Glutathionylation of cytosolic glyceraldehyde-3-phosphate dehydrogenase from the model plant Arabidopsis thaliana is reversed by both glutaredoxins and thioredoxins in vitro

Plants contain both cytosolic and chloroplastic GAPDHs (glyceraldehyde-3-phosphate dehydrogenases). In Arabidopsis thaliana, cytosolic GAPDH is involved in the glycolytic pathway and is represented by two differentially expressed isoforms (GapC1 and GapC2) that are 98% identical in amino acid sequence. In the present study we show that GapC1 is a phosphorylating NAD-specific GAPDH with enzymatic activity strictly dependent on Cys(149). Catalytic Cys(149) is the only solvent-exposed cysteine of the protein and its thiol is relatively acidic (pK(a)=5.7). This property makes GapC1 sensitive to oxidation by H(2)O(2), which appears to inhibit enzyme activity by converting the thiolate of Cys(149) (-S-) into irreversible oxidized forms (-SO(2)(-) and -SO(3)(-)) via a labile sulfenate intermediate (-SO(-)). GSH (reduced glutathione) prevents this irreversible process by reacting with Cys(149) sulfenates to give rise to a mixed disulfide (Cys(149)-SSG), as demonstrated by both MS and biotinylated GSH. Glutathionylated GapC1 can be fully reactivated either by cytosolic glutaredoxin, via a GSH-dependent monothiol mechanism, or, less efficiently, by cytosolic thioredoxins physiologically reduced by NADPH:thioredoxin reductase. The potential relevance of these findings is discussed in the light of the multiple functions of GAPDH in eukaryotic cells (e.g. glycolysis, control of gene expression and apoptosis) that appear to be influenced by the redox state of the catalytic Cys(149).     

Starch-related cytosolic heteroglycans in roots from Arabidopsis thaliana

Both photoautotrophic and heterotrophic plant cells are capable of accumulating starch inside the plastid. However, depending on the metabolic state of the respective cell the starch-related carbon fluxes are different. The vast majority of the transitory starch biosynthesis relies on the hexose phosphate pools derived from the reductive pentose phosphate cycle and, therefore, is restricted to ongoing photosynthesis. Transitory starch is usually degraded in the subsequent dark period and mainly results in the formation of neutral sugars, such as glucose and maltose, that both are exported into the cytosol. The cytosolic metabolism of the two carbohydrates includes reversible glucosyl transfer reactions to a heteroglycan that are mediated by two glucosyl transferases, DPE2 and PHS2 (or, in all other species, Pho2).In heterotrophic cells, accumulation of starch mostly depends on the long distance transport of reduced carbon compounds from source to sink organs and, therefore, includes as an essential step the import of carbohydrates from the cytosol into the starch forming plastids.In this communication, we focus on starch metabolism in heterotrophic tissues from Arabidopsis thaliana wild type plants (and in various starch-related mutants as well). By using hydroponically grown A. thaliana plants, we were able to analyse starch-related biochemical processes in leaves and roots from the same plants. Within the roots we determined starch levels and the morphology of native starch granules. Cytosolic and apoplastic heteroglycans were analysed in roots and compared with those from leaves of the same plants. A. thaliana mutants lacking functional enzymes either inside the plastid (such as phosphoglucomutase) or in the cytosol (disproportionating isoenzyme 2 or the phosphorylase isozyme, PHS2) were included in this study. In roots and leaves from the three mutants (and from the respective wild type organ as well), starch and heteroglycans as well as enzyme patterns were analysed.     

