acidification and alkalization of the cytosol and vacuole, respectively. Using heterologous expression in Escherichia coli strains lacking K(+) uptake systems, we provide evidence for K(+) ((86)Rb) transport mediated by CHX17 and CHX20. Finally, we show that CHX17 and CHX20 affected protein sorting as measured by carboxypeptidase Y secretion in yeast mutants grown at alkaline pH. In plant cells, CHX20-RFP co-localized with an endoplasmic reticulum marker, whereas RFP-tagged CHX17-CHX19 co-localized with prevacuolar compartment and endosome markers. Together, these results suggest that in response to environmental cues, multiple CHX transporters differentially modulate K(+) and pH homeostasis of distinct intracellular compartments, which alter membrane trafficking events likely to be critical for adaptation and survival.

Rice sodium-insensitive potassium transporter, OsHAK5, confers increased salt tolerance in tobacco BY2 cells

Potassium ion (K(+)) plays vital roles in many aspects of cellular homeostasis including competing with sodium ion (Na(+)) during potassium starvation and salt stress. Therefore, one way to engineer plant cells with improved salt tolerance is to enhance K(+) uptake activity of the cells, while keeping Na(+) out during salt stress. Here, in search for Na(+)-insensitive K(+) transporter for this purpose, bacterial expression system was used to characterize two K(+) transporters, OsHAK2 and OsHAK5, isolated from rice (Oryza sativa cv. Nipponbare). The two OsHAK transporters are members of a KT/HAK/KUP transporter family, which is one of the major K(+) transporter families in bacteria, fungi and plants. When expressed in an Escherichia coli K(+) transport mutant strain LB2003, both OsHAK transporters rescued the growth defect in K(+)-limiting conditions by significantly increasing the K(+) content of the cells. Under the condition with a large amount of extracellular Na(+), we found that OsHAK5 functions as a Na(+)-insensitive K(+) transporter, while OsHAK2 is sensitive to extracellular Na(+) and exhibits higher Na(+) over K(+) transport activities. Moreover, constitutive expression of OsHAK5 in cultured-tobacco BY2 (Nicotiana tabacum cv. Bright Yellow 2) cells enhanced the accumulation of K(+) but not Na(+) in the cells during salt stress and conferred increased salt tolerance to the cells. Transient expression experiment indicated that OsHAK5 is localized to the plant plasma membrane. These results suggest that the plasma-membrane localized Na(+) insensitive K(+) transporters, similar to OsHAK5 identified here, could be used as a tool to enhance salt tolerance in plant cells.

Pollen tubes lacking a pair of K+ transporters fail to target ovules in Arabidopsis

Flowering plant reproduction requires precise delivery of the sperm cells to the ovule by a pollen tube. Guidance signals from female cells are being identified; however, how pollen responds to those cues is largely unknown. Here, we show that two predicted cation/proton exchangers (CHX) in Arabidopsis thaliana, CHX21 and CHX23, are essential for pollen tube guidance. Male fertility was unchanged in single chx21 or chx23 mutants. However, fertility was impaired in chx21 chx23 double mutant pollen. Wild-type pistils pollinated with a limited number of single and double mutant pollen producing 62% fewer seeds than those pollinated with chx23 single mutant pollen, indicating that chx21 chx23 pollen is severely compromised. Double mutant pollen grains germinated and grew tubes down the transmitting tract, but the tubes failed to turn toward ovules. Furthermore, chx21 chx23 pollen tubes failed to enter the micropyle of excised ovules. Green fluorescent protein-tagged CHX23 driven by its native promoter was localized to the endoplasmic reticulum of pollen tubes. CHX23 mediated K(+) transport, as CHX23 expression in Escherichia coli increased K(+) uptake and growth in a pH-dependent manner. We propose that by modifying localized cation balance and pH, these transporters could affect steps in signal reception and/or transduction that are critical to shifting the axis of polarity and directing pollen growth toward the ovule.

Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

Bacterial Trk and Ktr, fungal Trk and plant HKT form a family of membrane transporters permeable to K(+) and/or Na(+) and characterized by a common structure probably derived from an ancestral K(+) channel subunit. This transporter family, specific of non-animal cells, displays a large diversity in terms of ionic permeability, affinity and energetic coupling (H(+)-K(+) or Na(+)-K(+) symport, K(+) or Na(+) uniport), which might reflect a high need for adaptation in organisms living in fluctuating or dilute environments. Trk/Ktr/HKT transporters are involved in diverse functions, from K(+) or Na(+) uptake to membrane potential control, adaptation to osmotic or salt stress, or Na(+) recirculation from shoots to roots in plants. Structural analyses of bacterial Ktr point to multimeric structures physically interacting with regulatory subunits. Elucidation of Trk/Ktr/HKT protein structures along with characterization of mutated transporters could highlight functional and evolutionary relationships between ion channels and transporters displaying channel-like features.

A novel potassium channel in photosynthetic cyanobacteria

Elucidation of the structure-function relationship of a small number of prokaryotic ion channels characterized so far greatly contributed to our knowledge on basic mechanisms of ion conduction. We identified a new potassium channel (SynK) in the genome of the cyanobacterium Synechocystis sp. PCC6803, a photosynthetic model organism. SynK, when expressed in a K(+)-uptake-system deficient E. coli strain, was able to recover growth of these organisms. The protein functions as a potassium selective ion channel when expressed in Chinese hamster ovary cells. The location of SynK in cyanobacteria in both thylakoid and plasmamembranes was revealed by immunogold electron microscopy and Western blotting of isolated membrane fractions. SynK seems to be conserved during evolution, giving rise to a TPK (two-pore K(+) channel) family member which is shown here to be located in the thylakoid membrane of Arabidopsis. Our work characterizes a novel cyanobacterial potassium channel and indicates the molecular nature of the first higher plant thylakoid cation channel, opening the way to functional studies.

Characterization of a tobacco TPK-type K+ channel as a novel tonoplast K+ channel using yeast tonoplasts

The tonoplast K(+) membrane transport system plays a crucial role in maintaining K(+) homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K(+) channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased approximately 2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K(+) over Na(+). NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca(2+), but a cytosolic concentration of 45 microM Ca(2+) resulted in a 2-fold increase in the amplitude of the K(+) current. Acidification of the cytosol to pH 5.5 also markedly increased NtTPK1-mediated K(+) currents. These results show that NtTPK1 is a novel tonoplast K(+) channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.

The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis

Caesium (Cs(+)) is a potentially toxic mineral element that is released into the environment and taken up by plants. Although Cs(+) is chemically similar to potassium (K(+)), and much is known about K(+) transport mechanisms, it is not clear through which K(+) transport mechanisms Cs(+) is taken up by plant roots. In this study, the role of AtHAK5 in high affinity K(+) and Cs(+) uptake was characterized. It is demonstrated that AtHAK5 is localized to the plasma membrane under conditions of K(+) deprivation, when it is expressed. Growth analysis showed that AtHAK5 plays a role during severe K(+) deprivation. Under K(+)-deficient conditions in the presence of Cs(+), Arabidopsis seedlings lacking AtHAK5 had increased inhibition of root growth and lower Cs(+) accumulation, and significantly higher leaf chlorophyll concentrations than wild type. These data indicate that, in addition to transporting K(+) in planta, AtHAK5 also transports Cs(+). Further experiments showed that AtHAK5 mediated Cs(+) uptake into yeast cells and that, although the K(+) deficiency-induced expression of AtHAK5 was inhibited by low concentrations of NH(4)(+) in planta, Cs(+) uptake by yeast was stimulated by low concentrations of NH(4)(+). Interestingly, the growth of the Arabidopsis atakt1-1 mutant was more sensitive to Cs(+) than the wild type. This may be explained, in part, by increased expression of AtHAK5 in the atakt1-1 mutant. It is concluded that AtHAK5 is a root plasma membrane uptake mechanism for K(+) and Cs(+) under conditions of low K(+) availability.

