Non-autonomous stomatal control by pavement cell turgor via the K+ channel subunit AtKC1

Stomata optimize land plants' photosynthetic requirements and limit water vapor loss. So far, all of the molecular and electrical components identified as regulating stomatal aperture are produced, and operate, directly within the guard cells. However, a completely autonomous function of guard cells is inconsistent with anatomical and biophysical observations hinting at mechanical contributions of epidermal origins. Here, potassium (K+) assays, membrane potential measurements, microindentation, and plasmolysis experiments provide evidence that disruption of the Arabidopsis thaliana K+ channel subunit gene AtKC1 reduces pavement cell turgor, due to decreased K+ accumulation, without affecting guard cell turgor. This results in an impaired back pressure of pavement cells onto guard cells, leading to larger stomatal apertures. Poorly rectifying membrane conductances to K+ were consistently observed in pavement cells. This plasmalemma property is likely to play an essential role in K+ shuttling within the epidermis. Functional complementation reveals that restoration of the wild-type stomatal functioning requires the expression of the transgenic AtKC1 at least in the pavement cells and trichomes. Altogether, the data suggest that AtKC1 activity contributes to the building of the back pressure that pavement cells exert onto guard cells by tuning K+ distribution throughout the leaf epidermis.

Mapping the cellular response to small molecules using chemogenomic fitness signatures

Genome-wide characterization of the in vivo cellular response to perturbation is fundamental to understanding how cells survive stress. Identifying the proteins and pathways perturbed by small molecules affects biology and medicine by revealing the mechanisms of drug action. We used a yeast chemogenomics platform that quantifies the requirement for each gene for resistance to a compound in vivo to profile 3250 small molecules in a systematic and unbiased manner. We identified 317 compounds that specifically perturb the function of 121 genes and characterized the mechanism of specific compounds. Global analysis revealed that the cellular response to small molecules is limited and described by a network of 45 major chemogenomic signatures. Our results provide a resource for the discovery of functional interactions among genes, chemicals, and biological processes.

AtKC1 is a general modulator of Arabidopsis inward Shaker channel activity

A functional Shaker potassium channel requires assembly of four α-subunits encoded by a single gene or various genes from the Shaker family. In Arabidopsis thaliana, AtKC1, a Shaker α-subunit that is silent when expressed alone, has been shown to regulate the activity of AKT1 by forming heteromeric AtKC1-AKT1 channels. Here, we investigated whether AtKC1 is a general regulator of channel activity. Co-expression in Xenopus oocytes of a dominant negative (pore-mutated) AtKC1 subunit with the inward Shaker channel subunits KAT1, KAT2 or AKT2, or the outward subunits SKOR or GORK, revealed that the three inward subunits functionally interact with AtKC1 while the outward ones cannot. Localization experiments in plant protoplasts showed that KAT2 was able to re-locate AtKC1 fused to GFP from endomembranes to the plasma membrane, indicating that heteromeric AtKC1-KAT2 channels are efficiently targeted to the plasma membrane. Functional properties of heteromeric channels involving AtKC1 and KAT1, KAT2 or AKT2 were analysed by voltage clamp after co-expression of the respective subunits in Xenopus oocytes. AtKC1 behaved as a regulatory subunit within the heterotetrameric channel, reducing the macroscopic conductance and negatively shifting the channel activation potential. Expression studies showed that AtKC1 and its identified Shaker partners have overlapping expression patterns, supporting the hypothesis of a general regulation of inward channel activity by AtKC1 in planta. Lastly, AtKC1 disruption appeared to reduce plant biomass production, showing that AtKC1-mediated channel activity regulation is required for normal plant growth.

