Insights into ascorbate regeneration in plants: investigating the redox and structural properties of dehydroascorbate reductases from Populus trichocarpa.

Dehydroascorbate reductases (DHARs), enzymes belonging to the GST superfamily, catalyse the GSH-dependent reduction of dehydroascorbate into ascorbate in plants. By maintaining a reduced ascorbate pool, they notably participate to H2O2 detoxification catalysed by ascorbate peroxidases (APXs). Despite this central role, the catalytic mechanism used by DHARs is still not well understood and there is no supportive 3D structure. In this context, we have performed a thorough biochemical and structural analysis of the three poplar DHARs and coupled this to the analysis of their transcript expression patterns and subcellular localizations. The transcripts for these genes are mainly detected in reproductive and green organs and the corresponding proteins are expressed in plastids, in the cytosol and in the nucleus, but not in mitochondria and peroxisomes where ascorbate regeneration is obviously necessary. Comparing the kinetic properties and the sensitivity to GSSG-mediated oxidation of DHAR2 and DHAR3A, exhibiting 1 or 3 cysteinyl residues respectively, we observed that the presence of additional cysteines in DHAR3A modifies the regeneration mechanism of the catalytic cysteine by forming different redox states. Finally, from the 3D structure of DHAR3A solved by NMR, we were able to map the residues important for the binding of both substrates (GSH and DHA), showing that DHAR active site is very selective for DHA recognition and providing further insights into the catalytic mechanism and the roles of the additional cysteines found in some DHARs.

Structural and spectroscopic insights into BolA-glutaredoxin complexes.

BolA proteins are defined as stress-responsive transcriptional regulators, but they also participate in iron metabolism. Although they can form [2Fe-2S]-containing complexes with monothiol glutaredoxins (Grx), structural details are lacking. Three Arabidopsis thaliana BolA structures were solved. They differ primarily by the size of a loop referred to as the variable [H/C] loop, which contains an important cysteine (BolA_C group) or histidine (BolA_H group) residue. From three-dimensional modeling and spectroscopic analyses of A. thaliana GrxS14-BolA1 holo-heterodimer (BolA_H), we provide evidence for the coordination of a Rieske-type [2Fe-2S] cluster. For BolA_C members, the cysteine could replace the histidine as a ligand. NMR interaction experiments using apoproteins indicate that a completely different heterodimer was formed involving the nucleic acid binding site of BolA and the C-terminal tail of Grx. The possible biological importance of these complexes is discussed considering the physiological functions previously assigned to BolA and to Grx-BolA or Grx-Grx complexes.

Arabidopsis chloroplastic glutaredoxin C5 as a model to explore molecular determinants for iron-sulfur cluster binding into glutaredoxins.

Unlike thioredoxins, glutaredoxins are involved in iron-sulfur cluster assembly and in reduction of specific disulfides (i.e. protein-glutathione adducts), and thus they are also important redox regulators of chloroplast metabolism. Using GFP fusion, AtGrxC5 isoform, present exclusively in Brassicaceae, was shown to be localized in chloroplasts. A comparison of the biochemical, structural, and spectroscopic properties of Arabidopsis GrxC5 (WCSYC active site) with poplar GrxS12 (WCSYS active site), a chloroplastic paralog, indicated that, contrary to the solely apomonomeric GrxS12 isoform, AtGrxC5 exists as two forms when expressed in Escherichia coli. The monomeric apoprotein possesses deglutathionylation activity mediating the recycling of plastidial methionine sulfoxide reductase B1 and peroxiredoxin IIE, whereas the dimeric holoprotein incorporates a [2Fe-2S] cluster. Site-directed mutagenesis experiments and resolution of the x-ray crystal structure of AtGrxC5 in its holoform revealed that, although not involved in its ligation, the presence of the second active site cysteine (Cys(32)) is required for cluster formation. In addition, thiol titrations, fluorescence measurements, and mass spectrometry analyses showed that, despite the presence of a dithiol active site, AtGrxC5 does not form any inter- or intramolecular disulfide bond and that its activity exclusively relies on a monothiol mechanism.

