Diego F. Gomez-Casati

Characterization of Arabidopsis Lines Deficient in GAPC-1, a Cytosolic NAD-Dependent Glyceraldehyde-3-Phosphate Dehydrogenase

Phosphorylating glyceraldehyde-3-P dehydrogenase (GAPC-1) is a highly conserved cytosolic enzyme that catalyzes the conversion of glyceraldehyde-3-P to 1,3-bis-phosphoglycerate; besides its participation in glycolysis, it is thought to be involved in additional cellular functions. To reach an integrative view on the many roles played by this enzyme, we characterized a homozygous gapc-1 null mutant and an as-GAPC1 line of Arabidopsis (Arabidopsis thaliana). Both mutant plant lines show a delay in growth, morphological alterations in siliques, and low seed number. Embryo development was altered, showing abortions and empty embryonic sacs in basal and apical siliques, respectively. The gapc-1 line shows a decrease in ATP levels and reduced respiratory rate. Furthermore, both lines exhibit a decrease in the expression and activity of aconitase and succinate dehydrogenase and reduced levels of pyruvate and several Krebs cycle intermediates, as well as increased reactive oxygen species levels. Transcriptome analysis of the gapc-1 mutants unveils a differential accumulation of transcripts encoding for enzymes involved in carbon partitioning. According to these studies, some enzymes involved in carbon flux decreased (phosphoenolpyruvate carboxylase, NAD-malic enzyme, glucose-6-P dehydrogenase) or increased (NAD-malate dehydrogenase) their activities compared to the wild-type line. Taken together, our data indicate that a deficiency in the cytosolic GAPC activity results in modifications of carbon flux and mitochondrial dysfunction, leading to an alteration of plant and embryo development with decreased number of seeds, indicating that GAPC-1 is essential for normal fertility in Arabidopsis plants.

Characterization of an Arabidopsis thaliana mutant lacking a cytosolic non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase

Non-phosphorylating glyceraldehyde- 3-phosphate dehydrogenase (NP-GAPDH) is a conserved cytosolic protein found in higher plants. In photosynthetic cells, the enzyme is involved in a shuttle transfer mechanism to export NADPH from the chloroplast to the cytosol. To investigate the role of this enzyme in plant tissues, we characterized a mutant from Arabidopsis thaliana having an insertion at the NP-GAPDH gene locus. The homozygous mutant was determined to be null respect to NP-GAPDH, as it exhibited undetectable levels of both transcription of NP-GAPDH mRNA, protein expression and enzyme activity. Transcriptome analysis demonstrated that the insertion mutant plant shows altered expression of several enzymes involved in carbohydrate metabolism. Significantly, cytosolic phosphorylating (NAD-dependent) glyceraldehyde-3-phosphate dehydrogenase mRNA levels are induced in the mutant, which correlates with an increase in enzyme activity. mRNA levels and enzymatic activity of glucose-6-phosphate dehydrogenase were also elevated, correlating with an increase in NADPH concentration. Moreover, increased ROS levels were measured in the mutant plants. Down-regulation of several glycolytic and photosynthetic genes suggests that NP-GAPDH is important for the efficiency of both metabolic processes. The results presented demonstrate that NP-GAPDH has a relevant role in plant growth and development.

Nitric oxide accumulation is required to protect against iron-mediated oxidative stress in frataxin-deficient Arabidopsis plants

Frataxin is a mitochondrial protein that is conserved throughout evolution. In yeast and mammals, frataxin is essential for cellular iron (Fe) homeostasis and survival during oxidative stress. In plants, frataxin deficiency causes increased reactive oxygen species (ROS) production and high sensitivity to oxidative stress. In this work we show that a knock‐down T‐DNA frataxin‐deficient mutant of Arabidopsis thaliana (atfh‐1) contains increased total and organellar Fe levels. Frataxin deficiency leads also to nitric oxide (NO) accumulation in both, atfh‐1 roots and frataxin null mutant yeast. Abnormally high NO production might be part of the defence mechanism against Fe‐mediated oxidative stress.