Comprehensive analysis of cytosolic Nudix hydrolases in Arabidopsis thaliana

Nudix hydrolases are a family of proteins that catalyze the hydrolysis of a variety of nucleoside diphosphate derivatives. Twenty-four genes of the Nudix hydrolase homologues (AtNUDTs) with predicted localizations in the cytosol, chloroplasts, and mitochondria exist in Arabidopsis thaliana. Here, we demonstrated the comprehensive analysis of nine types of cytosolic AtNUDT proteins (AtNUDT1, -2, -4, -5, -6, -7, -9, -10, and -11). The recombinant proteins of AtNUDT2, -6, -7, and -10 showed both ADP-ribose and NADH pyrophosphatase activities with significantly high affinities compared with those of animal and yeast enzymes. The expression of each AtNUDT is individually regulated in different tissues. These findings suggest that most cytosolic AtNUDTs may substantially function in the sanitization of potentially hazardous ADP-ribose and the regulation of the cellular NADH/NAD(+) ratio in plant cells. On the other hand, the AtNUDT1 protein had the ability to hydrolyze 8-oxo-dGTP with a K(m) value of 6.8 mum and completely suppress the increased frequency of spontaneous mutations in the Escherichia coli mutT(-) strain, indicating that AtNUDT1 is a functional homologue of E. coli MutT in A. thaliana and is involved in the prevention of spontaneous mutation. The results obtained here suggest that the plant Nudix family has evolved in a specific manner that differs from that of yeast and humans.     

CSR1, the Sole Target of Imidazolinone Herbicide in Arabidopsis thaliana

The above article was published     

The Arabidopsis thaliana genome encodes at least four thioredoxins m and a new prokaryotic-like thioredoxin

Screening of cDNA libraries at low stringency and complete sequencing of EST clones with homology to thioredoxins allowed us to characterize five new prokaryotic type Arabidopsis thaliana thioredoxins. All present N-terminal extensions with characteristics of transit peptides. Four are clustered in a phylogenetic tree with the chloroplastic thioredoxin m from red and green algae and higher plants, and their transit peptides have typical characteristics of chloroplastic transit peptides. One is clearly divergent and defines a new prokaryotic thioredoxin type that we have named thioredoxin x. Its transit peptide sequence presents characteristics of both chloroplastic and mitochondrial transit peptides. The five corresponding genes are expressed at different levels, but mostly in green tissues and in in-vitro cultivated cells.     

The Hsp90 family of proteins in Arabidopsis thaliana

The 90-kDa heat shock protein (Hsp90) is an essential molecular chaperone in eukaryotic cells, with key roles in the folding and activation of proteins involved in signal transduction and control of the cell cycle. A search for Hsp90 sequences in the Arabidopsis thaliana genome revealed that this family includes 7 members. The AtHsp90-1 through AtHsp90-4 proteins constitute the cytoplasmic subfamily, whereas the AtHsp90-5, AtHsp90-6, and AtHsp90-7 proteins are predicted to be within the plastidial, mitochondrial, and endoplasmic reticulum compartments, respectively. The deduced amino acid sequences of each of the cytoplasmic proteins contains the highly conserved C-terminal pentapeptide MEEVD. All of the AtHsp90 sequences include a conserved adenosine triphosphate-binding domain, whereas only the cytoplasmic and endoplasmic reticulum-resident sequences include an adjacent charged linker domain that is common in mammalian and yeast sequences. The occurrence of multiple AtHsp90 proteins in the cytoplasm and of family members in other subcellular compartments suggests a range of specific functions and target polypeptides.     

Two Related RNA-editing Proteins Target the Same Sites in Mitochondria of Arabidopsis thaliana

The facilitators for specific cytosine-to-uridine RNA-editing events in plant mitochondria and plastids are pentatricopeptide repeat (PPR)-containing proteins with specific additional C-terminal domains. Here we report the related PPR proteins mitochondrial editing factor 8 (MEF8) and MEF8S with only five such repeats each to be both involved in RNA editing at the same two sites in mitochondria of Arabidopsis thaliana. Mutants of MEF8 show diminished editing in leaves but not in pollen, whereas mutants of the related protein MEF8S show reduced RNA editing in pollen but not in leaves. Overexpressed MEF8 or MEF8S both increase editing at the two target sites in a mef8 mutant. Double mutants of MEF8 and MEF8S are not viable although both identified target sites are in mRNAs for nonessential proteins. This suggests that MEF8 and MEF8S may have other essential functions beyond these two editing sites in complex I mRNAs.     

Detection of membrane-bound cytokinin-binding proteins in Arabidopsis thaliana cells.

