Analytical ultracentrifugation and preliminary X-ray studies of the chloroplast envelope quinone oxidoreductase homologue from Arabidopsis thaliana.

International audienceQuinone oxidoreductases reduce a broad range of quinones and are widely distributed among living organisms. The chloroplast envelope quinone oxidoreductase homologue (ceQORH) from Arabidopsis thaliana binds NADPH, lacks a classical N-terminal and cleavable chloroplast transit peptide, and is transported through the chloroplast envelope membrane by an unknown alternative pathway without cleavage of its internal chloroplast targeting sequence. To unravel the fold of this targeting sequence and its substrate specificity, ceQORH from A. thaliana was overexpressed in Escherichia coli, purified and crystallized. Crystals of apo ceQORH were obtained and a complete data set was collected at 2.34 Å resolution. The crystals belonged to space group C2221, with two molecules in the asymmetric unit

Tyrosine metabolism: identification of a key residue in the acquisition of prephenate aminotransferase activity by 1β aspartate aminotransferase

International audienceAlternative routes for the post-chorismate branch of the biosynthetic pathway leading to tyrosine exist, the 4-hydroxyphenylpyruvate or the arogenate route. The arogenate route involves the transamination of prephenate into arogenate. In a previous study, we found that, depending on the microorganisms possessing the arogenate route, three different aminotransferases evolved to perform prephenate transamination, that is, 1β aspartate aminotransferase (1β AAT), N-succinyl-l,l-diaminopimelate aminotransferase, and branched-chain aminotransferase. The present work aimed at identifying molecular determinant(s) of 1β AAT prephenate aminotransferase (PAT) activity. To that purpose, we conducted X-ray crystal structure analysis of two PAT competent 1β AAT from Arabidopsis thaliana and Rhizobium meliloti and one PAT incompetent 1β AAT from R. meliloti. This structural analysis supported by site-directed mutagenesis, modeling, and molecular dynamics simulations allowed us to identify a molecular determinant of PAT activity in the flexible N-terminal loop of 1β AAT. Our data reveal that a Lys/Arg/Gln residue in position 12 in the sequence (numbering according to Thermus thermophilus 1β AAT), present only in PAT competent enzymes, could interact with the 4-hydroxyl group of the prephenate substrate, and thus may have a central role in the acquisition of PAT activity by 1β AAT

Identification of the Arabidopsis calmodulin-dependent NAD+ kinase that sustains the elicitor-induced oxidative burst

International audience17 NADP(H) is an essential cofactor of multiple metabolic processes in all living organisms. In plants, 18 NADP(H) is required as the substrate of Ca 2+-dependent NADPH oxidases which catalyze a reactive 19 oxygen species burst in response to various stimuli. While NADP + production in plants has long been 20 known to involve a Calmodulin and Calcium (CaM)/Ca 2+-dependent NAD + kinase, the nature of the 21 enzyme catalyzing this activity has remained enigmatic, as well as its role in plant physiology. Here, 22 thanks to a combination of proteomics, biochemistry, molecular biology and in vivo studies, we have 23 identified an Arabidopsis protein that catalyzes NADP + production exclusively in the presence of 24 CaM/Ca 2+. This new enzyme (NADKc) has a CaM-binding peptide located in its N-terminal region and 25 displays peculiar biochemical properties as well as different domain organization compared to known 26 plant NAD + kinases. In response to a pathogen elicitor, activity of NADKc, which is associated with the 27 mitochondrial periphery, contributes to an increase in the cellular NADP + concentration and to the 28 amplification of the elicitor-induced oxidative burst. Based on a phylogenetic analysis and enzymatic 29 assays, we propose that the CaM/Ca 2+-dependent NAD + kinase activity found in photosynthetic 3

Three different classes of aminotransferases evolved prephenate aminotransferase functionality in arogenate-competent microorganisms.