Gamma carbonic anhydrase like complex interact with plant mitochondrial complex I

We report the identification by two hybrid screens of two novel similar proteins, called Arabidopsis thaliana gamma carbonic anhydrase like1 and 2 (AtγCAL1 and AtγCAL2), that interact specifically with putative Arabidopsis thaliana gamma Carbonic Anhydrase (AtγCA) proteins in plant mitochondria. The interaction region that was located in the N-terminal 150 amino acids of mature AtγCA and AtγCA like proteins represents a new interaction domain. In vitro experiments indicate that these proteins are imported into mitochondria and are associated with mitochondrial complex I as AtγCAs. All plant species analyzed contain both AtγCA and AtγCAL sequences indicating that these genes were conserved throughout plant evolution. Structural modeling of AtγCAL sequences show a deviation of functionally important active site residues with respect to γCAs but could form active interfaces in the interaction with AtγCAs. We postulate a CA complex tightly associated to plant mitochondrial complex.

Functional and molecular characterization of the frataxin homolog from Arabidopsis thaliana

Frataxin is a highly conserved protein from bacteria to mammals that has been proposed to participate in iron–sulfur cluster assembly and mitochondrial iron homeostasis. In higher organisms, the frataxin gene is nuclear‐encoded and the protein is required for maintenance of normal mitochondrial iron levels and respiration. We describe here AtFH, a plant gene with significant homology to other members of the frataxin family. Plant frataxin has five segments of beta regions and two alpha helices, which are characteristics of human frataxin, as well as a potential N‐terminal targeting peptide for the mitochondrial localization. Transcription analysis showed that AtFH is ubiquitously expressed with high levels in flowers. Complementation of a Saccharomyces cerevisiae mutant (Δyfh) lacking the frataxin gene proved that AtFH is a functional protein, because it restored normal rates of respiration, growth and sensitivity to H2O2 of the null mutant. Our results support the involvement of AtFH in mitochondrial respiration and survival during oxidative stress in plants. This is the first report of a functional frataxin gene in plants.

Role of the N-terminal starch-binding domains in the kinetic properties of starch synthase III from Arabidopsis thaliana.

Starch synthase III (SSIII), one of the SS isoforms involved in plant starch synthesis, has been reported to play a regulatory role in the synthesis of transient starch. SSIII from Arabidopsis thaliana contains 1025 amino acid residues and has an N-terminal transit peptide for chloroplast localization which is followed by three repeated starch-binding domains (SBDs; SSIII residues 22-591) and a C-terminal catalytic domain (residues 592-1025) similar to bacterial glycogen synthase. In this work, we constructed recombinant full-length and truncated isoforms of SSIII, lacking one, two, or three SBDs, and recombinant proteins, containing three, two, or one SBD, to investigate the role of these domains in enzyme activity. Results revealed that SSIII uses preferentially ADPGlc, although UDPGlc can also be used as a sugar donor substrate. When ADPGlc was used, the presence of the SBDs confers particular properties to each isoform, increasing the apparent affinity and the V max for the oligosaccharide acceptor substrate. However, no substantial changes in the kinetic parameters for glycogen were observed when UDPGlc was the donor substrate. Under glycogen saturating conditions, the presence of SBDs increases progressively the apparent affinity and V max for ADPGlc but not for UDPGlc. Adsorption assays showed that the N-terminal region of SSIII, containing three, two, or one SBD module have increased capacity to bind starch depending on the number of SBD modules, with the D23 protein (containing the second and third SBD module) being the one that makes the greatest contribution to binding. The results presented here suggest that the N-terminal SBDs have a regulatory role, showing a starch binding capacity and modulating the catalytic properties of SSIII.

The starch‐binding capacity of the noncatalytic SBD2 region and the interaction between the N‐ and C‐terminal domains are involved in the modulation of the activity of starch synthase III from Arabidopsis thaliana