Structural genomics applied to the rust fungus Melampsora larici-populina reveals two candidate effector proteins adopting cystine knot and NTF2-like protein folds.

Rust fungi are plant pathogens that secrete an arsenal of effector proteins interfering with plant functions and promoting parasitic infection. Effectors are often species-specific, evolve rapidly, and display low sequence similarities with known proteins. How rust fungal effectors function in host cells remains elusive, and biochemical and structural approaches have been scarcely used to tackle this question. In this study, we produced recombinant proteins of eleven candidate effectors of the leaf rust fungus Melampsora larici-populina in Escherichia coli. We successfully purified and solved the three-dimensional structure of two proteins, MLP124266 and MLP124017, using NMR spectroscopy. Although both MLP124266 and MLP124017 show no sequence similarity with known proteins, they exhibit structural similarities to knottins, which are disulfide-rich small proteins characterized by intricate disulfide bridges, and to nuclear transport factor 2-like proteins, which are molecular containers involved in a wide range of functions, respectively. Interestingly, such structural folds have not been reported so far in pathogen effectors, indicating that MLP124266 and MLP124017 may bear novel functions related to pathogenicity. Our findings show that sequence-unrelated effectors can adopt folds similar to known proteins, and encourage the use of biochemical and structural approaches to functionally characterize effector candidates.

Surfactin-triggered small vesicle formation of negatively charged membranes: a novel membrane-lysis mechanism.

The molecular mode of action of the lipopeptide SF with zwitterionic and negatively charged model membranes has been investigated with solid-state NMR, light scattering, and electron microscopy. It has been found that this acidic lipopeptide (negatively charged) induces a strong destabilization of negatively charged micrometer-scale liposomes, leading to the formation of small unilamellar vesicles of a few 10s of nanometers. This transformation is detected for very low doses of SF (Ri = 200) and is complete for Ri = 50. The phenomenon has been observed for several membrane mixtures containing phosphatidylglycerol or phosphatidylserine. The vesicularization is not observed when the lipid negative charges are neutralized and a cholesterol-like effect is then evidenced, i.e., increase of gel membrane dynamics and decrease of fluid membrane microfluidity. The mechanism for small vesicle formation thus appears to be linked to severe changes in membrane curvature and could be described by a two-step action: 1), peptide insertion into membranes because of favorable van der Waals forces between the rather rigid cyclic and lipophilic part of SF and lipid chains and 2), electrostatic repulsion between like charges borne by lipid headgroups and the negatively charged SF amino acids. This might provide the basis for a novel mode of action of negatively charged lipopeptides.

Solution structure and dynamics of the reduced and oxidized forms of the N-terminal domain of PilB from Neisseria meningitidis.

The secreted form of the PilB protein was proposed to be involved in pathogen survival fighting against the defensive host's oxidative burst. PilB protein is composed of three domains. The central and the C-terminal domains display methionine sulfoxide reductase A and B activities, respectively. The N-terminal domain, which possesses a CXXC motif, was recently shown to regenerate in vitro the reduced forms of the methionine sulfoxide reductase domains of PilB from their oxidized forms, as does the thioredoxin 1 from E. coli, via a disulfide bond exchange. The thioredoxin-like N-terminal domain belongs to the cytochrome maturation protein structural family, but it possesses a unique additional segment (99)FLHE (102) localized in a loop. This segment covers one edge of the active site in the crystal structure of the reduced form of the N-terminal domain of PilB. We have determined the solution structure and the dynamics of the N-terminal domain from Neisseria meningitidis, in its reduced and oxidized forms. The FLHE loop adopts, in both redox states, a well-defined conformation. Subtle conformational and dynamic changes upon oxidation are highlighted around the active site, as well as in the FLHE loop. The functional consequences of the cytochrome maturation protein topology and those of the presence of FLHE loop are discussed in relation to the enzymatic properties of the N-terminal domain.

Solution structure and backbone dynamics of the cysteine 103 to serine mutant of the N-terminal domain of DsbD from Neisseria meningitidis.