Identification of mutations that alter the gating of the Escherichia coli mechanosensitive channel protein, MscK

Mechanosensitive channels allow bacteria to survive rapid increases in turgor pressure. Substantial questions remain as to how these channels sense and respond to mechanical stress. Here we describe a set of mutants with alterations in their MscK channel protein. The mutants were detected fortuitously by their enhanced ability to modify the accumulation of quinolinic acid. Some amino acid changes lie in the putative pore region of MscK, but others affect sequences that lie amino-terminal to the domain aligning with MscS. We demonstrate that the alterations in MscK cause the channel to open more frequently in the absence of excessive mechanical stress. This is manifested in changes in sensitivity to external K⁺ by cells expressing the mutant proteins. Single-channel analysis highlighted a range of gating behaviours: activation at lower pressures than the wild type, inability to achieve the fully open state or a modified requirement for K⁺. Thus, the dominant uptake phenotype of these mutants may result from a defect in their ability to regulate the gating of MscK. The locations of the substituted residues suggest that the overall gating mechanism of MscK is comparable to that of MscS, but with subtleties introduced by the additional protein sequences in MscK.

Mutation of His-157 in the second pore loop drastically reduces the activity of the Synechocystis Ktr-type transporter

Mutation of a conserved His-157 in the second pore loop of KtrB drastically reduced the activity of the K+ transporter from Synechocystis sp. strain PCC 6803. This result suggests that His-157 plays an essential role in the K+ transport activity of the transporter system.

Properties of shaker-type potassium channels in higher plants

Potassium (K(+)), the most abundant cation in biological organisms, plays a crucial role in the survival and development of plant cells, modulation of basic mechanisms such as enzyme activity, electrical membrane potentials, plant turgor and cellular homeostasis. Due to the absence of a Na(+)/K(+) exchanger, which widely exists in animal cells, K(+) channels and some type of K(+) transporters function as K(+) uptake systems in plants. Plant voltage-dependent K(+) channels, which display striking topological and functional similarities with the voltage-dependent six-transmembrane segment animal Shaker-type K(+) channels, have been found to play an important role in the plasma membrane of a variety of tissues and organs in higher plants. Outward-rectifying, inward-rectifying and weakly-rectifying K(+) channels have been identified and play a crucial role in K(+) homeostasis in plant cells. To adapt to the environmental conditions, plants must take advantage of the large variety of Shaker-type K(+) channels naturally present in the plant kingdom. This review summarizes the extensive data on the structure, function, membrane topogenesis, heteromerization, expression, localization, physiological roles and modulation of Shaker-type K(+) channels from various plant species. The accumulated results also help in understanding the similarities and differences in the properties of Shaker-type K(+) channels in plants in comparison to those of Shaker channels in animals and bacteria.

Cloning and functional characterization of a superfamily of microbial inwardly rectifying potassium channels

Our understanding of the mammalian inwardly rectifying family of K+ channels (Kir family) has recently been advanced by X-ray crystal structures of two homologous prokaryotic orthologs (KirBac1.1 and KirBac3.1). However, the functional properties of these KirBac channels are still poorly understood. To address this problem, we cloned and characterized genes encoding KirBac orthologs from a wide variety of different prokaryotes and a simple unicellular eukaryote. The functional properties of these KirBacs were then examined by growth complementation in a K+ uptake-deficient strain of Escherichia coli (TK2420). Whereas some KirBac genes exhibited robust growth complementation, others either did not complement or showed temperature-dependent complementation including KirBac1.1 and KirBac3.1. In some cases, KirBac expression was also toxic to the growth of E. coli. The KirBac family exhibited a range of sensitivity to the K+ channel blockers Ba2+ and Cs+ as well as differences in their ability to grow on very low-K+ media, thus demonstrating major differences in their permeation properties. These results reveal the existence of a functionally diverse superfamily of microbial KirBac genes and present an excellent resource for the structural and functional analysis of this class of K+ channels. Furthermore, the complementation assay used in this study provides a simple and robust method for the functional characterization of a range of prokaryotic K+ channels that are difficult to study by traditional methods.