International audienceThe aromatic amino acids phenylalanine and tyrosine represent essential sources of high value natural aromatic compounds for human health and industry. Depending on the organism, alternative routes exist for their synthesis. Phenylalanine and tyrosine are synthesized either via phenylpyruvate/4-hydroxyphenylpyruvate or via arogenate. In arogenate-competent microorganisms, an aminotransferase is required for the transamination of prephenate into arogenate, but the identity of the genes is still unknown. We present here the first identification of prephenate aminotransferases (PATs) in seven arogenate-competent microorganisms and the discovery that PAT activity is provided by three different classes of aminotransferase, which belong to two different fold types of pyridoxal phosphate enzymes: an aspartate aminotransferase subgroup 1β in tested α- and β-proteobacteria, a branched-chain aminotransferase in tested cyanobacteria, and an N-succinyldiaminopimelate aminotransferase in tested actinobacteria and in the β-proteobacterium Nitrosomonas europaea. Recombinant PAT enzymes exhibit high activity toward prephenate, indicating that the corresponding genes encode bona fide PAT. PAT functionality was acquired without other modification of substrate specificity and is not a general catalytic property of the three classes of aminotransferases

Identification of a plant gene encoding glutamate/aspartate-prephenate aminotransferase : the last homeless enzyme of aromatic amino acids biosynthesis

In all organisms synthesising phenylalanine and/or tyrosine via arogenate, a prephenate aminotransferase is required for the transamination of prephenate into arogenate. The identity of the gene encoding this enzyme in the organisms where this activity occurs is still unknown. Glutamate/aspartate-prephenate aminotransferase (PAT) is thus the last homeless enzyme in the aromatic amino acids pathway. We report on the purification, mass spectrometry identification and biochemical characterization of Arabidopsis thaliana prephenate aminotransferase. Our data revealed that this activity is housed by the prokaryotic-type plastidic aspartate aminotransferase (At2g22250). This represents the first identification of a gene encoding PAT

Crystal Structure of the Chloroplastic Oxoene Reductase ceQORH from Arabidopsis thaliana

International audienceEnzymatic and non-enzymatic peroxidation of polyunsaturated fatty acids give rise to accumulation of aldehydes, ketones, and alpha, beta-unsaturated carbonyls of various lengths, known as oxylipins. Oxylipins with alpha, beta-unsaturated carbonyls are reactive electrophile species and are toxic. Cells have evolved several mechanisms to scavenge reactive electrophile oxylipins and decrease their reactivity such as by coupling with glutathione, or by reduction using NAD(P)H-dependent reductases and dehydrogenases of various substrate specificities. Plant cell chloroplasts produce reactive electrophile oxylipins named gamma-ketols downstream of enzymatic lipid peroxidation. The chloroplast envelope quinone oxidoreductase homolog (ceQORH) from Arabidopsis thaliana was previously shown to reduce the reactive double bond of gamma-ketols. In marked difference with its cytosolic homolog alkenal reductase (AtAER) that displays a high activity toward the ketodiene 13-oxo-9(Z), 11(E)-octadecadienoic acid (13-KODE) and the ketotriene 13-oxo-9(Z), 11(E), 15(Z)-octadecatrienoic acid (13-KOTE), ceQORH binds, but does not reduce, 13-KODE and 13-KOTE. Crystal structures of apo-ceQORH and ceQORH bound to 13-KOTE or to NADP(+) and 13-KOTE have been solved showing a large ligand binding site, also observed in the structure of the cytosolic alkenal/one reductase. Positioning of the alpha, beta-unsaturated carbonyl of 13-KOTE in ceQORH-NADP(+)-13-KOTE, far away from the NADP(+) nicotinamide ring, provides a rational for the absence of activity with the ketodienes and ketotrienes. ceQORH is a monomeric enzyme in solution whereas other enzymes from the quinone oxidoreductase family are stable dimers and a structural explanation of this difference is proposed. A possible in vivo role of ketodienes and ketotrienes binding to ceQORH is also discussed

Identification of Two Conserved Residues Involved in Copper Release from Chloroplast PIB-1-ATPases.