The DsbD protein is essential for electron transfer from the cytoplasm to the periplasm of Gram-negative bacteria. Its N-terminal domain dispatches electrons coming from cytoplasmic thioredoxin (Trx), via its central transmembrane and C-terminal domains, to its periplasmic partners: DsbC, DsbE/CcmG, and DsbG. Previous structural studies described the latter proteins as Trx-like folds possessing a characteristic C-X-X-C motif able to generate a disulfide bond upon oxidation. The Escherichia coli nDsbD displays an immunoglobulin-like fold in which two cysteine residues (Cys103 and Cys109) allow a disulfide bond exchange with its biological partners.We have determined the structure in solution and the backbone dynamics of the C103S mutant of the N-terminal domain of DsbD from Neisseria meningitidis. Our results highlight significant structural changes concerning the beta-sheets and the local topology of the active site compared with the oxidized form of the E. coli nDsbD. The structure reveals a "cap loop" covering the active site, similar to the oxidized E. coli nDsbD X-ray structure. However, regions featuring enhanced mobility were observed both near to and distant from the active site, revealing a capacity of structural adjustments in the active site and in putative interaction areas with nDsbD biological partners. Results are discussed in terms of functional consequences.

Interfacial behaviour of bovine testis hyaluronidase.

The interfacial properties of bovine testicular hyaluronidase were investigated by demonstrating the association of hyaluronidase activity with membranes prepared from bovine testis. Protein adsorption to the air/water interface was investigated using surface pressure-area isotherms. In whichever way the interfacial films were obtained (protein injection or deposition), the hyaluronidase exhibited a significant affinity for the air/water interface. The isotherm obtained 180 min after protein injection into a pH 5.3 subphase was similar to the isotherm obtained after spreading the same amount of protein onto the same subphase, indicating that bovine testicular hyaluronidase molecules adopted a similar arrangement and/or conformation at the interface. Increasing the subphase pH from 5.3 to 8 resulted in changes of the protein isotherms. These modifications, which could correspond to the small pH-induced conformational changes observed by Fourier-transform IR spectroscopy, were discussed in relation to the pH influence on the hyaluronidase activity. Adding hyaluronic acid, the enzyme substrate, to the subphase tested the stability of the interfacial properties of hyaluronidase. The presence of hyaluronic acid in the subphase did not modify the protein adsorption and allowed substrate binding to a preformed film of hyaluronidase at pH 5.3, the optimal pH for the enzyme activity. Such effects of hyaluronic acid were not observed when the subphase was constituted of pure water, a medium where the enzyme activity was negligible. These influences of hyaluronic acid were discussed in relation to the modelled structure of bovine testis hyaluronidase where a hydrophobic region was proposed to be opposite of the catalytic site.

NMR structure determination of a synthetic analogue of bacillomycin Lc reveals the strategic role of L-Asn1 in the natural iturinic antibiotics.

Iturins are a group of antifungal produced by Bacillus subtilis. All are cyclic lipopeptides with seven alpha-amino acids of configuration LDDLLDL and one beta-amino fatty acid. The bacillomycin L is a member of this family and its NMR structure was previously resolved using the sequence Asp-Tyr-Asn-Ser-Gln-Ser-Thr. In this work, we carefully examined the NMR spectra of this compound and detected an error in the sequence. In fact, Asp1 and Gln5 need to be changed into Asn1 and Glu5, which therefore makes it identical to bacillomycin Lc. As a consequence, it now appears that all iturinic peptides with antibiotic activity share the common beta-amino fatty acid 8-L-Asn1-D-Tyr2-D-Asn3 sequence. To better understand the conformational influence of the acidic residue L-Asp1, present, for example in the inactive iturin C, the NMR structure of the synthetic analogue SCP [cyclo (L-Asp1-D-Tyr2-D-Asn3-L-Ser4-L-Gln5-D-Ser6-L-Thr7-beta-Ala8)] was determined and compared with bacillomycin Lc recalculated with the corrected sequence. In both cases, the conformers obtained were separated into two families of similar energy which essentially differ in the number and type of turns. A detailed analysis of both cyclopeptide structures is presented here. In addition, CD and FTIR spectra were performed and confirmed the conformational differences observed by NMR between both cyclopeptides.

Probing the structure-function relationship of polyene macrolides: engineered biosynthesis of soluble nystatin analogues.