Starch synthase III from Arabidopsis thaliana contains an N‐terminal region, including three in‐tandem starch‐binding domains, followed by a C‐terminal catalytic domain. We have reported previously that starch‐binding domains may be involved in the regulation of starch synthase III function. In this work, we analyzed the existence of protein interactions between both domains using pull‐down assays, far western blotting and co‐expression of the full and truncated starch‐binding domains with the catalytic domain. Pull‐down assays and co‐purification analysis showed that the D(316–344) and D(495–535) regions in the D2 and D3 domains, respectively, but not the individual starch‐binding domains, are involved in the interaction with the catalytic domain. We also determined that the residues W366 and Y394 in the D2 domain are important in starch binding. Moreover, the co‐purified catalytic domain plus site‐directed mutants of the D123 protein lacking these aromatic residues showed that W366 was key to the apparent affinity for the polysaccharide substrate of starch synthase III, whereas either of these amino acid residues altered ADP‐glucose kinetics. In addition, the analysis of full‐length and truncated proteins showed an almost complete restoration of the apparent affinity for the substrates and Vmax of starch synthase III. The results presented here suggest that the interaction of the N‐terminal starch‐binding domains, particularly the D(316–344) and D(495–535) regions, with the catalytic domains, as well as the full integrity of the starch‐binding capacity of the D2 domain, are involved in the modulation of starch synthase III activity.

A mitochondrial dysfunction induces the expression of nuclear-encoded complex I genes in engineered male sterile Arabidopsis thaliana

To study the effect of a mitochondrial dysfunction induced by the expression of the unedited form of the subunit 9 of ATP synthase gene (u‐atp9) in Arabidopsis, we constructed transgenic plants expressing u‐atp9 under the control of three different promoters: CaMV 35S, apetala 3 and A9. The size and shape of transgenic plants bearing the apetala3::u‐atp9 and A9::u‐atp9 genes looked normal while the 35S::u‐atp9 transformed plants showed a dwarf morphology. All u‐atp9 expressing plants, independent of the promoter used, exhibited a male sterile phenotype. Molecular analysis of male sterile plants revealed the induction of the mitochondrial nuclear complex I (nCI) genes, psst, tyky and nadh binding protein (nadhbp), associated with a mitochondrial dysfunction. These results support the hypothesis that the expression of u‐atp9 can induce male sterility and reveal that the apetala3::u‐atp9 and A9::u‐atp9 plants induced the sterile phenotype without affecting the vegetative development of Arabidopsis plants. Moreover, male sterile plants produced by this procedure are an interesting model to study the global changes generated by an engineered mitochondrial dysfunction at the transcriptome and proteome levels in Arabidopsis plants.

Functional and structural characterization of the catalytic domain of the starch synthase III from Arabidopsis thaliana.

Glycogen and starch are the major energy storage compounds in most living organisms. The metabolic pathways leading to their synthesis involve the action of several enzymes, among which glycogen synthase (GS) or starch synthase (SS) catalyze the elongation of the α‐1,4‐glucan backbone. At least five SS isoforms were described in Arabidopsis thaliana; it has been reported that the isoform III (SSIII) has a regulatory function on the synthesis of transient plant starch. The catalytic C‐terminal domain of A. thaliana SSIII (SSIII‐CD) was cloned and expressed. SSIII‐CD fully complements the production of glycogen by an Agrobacterium tumefaciens glycogen synthase null mutant, suggesting that this truncated isoform restores in vivo the novo synthesis of bacterial glycogen. In vitro studies revealed that recombinant SSIII‐CD uses with more efficiency rabbit muscle glycogen than amylopectin as primer and display a high apparent affinity for ADP‐Glc. Fold class assignment methods followed by homology modeling predict a high global similarity to A. tumefaciens GS showing a fully conservation of the ADP‐binding residues. On the other hand, this comparison revealed important divergences of the polysaccharide binding domain between AtGS and SSIII‐CD. Proteins 2008.

Starch‐synthase III family encodes a tandem of three starch‐binding domains

The starch‐synthase III (SSIII), with a total of 1025 residues, is one of the enzymes involved in plants starch synthesis. SSIII from Arabidopsis thaliana contains a putative N‐terminal transit peptide followed by a 557‐amino acid SSIII‐specific domain (SSIII‐SD) with three internal repeats and a C‐terminal catalytic domain of 450 amino acids. Here, using computational characterization techniques, we show that each of the three internal repeats encodes a starch‐binding domain (SBD). Although the SSIII from A. thaliana and its close homologous proteins show no detectable sequence similarity with characterized SBD sequences, the amino acid residues known to be involved in starch binding are well conserved. Proteins 2006.