In order to isolate cytokinin-binding proteins (CBPs), we have developed new affinity probes constituted of a cytokinin such as zeatin riboside ([9R]Z) conjugated to a carrier protein. These probes were used for detecting CBPs in an ELISA procedure. The efficiency of the cytokinin conjugate in detecting CBPs was controlled with protein model: proteins having an affinity for cytokinin such as the monoclonal anti-[9R]Z antibodies did bind the cytokinin conjugate whereas proteins unable to bind cytokinin such as bovine serum albumin did not. Using these new affinity probes, we showed that CBPs are present in the membrane fraction of in vitro cultured Arabidopsis thaliana cells. The nature of the protein at the detected binding sites was demonstrated by submitting the microsomal proteins to a proteolytic treatment, which was found to eradicate the binding. Free biologically active cytokinins or monoclonal anti-[9R]Z antibodies inhibited the binding, thus showing the specificity of the interaction. The detected CBPs were partially solubilized from the membranes with potassium chloride, indicating their peripheral membrane location. The separation by anion exchange chromatography of solubilized microsomal proteins revealed the existence of two different CBPs. They were present at higher levels in cells during the exponential growth phase.     

Mitochondrial localization of Arabidopsis thaliana Isu Fe-S scaffold proteins

Isu are scaffold proteins involved in iron-sulfur cluster biogenesis and playing a key role in yeast mitochondria and Escherichia coli. In this work, we have characterized the Arabidopsis thaliana Isu gene family. AtIsu1,2,3 genes encode polypeptides closely related to their bacterial and eukaryotic counterparts. AtIsu expression in a Saccharomyces cerevisiae Deltaisu1Deltanfu1 thermosensitive mutant led to the growth restoration of this strain at 37 degrees C. Using Isu-GFP fusions expressed in leaf protoplasts and immunodetection in organelle extracts, we have shown that Arabidopsis Isu proteins are located only into mitochondria, supporting the existence of an Isu-independent Fe-S assembly machinery in plant plastids.     

MscS-like proteins control plastid size and shape in Arabidopsis thaliana

BACKGROUND: Mechanosensitive (MS) ion channels provide a mechanism for the perception of mechanical stimuli such as sound, touch, and osmotic pressure. The bacterial MS ion channel MscS opens in response to increased membrane tension and serves to protect against cellular lysis during osmotic downshock. MscS-like proteins are found widely in bacterial and archaeal species and have also been identified in fission yeast and plants. None of the eukaryotic members of the family have yet been characterized. RESULTS: Here, we characterize two MscS-like (MSL) proteins from Arabidopsis thaliana, MSL2 and MSL3. MSL3 can rescue the osmotic-shock sensitivity of a bacterial mutant lacking MS-ion-channel activity, suggesting that it functions as a mechanosensitive ion channel. Arabidopsis plants harboring insertional mutations in both MSL3 and MSL2 show abnormalities in the size and shape of plastids, which are plant-specific endosymbiotic organelles responsible for photosynthesis, gravity perception, and numerous metabolic reactions. MSL2-GFP and MSL3-GFP are localized to discrete foci on the plastid envelope and colocalize with the plastid division protein AtMinE. CONCLUSIONS: Our data support a model wherein MSL2 and MSL3 control plastid size, shape, and perhaps division during normal plant development by altering ion flux in response to changes in membrane tension. We propose that MscS family members have evolved new roles in plants since the endosymbiotic event that gave rise to plastids.     

Arabidopsis thaliana LSM proteins function in mRNA splicing and degradation

Sm-like (Lsm) proteins have been identified in all organisms and are related to RNA metabolism. Here, we report that Arabidopsis nuclear AtLSM8 protein, as well as AtLSM5, which localizes to both the cytoplasm and nucleus, function in pre-mRNA splicing, while AtLSM5 and the exclusively cytoplasmic AtLSM1 contribute to 5′–3′ mRNA decay. In lsm8 and sad1/lsm5 mutants, U6 small nuclear RNA (snRNA) was reduced and unspliced mRNA precursors accumulated, whereas mRNA stability was mainly affected in plants lacking AtLSM1 and AtLSM5. Some of the mRNAs affected in lsm1a lsm1b and sad1/lsm5 plants were also substrates of the cytoplasmic 5′–3′ exonuclease AtXRN4 and of the decapping enzyme AtDCP2. Surprisingly, a subset of substrates was also stabilized in the mutant lacking AtLSM8, which supports the notion that plant mRNAs are actively degraded in the nucleus. Localization of LSM components, purification of LSM-interacting proteins as well as functional analyses strongly suggest that at least two LSM complexes with conserved activities in RNA metabolism, AtLSM1-7 and AtLSM2-8, exist also in plants.     