Expression of plant cyclic nucleotide-gated cation channels in yeast

The functional properties of inwardly conducting plant cyclic nucleotide-gated cation channels (CNGCs) have not been thoroughly characterized due in part to the recalcitrance of their functional expression in heterologous systems. Here, K+ uptake-deficient mutants of yeast (trk1,2) and Escherichia coli (LB650), as well as the Ca2+-uptake yeast mutant mid1,cch1, were used for functional characterization of Arabidopsis thaliana CNGCs, with the aim of identifying some of the cultural and physiological conditions that impact on plant CNGC function in heterologous systems. Use of the Ca2+-uptake yeast mutant provided the first evidence consistent with Ca2+ conduction by the A. thaliana CNGC AtCNGC1. Expression of AtCNGC1 in LB650 demonstrated that mutants of Escherichia coli (which has no endogenous calmodulin) can also be used to study functional properties of CNGCs. Expression of AtCNGC2 and AtCNGC4 enhanced growth of trk1,2 in the presence of hygromycin; AtCNGC1 has less of an effect. Deletion of the AtCNGC1 calmodulin-binding domain enhanced growth of trk1,2 at low external K+ but not of LB650, suggesting that yeast calmodulin may bind to, and down-regulate this plant channel. In vitro binding studies confirmed this physical interaction. Northern analysis, green fluorescent protein:AtCNGC1 fusion protein expression, as well as an antibody raised against a portion of AtCNGC1, were used to monitor expression of AtCNGC1 and deletion constructs of the channel in the heterologous systems. In the presence of the activating ligand cAMP, expression of the AtCNGC1 channel with the calmodulin-binding domain deleted increased intracellular [K+] of trk1,2. Trk1,2 is hypersensitive to the toxic cations spermine, tetramethylamine, and NH4+. These compounds, as well as amiloride, inhibited trk1,2 growth and thereby improved the efficacy of this yeast mutant as a heterologous expression system for CNGCs. In addition to characterizing mutants of yeast and E. coli as assay systems for plant CNGCs, work presented in this report demonstrates, for the first time, that a plant CNGC can retain ion channel function despite (partial) deletion of its calmodulin-binding domain and that yeast calmodulin can bind to and possibly down-regulate a plant CNGC.

Enhanced salt tolerance mediated by AtHKT1 transporter-induced Na unloading from xylem vessels to xylem parenchyma cells

AtHKT1 is a sodium (Na+) transporter that functions in mediating tolerance to salt stress. To investigate the membrane targeting of AtHKT1 and its expression at the translational level, antibodies were generated against peptides corresponding to the first pore of AtHKT1. Immunoelectron microscopy studies using anti-AtHKT1 antibodies demonstrate that AtHKT1 is targeted to the plasma membrane in xylem parenchyma cells in leaves. AtHKT1 expression in xylem parenchyma cells was also confirmed by AtHKT1 promoter-GUS reporter gene analyses. Interestingly, AtHKT1 disruption alleles caused large increases in the Na+ content of the xylem sap and conversely reduced the Na+ content of the phloem sap. The athkt1 mutant alleles had a smaller and inverse influence on the potassium (K+) content compared with the Na+ content of the xylem, suggesting that K+ transport may be indirectly affected. The expression of AtHKT1 was modulated not only by the concentrations of Na+ and K+ but also by the osmolality of non-ionic compounds. These findings show that AtHKT1 selectively unloads sodium directly from xylem vessels to xylem parenchyma cells. AtHKT1 mediates osmolality balance between xylem vessels and xylem parenchyma cells under saline conditions. Thus AtHKT1 reduces the sodium content in xylem vessels and leaves, thereby playing a central role in protecting plant leaves from salinity stress.

Na+-dependent K+ uptake Ktr system from the cyanobacterium Synechocystis sp. PCC 6803 and its role in the early phases of cell adaptation to hyperosmotic shock