International audienceCopper is an essential transition metal for living organisms. In the plant model Arabidopsis thaliana, half of the copper content is localized in the chloroplast and, as a cofactor of plastocyanin, copper is essential for photosynthesis. Within the chloroplast, copper delivery to plastocyanin involves two transporters of the PIB-1-ATPases subfamily, HMA6 at the chloroplast envelope and HMA8, in the thylakoid membranes. Both proteins are high affinity copper transporters but share distinct enzymatic properties. In the present work, the comparison of 140 sequences of PIB-1-ATPases revealed a conserved region unusually rich in histidine and cysteine residues in the TMA-L1 region of eukaryotic chloroplast copper ATPases. To evaluate the role of these residues, we mutated them in HMA6 and HMA8. Mutants of interest were selected from phenotypic tests in yeast and produced in Lactococcus lactis for further biochemical characterizations using phosphorylation assays from ATP and Pi. Combining functional and structural data, we highlight the importance of the cysteine and the first histidine of the Cx3Hx2H motif in the process of copper release from HMA6 and HMA8, and propose a copper pathway through the membrane domain of these transporters. Finally our work suggests a more general role of the histidine residue in the transport of copper by PIB-1-ATPases

The chloroplast membrane associated ceQORH putative quinone oxidoreductase reduces long-chain, stress-related oxidized lipids.

International audienceUnder oxidative stress conditions the lipid constituents of cells can undergo oxidation whose frequent consequence is the production of highly reactive alpha,beta-unsaturated carbonyls. These molecules are toxic because they can add to biomolecules (such as proteins and nucleic acids) and several enzyme activities cooperate to eliminate these reactive electrophile species. CeQORH (chloroplast envelope Quinone Oxidoreductase Homolog, At4g13010) is associated with the inner membrane of the chloroplast envelope and imported into the organelle by an alternative import pathway. In the present study, we show that the recombinant ceQORH exhibits the activity of a NADPH-dependent alpha,beta-unsaturated oxoene reductase reducing the double bond of medium-chain (C >= 9) to long-chain (18 carbon atoms) reactive electrophile species deriving from poly-unsaturated fatty acid peroxides. The best substrates of ceQORH are 13-lipoxygenase-derived gamma-ketols. gamma-Ketols are spontaneously produced in the chloroplast from the unstable allene oxide formed in the biochemical pathway leading to 12-oxo-phytodienoic acid, a precursor of the defense hormone jasmonate. In chloroplasts, ceQORH could detoxify 13-lipoxygenase-derived gamma-ketols at their production sites in the membranes. This finding opens new routes toward the understanding of gamma-ketols role and detoxification

HMA6 and HMA8 are two chloroplast Cu+-ATPases with different enzymatic properties.

The final version of record is available at http://www.bioscirep.org/content/35/3/e00201International audienceCopper (Cu) plays a key role in the photosynthetic process as cofactor of the plastocyanin (PC), an essential component of the chloroplast photosynthetic electron transfer chain. Encoded by the nuclear genome, PC is translocated in its apo-form into the chloroplast and the lumen of thylakoids where it is processed to its mature form and acquires Cu. In Arabidopsis, Cu delivery into the thylakoids involves two transporters of the PIB-1 ATPases family, heavy metal associated protein 6 (HMA6) located at the chloroplast envelope and HMA8 at the thylakoid membrane. To gain further insight into the way Cu is delivered to PC, we analysed the enzymatic properties of HMA8 and compared them with HMA6 ones using in vitro phosphorylation assays and phenotypic tests in yeast. These experiments reveal that HMA6 and HMA8 display different enzymatic properties: HMA8 has a higher apparent affinity for Cu+ but a slower dephosphorylation kinetics than HMA6. Modelling experiments suggest that these differences could be explained by the electrostatic properties of the Cu+ releasing cavities of the two transporters and/or by the different nature of their cognate Cu+ acceptors (metallochaperone/PC)

Tailoring confocal microscopy for real-time analysis of photosynthesis at single-cell resolution

International audiencePhotoautotrophs’ environmental responses have been extensively studied at the organism and ecosystemlevel. However, less is known about their photosynthesis at the single-cell level. This information is neededto understand photosynthetic acclimation processes, as light changes as it penetrates cells, layers of cells, ororgans. Furthermore, cells within the same tissue may behave differently, being at different developmental/physiological stages. Here, we describe an approach for single-cell and subcellular photophysiology basedon the customization of confocal microscopy to assess chlorophyll fluorescence quenching by the saturationpulse method. We exploit this setup to (1) reassess the specialization of photosynthetic activities in developingtissues of non-vascular plants; (2) identify a specific subpopulation of phytoplankton cells in marinephotosymbiosis, which consolidate energetic connections with their hosts; and (3) examine the link betweenlight penetration and photoprotection responses inside the different tissues that constitute a plant leafanatomy