An Arabidopsis thaliana protein homologous to cyanobacterial high-light-inducible proteins

An Arabidopsis thaliana cDNA clone encoding a novel 110 amino acid thylakoid protein has been sequenced. The in vitro synthesized protein is taken up by intact chloroplasts, inserted into the thylakoid membrane and the transit peptide is cleaved off during this process. The mature protein is predicted to contain 69 amino acids, to form one membrane-spanning -helix and to have its N-terminus at the stromal side of the thylakoid membrane. The protein showed similarity to the LHC, ELIP and PsbS proteins of higher plants, but more pronounced to the high-light-inducible proteins (HLIPs) of cyanobacteria and red algae, to which no homologue previously has been detected in higher plants. As for HLIP and ELIP, high light increases the mRNA levels of the corresponding gene. Sequence comparisons indicate that the protein may bind chlorophyll and form dimers in the thylakoid membrane. The level of expression of the protein seems to be far lower than that of normal PSI and PSII subunits.     

Sugar-induced increase in cytosolic Ca(2+) in Arabidopsis thaliana whole plants.

Using Ca(2+)-dependent photoprotein aequorin-transformed Arabidopsis thaliana, sugar-induced increase in cytosolic free Ca(2+ )concentration ([Ca(2+)](cyt))( )was investigated by luminescence imaging technique. When 0.1 M sucrose was fed to roots of autotrophically grown intact whole plants whose roots had been incubated overnight with coelenterazine to reconstitute aequorin systemically, strong and transient (within 20 s) luminescence was observed in the roots; that luminescence was followed by weak luminescence moving from the lower leaves to the upper leaves. The moving rate of luminescence was roughly comparable to that of [(14)C]sucrose. Application of 0.1 M glucose or fructose induced transient luminescence in excised leaves. No such luminescence was observed in heterotrophically grown (with sucrose) whole plants or in excised tissues. mRNA levels of sucrose-H(+) symporter genes AtSUC1 and AtSUC2 were higher in autotrophic plants than in heterotrophic plants. These results indicate that influx of transported sucrose together with H(+) into the mesophyll cells of autotrophic plants may depolarize the membrane potential, and subsequently activate a voltage-gated Ca(2+) channel on the plasma membrane, resulting in a [Ca(2+)](cyt) increase. The [Ca(2+)](cyt) increase might initiate Ca(2+ )signaling leading to the expression of genes related to biosynthesis of storage carbohydrates. Hexoses, when applied, might also be involved in the [Ca(2+)](cyt) increase mediated by monosaccharide-H(+) co-transporters.     

Arabinogalactan proteins as molecular markers in Arabidopsis thaliana sexual reproduction

Some of the most important changes that occur in plants during sexual reproduction involve the transition from a sporophytic to a gametophytic type of development. In this paper, these changes were evaluated for Arabidopsis thaliana. The results obtained clearly show differences in the pattern of distribution of specific arabinogalactan protein (AGP) sugar epitopes, during anther and ovule development. AGPs are hydroxyproline-rich glycoproteins that are massively glycosylated and ubiquitous in plants. The molecular mechanism of action of AGPs is still unknown, mainly due to the difficulties posed by the complex saccharide chains. However, the complex structure of the sugar fraction of AGPs makes them a potential source of signalling molecules. The selective labelling obtained with AGP mAbs JIM8, JIM13, MAC207, and LM2, during Arabidopsis pollen and pistil development, suggests that some AGPs can work as markers for gametophytic cell differentiation. Specific labelling of the first gametophytic cells in the pistil, the strong labelling of the secretory cells of the embryo sac, the synergid cells, and the labelling of the integument micropylar cells, apparently outlining the pollen tube pathway into its final target, the embryo sac, have all been shown. In the anthers, the specific labelling of gametophytic cells, and of the male gametes that travel along the pollen tube, may indicate AGP epitopes acting as signals for the pollen tube to reach its final destiny. The specific labelling of cells destined to go into programmed cell death is also discussed.