A phosphorylation in the c-terminal auto-inhibitory domain of the plant plasma membrane H+-ATPase activates the enzyme with no requirement for regulatory 14-3-3 proteins

The plant plasma membrane H(+)-ATPase is regulated by an auto-inhibitory C-terminal domain that can be displaced by phosphorylation of the penultimate residue, a Thr, and the subsequent binding of 14-3-3 proteins. By mass spectrometric analysis of plasma membrane H(+)-ATPase isoform 2 (PMA2) isolated from Nicotiana tabacum plants and suspension cells, we identified a new phosphorylation site, Thr-889, in a region of the C-terminal domain upstream of the 14-3-3 protein binding site. This residue was mutated into aspartate or alanine, and the mutated H(+)-ATPases expressed in the yeast Saccharomyces cerevisiae. Unlike wild-type PMA2, which could replace the yeast H(+)-ATPases, the PMA2-Thr889Ala mutant did not allow yeast growth, whereas the PMA2-Thr889Asp mutant resulted in improved growth and increased H(+)-ATPase activity despite reduced phosphorylation of the PMA2 penultimate residue and reduced 14-3-3 protein binding. To determine whether the regulation taking place at Thr-889 was independent of phosphorylation of the penultimate residue and 14-3-3 protein binding, we examined the effect of combining the PMA2-Thr889Asp mutation with mutations of other residues that impair phosphorylation of the penultimate residue and/or binding of 14-3-3 proteins. The results showed that in yeast, PMA2 Thr-889 phosphorylation could activate H(+)-ATPase if PMA2 was also phosphorylated at its penultimate residue. However, binding of 14-3-3 proteins was not required, although 14-3-3 binding resulted in further activation. These results were confirmed in N. tabacum suspension cells. These data define a new H(+)-ATPase activation mechanism that can take place without 14-3-3 proteins.

The proteome complement of Nicotiana tabacum Bright-Yellow-2 culture cells

The Nicotiana tabacum Bright-Yellow-2 (BY2) cell line is one of most commonly used plant suspension cell lines and offers interesting properties, such as fast growth, amenability to genetic transformation, and synchronization of cell division. To build a proteome reference map of BY2 cell proteins, we isolated the soluble proteins from N. tabacum BY2 cells at the end of the exponential growth phase and analyzed them by 2-DE and MALDI TOF-TOF. Of the 1422 spots isolated, 795 were identified with a significant score, corresponding to 532 distinct proteins.

Activation of the plasma membrane H (+) -ATPase by acid stress: antibodies as a tool to follow the phosphorylation status of the penultimate activating Thr

Tight regulation of the plasma membrane proton pump ATPase (H (+) -ATPase) is necessary for controlling the membrane potential that energizes secondary transporters. This regulation relies on the phosphorylation of the H (+) -ATPase penultimate residue, a theonine, and the subsequent binding of regulatory 14-3-3 proteins, which results in enzyme activation. Using phospho-specific antibodies directed against the phosphorylable Thr of either PMA2 (Plasma membrane H (+) -ATPase from N. plumbaginifolia) or PMA4, we showed that the kinetics and extent of phosphorylation differ between both isoforms according to the growth or environmental conditions like cold stress. (1) Here, we used phospho-specific antibodies to follow PMA2 Thr phosphorylation upon acidification of the cytosol by incubating N. tabacum BY2 cells with four different weak organic acids. Increased PMA2 phosphorylation was observed for three of them, thus highlighting the role of the H (+) -ATPase in cell pH homeostasis.

Two widely expressed plasma membrane H(+)-ATPase isoforms of Nicotiana tabacum are differentially regulated by phosphorylation of their penultimate threonine

The plasma membrane H(+)-ATPases PMA2 and PMA4 are the most widely expressed in Nicotiana plumbaginifolia, and belong to two different subfamilies. Both are activated by phosphorylation of a Thr at the penultimate position and the subsequent binding of 14-3-3 proteins. Their expression in Saccharomyces cerevisiae revealed functional and regulatory differences. To determine whether different regulatory properties between PMA2 and PMA4 exist in plants, we generated two monoclonal antibodies able to detect phosphorylation of the penultimate Thr of either PMA2 or PMA4 in a total protein extract. We also raised Nicotiana tabacum transgenic plants expressing 6-His-tagged PMA2 or PMA4, enabling their individual purification. Using these tools we showed that phosphorylation of the penultimate Thr of both PMAs was high during the early exponential growth phase of an N. tabacum cell culture, and then progressively declined. This decline correlated with decreased 14-3-3 binding and decreased plasma membrane ATPase activity. However, the rate and extent of the decrease differed between the two isoforms. Cold stress of culture cells or leaf tissues reduced the Thr phosphorylation of PMA2, whereas no significant changes in Thr phosphorylation of PMA4 were seen. These results strongly suggest that PMA2 and PMA4 are differentially regulated by phosphorylation. Analysis of the H(+)-ATPase phosphorylation status in leaf tissues indicated that no more than 44% (PMA2) or 32% (PMA4) was in the activated state under normal growth conditions. Purification of either isoform showed that, when activated, the two isoforms did not form hetero-oligomers, which is further support for these two H(+)-ATPase subfamilies having different properties.