Transmembrane ion transport processes play a key role in the adaptation of cells to hyperosmotic conditions. Previous work has shown that the disruption of a ktrB/ntpJ-like putative Na(+)/K(+) transporter gene in the cyanobacterium Synechocystis sp. PCC 6803 confers increased Na(+) sensitivity, and inhibits HCO(3)(-) uptake. Here, we report on the mechanistic basis of this effect. Heterologous expression experiments in Escherichia coli show that three Synechocystis genes are required for K(+) transport activity. They encode an NAD(+)-binding peripheral membrane protein (ktrA; sll0493), an integral membrane protein, belonging to a superfamily of K(+) transporters (ktrB; formerly ntpJ; slr1509), and a novel type of ktr gene product, not previously found in Ktr systems (ktrE; slr1508). In E. coli, Synechocystis KtrABE-mediated K(+) uptake occurred with a moderately high affinity (K(m) of about 60 microm), and depended on both Na(+) and a high membrane potential, but not on ATP. KtrABE neither mediated Na(+) uptake nor Na(+) efflux. In Synechocystis sp. PCC 6803, KtrB-mediated K(+) uptake required Na(+) and was inhibited by protonophore. A Delta ktrB strain was sensitive to long term hyperosmotic stress elicited by either NaCl or sorbitol. Hyperosmotic shock led initially to loss of net K(+) from the cells. The Delta ktrB cells shocked with sorbitol failed to reaccumulate K(+) up to its original level. These data indicate that in strain PCC 6803 K(+) uptake via KtrABE plays a crucial role in the early phase of cell turgor regulation after hyperosmotic shock.

Addition of a peptide tag at the C terminus of AtHKT1 inhibits its Na+ transport

Eight transmembrane segments in the Arabidopsis Na+ transporter, AtHKT1, have been proposed to be arranged around the axis of the pore. We report here that the addition of a GFP or a FLAG tag to the C terminus of AtHKT1 impairs the Na+ transport in Xenopus ooytes as detected by electrophysiological measurements.

A nodule-specific dicarboxylate transporter from alder is a member of the peptide transporter family

Alder (Alnus glutinosa) and more than 200 angiosperms that encompass 24 genera are collectively called actinorhizal plants. These plants form a symbiotic relationship with the nitrogen-fixing actinomycete Frankia strain HFPArI3. The plants provide the bacteria with carbon sources in exchange for fixed nitrogen, but this metabolite exchange in actinorhizal nodules has not been well defined. We isolated an alder cDNA from a nodule cDNA library by differential screening with nodule versus root cDNA and found that it encoded a transporter of the PTR (peptide transporter) family, AgDCAT1. AgDCAT1 mRNA was detected only in the nodules and not in other plant organs. Immunolocalization analysis showed that AgDCAT1 protein is localized at the symbiotic interface. The AgDCAT1 substrate was determined by its heterologous expression in two systems. Xenopus laevis oocytes injected with AgDCAT1 cRNA showed an outward current when perfused with malate or succinate, and AgDCAT1 was able to complement a dicarboxylate uptake-deficient Escherichia coli mutant. Using the E. coli system, AgDCAT1 was shown to be a dicarboxylate transporter with a K(m) of 70 microm for malate. It also transported succinate, fumarate, and oxaloacetate. To our knowledge, AgDCAT1 is the first dicarboxylate transporter to be isolated from the nodules of symbiotic plants, and we suggest that it may supply the intracellular bacteria with dicarboxylates as carbon sources.

MjK1, a K+ channel from M. jannaschii, mediates K+ uptake and K+ sensitivity in E. coli

The methanogenic and hyperthermophilic deep-sea archaeon Methanococcus jannaschii has three putative K+ channels, MVP (Mj0139), MjK1 (Mj0138.1) and MjK2 (Mj1357). The physiological function of these K+ channels was examined in a viability assay, using the Escherichia coli mutant LB2003 (kup1, DeltakdpABC5, DeltatrkA). While MjK2 expression had no effects on the potassium-dependent phenotype of LB2003, MVP and MjK1 complemented the deficiency at a concentration of 1 mM KCl. In contrast to KcsA, MthK and MVP, MjK1 strongly affected host cell viability at 10 and 100 mM KCl. The toxic effects were less pronounced when growth media were supplemented with the K+ channel blocker BaCl2.