Although polyene macrolides are efficient antifungal agents with fungicidal mode of action, their use in medical practice is problematic due to their low solubility and significant human toxicity. In an attempt to address the solubility problem, we have obtained two analogues of nystatin with hydroxy groups at positions C31 and C33 through manipulation of the nystatin polyketide synthase in the producing organism Streptomyces noursei. Structures of the analogues were confirmed by nuclear magnetic resonance (NMR), and their solubility was found to be more than 2000 times higher than that of nystatin. However, both analogues were shown to have lost antifungal activity, implying that the integrity of the hydrophobic polyene region of the nystatin molecule is crucial for the fungicidal action. NMR data and computer modeling performed for the new analogues suggested conformational changes together with a significantly increased structural disorder, which may account for both increased solubility and the loss of activity.

The Poplar Rust-Induced Secreted Protein (RISP) Inhibits the Growth of the Leaf Rust Pathogen Melampsora larici-populina and Triggers Cell Culture Alkalinisation.

Plant cells secrete a wide range of proteins in extracellular spaces in response to pathogen attack. The poplar rust-induced secreted protein (RISP) is a small cationic protein of unknown function that was identified as the most induced gene in poplar leaves during immune responses to the leaf rust pathogen Melampsora larici-populina, an obligate biotrophic parasite. Here, we combined in planta and in vitro molecular biology approaches to tackle the function of RISP. Using a RISP-mCherry fusion transiently expressed in Nicotiana benthamiana leaves, we demonstrated that RISP is secreted into the apoplast. A recombinant RISP specifically binds to M. larici-populina urediniospores and inhibits their germination. It also arrests the growth of the fungus in vitro and on poplar leaves. Interestingly, RISP also triggers poplar cell culture alkalinisation and is cleaved at the C-terminus by a plant-encoded mechanism. Altogether our results indicate that RISP is an antifungal protein that has the ability to trigger cellular responses.

Membrane fluidity does not explain how solvents act on the middle-ear reflex.

Some volatile aromatic solvents have similar or opposite effects to anesthetics in the central nervous system. Like for anesthetics, the mechanisms of action involved are currently the subject of debate. This paper presents an in vivo study to determine whether direct binding or effects on membrane fluidity best explain how solvents counterbalance anesthesia's depression of the middle-ear reflex (MER). Rats were anesthetized with a mixture of ketamine and xylazine while also exposed to solvent vapors (toluene, ethylbenzene, or one of the three xylene isomers) and the amplitude of their MER was monitored. The depth of anesthesia was standardized based on the magnitude of the contraction of the muscles involved in the MER, determined by measuring cubic distortion product oto-acoustic emissions (DPOAEs) while triggering the bilateral reflex with contralateral acoustic stimulation. The effects of the aromatic solvents were quantified based on variations in the amplitude of the DPOAEs. The amplitude of the alteration to the MER measured in anesthetized rats did not correlate with solvent lipophilocity (as indicated by logKow values). Results obtained with the three xylene isomers indicated that the positions of two methyl groups around the benzene ring played a determinant role in solvent/neuronal cell interaction. Additionally, Solid-state Nuclear Magnetic Resonance (NMR) spectra for brain microsomes confirmed that brain lipid fluidity was unaffected by solvent exposure, even after three days (6h/day) at an extremely high concentration (3000ppm). Therefore, aromatic solvents appear to act directly on the neuroreceptors involved in the acoustic reflex circuit, rather than on membrane fluidity. The affinity of this interaction is determined by stereospecific parameters rather than lipophilocity.

NMR of redox proteins of plants, yeasts and photosynthetic bacteria.