Activation of plant plasma membrane H+-ATPase by 14-3-3 proteins is negatively controlled by two phosphorylation sites within the H+-ATPase C-terminal region

The proton pump ATPase (H(+)-ATPase) of the plant plasma membrane is regulated by an autoinhibitory C-terminal domain, which can be displaced by phosphorylation of the penultimate Thr residue and the subsequent binding of 14-3-3 proteins. We performed a mass spectrometric analysis of PMA2 (plasma membrane H(+)-ATPase isoform 2) isolated from Nicotiana tabacum suspension cells and identified two new phosphorylated residues in the enzyme 14-3-3 protein binding site: Thr(931) and Ser(938). When PMA2 was expressed in Saccharomyces cerevisiae, mutagenesis of each of these two residues into Asp prevented growth of a yeast strain devoid of its own H(+)-ATPases. When the Asp mutations were individually introduced in a constitutively activated mutant of PMA2 (E14D), they still allowed yeast growth but at a reduced rate. Purification of His-tagged PMA2 showed that the T931D or S938D mutation prevented 14-3-3 protein binding, although the penultimate Thr(955) was still phosphorylated, indicating that Thr(955) phosphorylation is not sufficient for full enzyme activation. Expression of PMA2 in an N. tabacum cell line also showed an absence of 14-3-3 protein binding resulting from the T931D or S938D mutation. Together, the data show that activation of H(+)-ATPase by the binding of 14-3-3 proteins is negatively controlled by phosphorylation of two residues in the H(+)-ATPase 14-3-3 protein binding site. The data also show that phosphorylation of the penultimate Thr and 14-3-3 binding each contribute in part to H(+)-ATPase activation.

Heteromerization of Arabidopsis Kv channel alpha-subunits: Data and prospects

Potassium translocation in plants is accomplished by a large variety of transport systems. Most of the available molecular information on these proteins concerns voltage-gated potassium channels (Kv channels). The Arabidopsis genome comprises nine genes encoding alpha-subunits of Kv channels. Based on knowledge of their animal homologues, and on biochemical investigations, it is broadly admitted that four such polypeptides must assemble to yield a functional Kv channel. The intrinsic functional properties of Kv channel alpha-subunits have been described by expressing them in suitable heterologous contexts where homo-tetrameric channels could be characterized. However, due to the high similarity of both the polypeptidic sequence and the structural scheme of Kv channel alpha-subunits, formation of heteromeric Kv channels by at least two types of alpha-subunits is conceivable. Several examples of such heteromeric plant Kv channels have been studied in heterologous expression systems and evidence that heteromerization actually occurs in planta has now been published. It is therefore challenging to uncover the physiological role of this heteromerization. Fine tuning of Kv channels by heteromerisation could be relevant not only to potassium transport but also to electrical signaling within the plant.

The plant plasma membrane proton pump ATPase: a highly regulated P-type ATPase with multiple physiological roles

Around 40 P-type ATPases have been identified in Arabidopsis and rice, for which the genomes are known. None seems to exchange sodium and potassium, as does the animal Na(+)/K(+)-ATPase. Instead, plants, together with fungi, possess a proton pumping ATPase (H(+)-ATPase), which couples ATP hydrolysis to proton transport out of the cell, and so establishes an electrochemical gradient across the plasma membrane, which is dissipated by secondary transporters using protons in symport or antiport, as sodium is used in animal cells. Additional functions, such as stomata opening, cell growth, and intracellular pH homeostasis, have been proposed. Crystallographic data and homology modeling suggest that the H(+)-ATPase has a broadly similar structure to the other P-type ATPases but has an extended C-terminal region, which is involved in enzyme regulation. Phosphorylation of the penultimate residue, a Thr, and the subsequent binding of regulatory 14-3-3 proteins result in the formation of a dodecamer (six H(+)-ATPase and six 14-3-3 molecules) and enzyme activation. This type of regulation is unique to the P-type ATPase family. However, the recent identification of additional phosphorylated residues suggests further regulatory features.