Molecular dissection of the contribution of negatively and positively charged residues in S2, S3, and S4 to the final membrane topology of the voltage sensor in the K+ channel, KAT1

Voltage-dependent ion channels control changes in ion permeability in response to membrane potential changes. The voltage sensor in channel proteins consists of the highly positively charged segment, S4, and the negatively charged segments, S2 and S3. The process involved in the integration of the protein into the membrane remains to be elucidated. In this study, we used in vitro translation and translocation experiments to evaluate interactions between residues in the voltage sensor of a hyperpolarization-activated potassium channel, KAT1, and their effect on the final topology in the endoplasmic reticulum (ER) membrane. A D95V mutation in S2 showed less S3-S4 integration into the membrane, whereas a D105V mutation allowed S4 to be released into the ER lumen. These results indicate that Asp(95) assists in the membrane insertion of S3-S4 and that Asp(105) helps in preventing S4 from being releasing into the ER lumen. The charge reversal mutation, R171D, in S4 rescued the D105R mutation and prevented S4 release into the ER lumen. A series of constructs containing different C-terminal truncations of S4 showed that Arg(174) was required for correct integration of S3 and S4 into the membrane. Interactions between Asp(105) and Arg(171) and between negative residues in S2 or S3 and Arg(174) may be formed transiently during membrane integration. These data clarify the role of charged residues in S2, S3, and S4 and identify posttranslational electrostatic interactions between charged residues that are required to achieve the correct voltage sensor topology in the ER membrane.

Requirement of negative residues, Asp 95 and Asp 105, in S2 on membrane integration of a voltage-dependent K+ channel, KAT1

Voltage-dependent K+ channels consist of a voltage-sensing region and a pore-forming region. Here we have identified the negative residues of the second transmembrane segment in the plant voltage-dependent K+ channel, KAT1, which involves the function of voltage sensing. Point mutations at D95 and D105 but not D89 and D116 failed to complement the K+ uptake deficient properties of the mutant yeast. In vitro translation and translocation experiments showed that the membrane integration of the third and fourth segments involving voltage sensor were impaired by the replacement of D95 or D105 by serine. These data show that both the residues play a crucial role in the membrane topogenesis of the voltage sensor in KAT1.

Specific lipid requirements of membrane proteins--a putative bottleneck in heterologous expression

Membrane proteins are mostly protein-lipid complexes. For more than 30 examples of membrane proteins from prokaryotes, yeast, plant and mammals, the importance of phospholipids and sterols for optimal activity is documented. All crystallized membrane protein complexes show defined lipid-protein contacts. In addition, lipid requirements may also be transitory and necessary only for correct folding and intercellular transport. With respect to specific lipid requirements of membrane proteins, the phospholipid and glycolipid as well as the sterol content of the host cell chosen for heterologous expression should be carefully considered. The lipid composition of bacteria, archaea, yeasts, insects,Xenopus oocytes, and typical plant and mammalian cells are given in this review. A few examples of heterologous expression of membrane proteins, where problems of specific lipid requirements have been noticed or should be thought of, have been chosen.

Integration of Shaker-type K+ channel, KAT1, into the endoplasmic reticulum membrane: synergistic insertion of voltage-sensing segments, S3-S4, and independent insertion of pore-forming segments, S5-P-S6

KAT1 is a member of the Shaker family of voltage-dependent K(+) channels, which has six transmembrane segments (called S1-S6), including an amphipathic S4 with several positively charged residues and a hydrophobic pore-forming region (called P) between S5 and S6. In this study, we systematically evaluated the function of individual and combined transmembrane segments of KAT1 to direct the final topology in the endoplasmic reticulum membrane by in vitro translation and translocation experiments. The assay with single-transmembrane constructs showed that S1 possesses the type II signal-anchor function, whereas S2 has the stop-transfer function. The properties fit well with the results derived from combined insertion of S1 and S2. S3 and S4 failed to integrate into the membrane by themselves. The inserted glycosylation sequence at the S3-S4 loop neither prevented the translocation of S3 and S4 nor impaired the function of voltage-dependent K(+) transport regardless of the changed length of the S3-S4 loop. S3 and S4 are likely to be posttranslationally integrated into the membrane only when somewhat specific interaction occurs between them. S5 had the ability of translocation reinitiation, and S6 had a strong preference for N(exo)/C(cyt) orientation. The pore region resided outside because of its lack of its transmembrane-spanning property. According to their own topogenic function, combined constructs of S5-P-S6 conferred the membrane-pore-membrane topology. This finding supports the notion that a set of S5-P-S6 can be independently integrated into the membrane. The results in this study provide the fundamental topogenesis mechanism of transmembrane segments involving voltage sensor and pore region in KAT1.