AtKC1, a conditionally targeted Shaker-type subunit, regulates the activity of plant K+ channels

Amongst the nine voltage-gated K(+) channel (Kv) subunits expressed in Arabidopsis, AtKC1 does not seem to form functional Kv channels on its own, and is therefore said to be silent. It has been proposed to be a regulatory subunit, and to significantly influence the functional properties of heteromeric channels in which it participates, along with other Kv channel subunits. The mechanisms underlying these properties of AtKC1 remain unknown. Here, the transient (co-)expression of AtKC1, AKT1 and/or KAT1 genes was obtained in tobacco mesophyll protoplasts, which lack endogenous inward Kv channel activity. Our experimental conditions allowed both localization of expressed polypeptides (GFP-tagging) and recording of heterologously expressed Kv channel activity (untagged polypeptides). It is shown that AtKC1 remains in the endoplasmic reticulum unless it is co-expressed with AKT1. In these conditions heteromeric AtKC1-AKT1 channels are obtained, and display functional properties different from those of homomeric AKT1 channels in the same context. In particular, the activation threshold voltage of the former channels is more negative than that of the latter ones. Also, it is proposed that AtKC1-AKT1 heterodimers are preferred to AKT1-AKT1 homodimers during the process of tetramer assembly. Similar results are obtained upon co-expression of AtKC1 with KAT1. The whole set of data provides evidence that AtKC1 is a conditionally-targeted Kv subunit, which probably downregulates the physiological activity of other Kv channel subunits in Arabidopsis.

Expression of a constitutively activated plasma membrane H+-ATPase alters plant development and increases salt tolerance

The plasma membrane proton pump ATPase (H(+)-ATPase) plays a major role in the activation of ion and nutrient transport and has been suggested to be involved in several physiological processes, such as cell expansion and salt tolerance. Its activity is regulated by a C-terminal autoinhibitory domain that can be displaced by phosphorylation and the binding of regulatory 14-3-3 proteins, resulting in an activated enzyme. To better understand the physiological consequence of this activation, we have analyzed transgenic tobacco (Nicotiana tabacum) plants expressing either wild-type plasma membrane H(+)-ATPase4 (wtPMA4) or a PMA4 mutant lacking the autoinhibitory domain (DeltaPMA4), generating a constitutively activated enzyme. Plants showing 4-fold higher expression of wtPMA4 than untransformed plants did not display any unusual phenotype and their leaf and root external acidification rates were not modified, while their in vitro H(+)-ATPase activity was markedly increased. This indicates that, in vivo, H(+)-ATPase overexpression is compensated by down-regulation of H(+)-ATPase activity. In contrast, plants that expressed DeltaPMA4 were characterized by a lower apoplastic and external root pH, abnormal leaf inclination, and twisted stems, suggesting alterations in cell expansion. This was confirmed by in vitro leaf extension and curling assays. These data therefore strongly support a direct role of H(+)-ATPase in plant development. The DeltaPMA4 plants also displayed increased salt tolerance during germination and seedling growth, supporting the hypothesis that H(+)-ATPase is involved in salt tolerance.

Structure of a 14-3-3 coordinated hexamer of the plant plasma membrane H+ -ATPase by combining X-ray crystallography and electron cryomicroscopy

Regulatory 14-3-3 proteins activate the plant plasma membrane H(+)-ATPase by binding to its C-terminal autoinhibitory domain. This interaction requires phosphorylation of a C-terminal, mode III, recognition motif as well as an adjacent span of approximately 50 amino acids. Here we report the X-ray crystal structure of 14-3-3 in complex with the entire binding motif, revealing a previously unidentified mode of interaction. A 14-3-3 dimer simultaneously binds two H(+)-ATPase peptides, each of which forms a loop within the typical 14-3-3 binding groove and therefore exits from the center of the dimer. Several H(+)-ATPase mutants support this structure determination. Accordingly, 14-3-3 binding could result in H(+)-ATPase oligomerization. Indeed, by using single-particle electron cryomicroscopy, the 3D reconstruction of the purified H(+)-ATPase/14-3-3 complex demonstrates a hexameric arrangement. Fitting of 14-3-3 and H(+)-ATPase atomic structures into the 3D reconstruction map suggests the spatial arrangement of the holocomplex.

A procedure for localisation and electrophysiological characterisation of ion channels heterologously expressed in a plant context

BACKGROUND

In silico analyses based on sequence similarities with animal channels have identified a large number of plant genes likely to encode ion channels. The attempts made to characterise such putative plant channels at the functional level have most often relied on electrophysiological analyses in classical expression systems, such as Xenopus oocytes or mammalian cells. In a number of cases, these expression systems have failed so far to provide functional data and one can speculate that using a plant expression system instead of an animal one might provide a more efficient way towards functional characterisation of plant channels, and a more realistic context to investigate regulation of plant channels.

RESULTS

With the aim of developing a plant expression system readily amenable to electrophysiological analyses, we optimised experimental conditions for preparation and transformation of tobacco mesophyll protoplasts and engineered expression plasmids, that were designed to allow subcellular localisation and functional characterisation of ion channels eventually in presence of their putative (possibly over-expressed) regulatory partners. Two inward K+ channels from the Shaker family were functionally expressed in this system: not only the compliant KAT1 but also the recalcitrant AKT1 channel, which remains electrically silent when expressed in Xenopus oocytes or in mammalian cells.

CONCLUSION

The level of endogenous currents in control protoplasts seems compatible with the use of the described experimental procedures for the characterisation of plant ion channels, by studying for instance their subcellular localisation, functional properties, structure-function relationships, interacting partners and regulation, very likely in a more realistic context than the classically used animal systems.

Targeting of a Nicotiana plumbaginifolia H+ -ATPase to the plasma membrane is not by default and requires cytosolic structural determinants

The structural determinants involved in the targeting of multitransmembrane-span proteins to the plasma membrane (PM) remain poorly understood. The plasma membrane H+ -ATPase (PMA) from Nicotiana plumbaginifolia, a well-characterized 10 transmembrane-span enzyme, was used as a model to identify structural elements essential for targeting to the PM. When PMA2 and PMA4, representatives of the two main PMA subfamilies, were fused to green fluorescent protein (GFP), the chimeras were shown to be still functional and to be correctly and rapidly targeted to the PM in transgenic tobacco. By contrast, chimeric proteins containing various combinations of PMA transmembrane spanning domains accumulated in the Golgi apparatus and not in the PM and displayed slow traffic properties through the secretory pathway. Individual deletion of three of the four cytosolic domains did not prevent PM targeting, but deletion of the large loop or of its nucleotide binding domain resulted in GFP fluorescence accumulating exclusively in the endoplasmic reticulum. The results show that, at least for this polytopic protein, the PM is not the default pathway and that, in contrast with single-pass membrane proteins, cytosolic structural determinants are required for correct targeting.

A non-essential function for yeast frataxin in iron-sulfur cluster assembly

Friedreich's ataxia is caused by a deficit in frataxin, a small mitochondrial protein of unknown function that has been conserved during evolution. Previous studies have pointed out a role for frataxin in mitochondrial iron-sulfur (Fe-S) metabolism. Here, we have analyzed the incorporation of Fe-S clusters into yeast ferredoxin imported into isolated energized mitochondria from cells grown in the presence of glycerol, an obligatory respiratory carbon source. Similar amounts of apo-ferredoxin precursor were imported into mitochondria and processed in wild-type and yfh1-deleted (delta YF111) strains. However, the incorporation of Fe-S clusters into apo-ferredoxin was significantly reduced in delta YFH1 mitochondria. The newly assembled ferredoxin was stable, excluding the possibility that the decreased incorporation was a result of increased oxidative damage. When delta YFH1 cells were grown in raffinose medium, the formation of holo-ferredoxin was low, as a consequence of the decrease in ferredoxin precursor import into mitochondria. However, the decrease in the conversion rate of apo- into holo-ferredoxin was in the same range as for glycerol-grown cells, indicating that the extent of the defect in Fe-S protein assembly is similar under different physiological conditions. These data show that frataxin is not essential for Fe-S protein assembly, but improves the efficiency of the process. The large variations observed in the activity of Fe-S cluster proteins under different physiological conditions result from secondary defects in the physiology of delta YFH1 cells.

Structure requirement and identification of a cryptic cleavage site in the mitochondrial processing of a plant F1-ATPase beta-subunit presequence

We sought to determine the structural features involved in the processing of the mitochondrial F1-ATPase beta-subunit (F1beta) presequence (54 residues) from Nicotiana plumbaginifolia. The cleavage efficiency of F1beta presequence mutants linked to the green fluorescent protein (GFP) was evaluated in vivo in tobacco by in situ microscopy and Western blotting. The residue at position -1 (Tyr) was required to be an aromatic residue and the residue at position +2 (Thr) was found to be important for F1beta processing, while, unexpectedly, changing the distal (Arg-15) and proximal (Arg-5) arginine residues did not strongly reduce processing. In addition, results also supported the requirement of a helical structure around the cleavage position. Sequencing of the mature form of a precursor containing the first 30 residues of the F1beta presequence linked to GFP revealed the presence of a cryptic cleavage site between residues 26 and 27, which showed the features of a classical mitochondrial processing site, suggesting dual processing of the F1beta presequence. In vitro processing confirmed these data and showed that processing was sensitive to o-phenanthroline, thus catalyzed by mitochondrial processing peptidase.

Hydrophobic residues within the predicted N-terminal amphiphilic alpha-helix of a plant mitochondrial targeting presequence play a major role in in vivo import

A deletion and mutagenesis study was performed on the mitochondrial presequence of the beta-subunit of the F(1)-ATP synthase from Nicotiana plumbaginifolia linked to the green fluorescent protein (GFP). The various constructs were tested in vivo by transient expression in tobacco protoplasts. GFP distribution in transformed cells was analysed in situ by confocal microscopy, and in vitro in subcellular fractions by Western blotting. Despite its being highly conserved in different species, deletion of the C-terminal region (residues 48-54) of the presequence did not affect mitochondrial import. Deletion of the conserved residues 40-47 and the less conserved intermediate region (residues 18-39) resulted in 60% reduction in GFP import, whereas mutation of conserved residues within these regions had little effect. Further shortening of the presequence progressively reduced import, with the construct retaining the predicted N-terminal amphiphilic alpha-helix (residues 1-12) being unable to mediate mitochondrial import. However, point mutation showed that this last region plays an important role through its basic residues and amphiphilicity, but also through its hydrophobic residues. Replacing Arg4 and Arg5 by alanine residues and shifting the Arg5 and Leu6 (in order to disturb amphiphilicity) resulted in reduction of the presequence import efficiency. The most dramatic effects were seen with single or double mutations of the four Leu residues (positions 5, 6, 10 and 11), which resulted in marked reduction or abolition of GFP import, respectively. We conclude that the N-terminal helical structure of the presequence is necessary but not sufficient for efficient mitochondrial import, and that its hydrophobic residues play an essential role in in vivo mitochondrial targeting.

Tobacco VDL gene encodes a plastid DEAD box RNA helicase and is involved in chloroplast differentiation and plant morphogenesis

The recessive nuclear vdl (for variegated and distorted leaf) mutant of tobacco was obtained by T-DNA insertion and characterized by variegated leaves and abnormal roots and flowers. Affected leaf tissues were white and distorted, lacked palisadic cells, and contained undifferentiated plastids. The variegation was due to phenotypic, rather than genetic, instability. Genomic and cDNA clones were obtained for both the mutant and wild-type VDL alleles. Three transcripts, resulting from alternate intron splicing or polyadenylation, were found for the wild type. The transcripts potentially encode a set of proteins (53, 19, and 15 kD) sharing the same N-terminal region that contains a chloroplast transit peptide capable of importing the green fluorescent protein into chloroplasts. The predicted 53-kD product belongs to the DEAD box RNA helicase family. In the homozygous vdl mutant, T-DNA insertion resulted in accumulation of the shortest transcript and the absence of the RNA helicase-encoding transcript. Genetic transformation of the homozygous mutant by the 53-kD product-encoding cDNA fully restored the wild-type phenotype. These data suggest that a plastid RNA helicase controls early plastid differentiation and plant morphogenesis.