NMR spectroscopy has evolved dramatically over the past 15 years, establishing a new, reliable methodology for studying biomacromolecules at atomic resolution. The three-dimensional structure and dynamics of a biomolecule or a biomolecular complex is only one of the main types of information available using NMR. The spectral assignment to the specific nuclei of a biostructure is a very precise reflection of their electronic environment. Any change in this environment due to a structural change, the binding of a ligand or the redox state of a redox cofactor, will be very sensitively reported by changes in the different NMR parameters. The capabilities of the NMR method are currently expanding dramatically and it is turning into a powerful means to study biosystems dynamically in exchange between different conformations, exchanging ligands, transient complexes, or the activation/inhibition of regulated enzymes. We review here several NMR studies that have appeared during the past 5 or 6 years in the field of redox proteins of plants, yeasts and photosynthetic bacteria. These new results illustrate the recent biomolecular NMR evolution and provide new physiological models for understanding the different types of electron transfer, including glutaredoxins, thioredoxins and their dependent enzymes, the ferredoxin-NADP oxidoreductase complex, flavodoxins, the plastocyanin-cytochrome f complex, and cytochromes c.

Hyaluronidase binds differently DPPC, DPPS or GlcNAc-bearing glycolipid biomimetic monolayers.

Bovine testis hyaluronidase (btHyal) had been shown to have direct effects on cancer cells and to be a useful adjuvant in several medicines. Furthermore this enzyme had been found to be membrane-associated. Thus, in this work, the interactions between btHyal and membranes were analyzed by using lipid monolayers at the air-water interface as a biomimetic membrane system. This allowed us to define the btHyal interactions with two residues of hyaluronic acid (a btHyal substrate), GlcNAc and carboxylic group, which are present in cholesteryl-triethoxy-N-acetylglucosamine (Chol-E3-GlcNAc) and in DPPS, respectively. btHyal bound preferentially Chol-E3-GlcNAc monolayers and showed a decreasing affinity for Chol-E3-GlcNAc-DPPC monolayers containing decreasing amount of glycolipid, suggesting a crucial role of the glycolipid GlcNAc. Furthermore the significant btHyal binding to DPPS was not affected by the presence of free GlcNAc in the subphase. These results and the absence of significant binding of btHyal to pure DPPC monolayer suggest that the protein interacts with the lipid monolayer by mimicking the enzyme-substrate interactions or by electrostatic interactions.

Formation of the complex between DsbD and PilB N-terminal domains from Neisseria meningitidis necessitates an adaptability of nDsbD.

DsbD transmembrane protein dispatches electrons to periplasmic Trx/DsbE-like partners via specific interactions with its N-terminal domain, nDsbD. In the present study, PilB N-terminal domain (NterPilB) is shown to efficiently accept electrons coming from nDsbD from Neisseria meningitidis. Using an NMR-driven docking approach, we have modeled the structure of a mixed disulfide complex between NterPilB and nDsbD. We show the needed opening of nDsbD cap-loop whereas NterPilB FLHE loop does not seem essential in the formation and stabilization of the complex. Relaxation analysis performed on backbone amide groups highlights a kind of dynamics transfer from nDsbD cap-loop on NterPilB alpha1 helix, suggesting that a mobility contribution is required not only for the formation of the mixed disulfide complex, but also for its disruption. Taking into account previous X-ray data on covalent complexes involving nDsbD, a cartoon of interactions between Trx-like partners and nDsbD is proposed that illustrates the adaptability of nDsbD.

1H, 13C, and 15N resonance assignment of the C103S mutant of the N-terminal domain of DsbD from Neisseria meningitidis.

We report the nearly complete 1H, 13C, and 15N resonance assignments of the C103S mutant of the N-terminal domain of DsbD from Neisseria meningitides. Secondary structure determination using CSI method leads to the prediction of nine beta-sheet parts.

Structure and dynamics of surfactin studied by NMR in micellar media.

The NMR structure of the cyclic lipopeptide surfactin from Bacillus subtilis was determined in sodium dodecyl sulfate (SDS) micellar solution. The two negatively charged side chains of surfactin form a polar head opposite to most hydrophobic side chains, accounting for its amphiphilic nature and its strong surfactant properties. Disorder was observed around the fatty acid chain, and 15N relaxation studies were performed to investigate whether it originates from a dynamic phenomenon. A very large exchange contribution to transverse relaxation rate R(2) was effectively observed in this region, indicating slow conformational exchange. Temperature variation and Carr-Purcell-Meiboom-Gill (CPMG) delay variation relaxation studies provided an estimation of the apparent activation energy around 35-43 kJ x mol(-1) and an exchange rate of about 200 ms(-1) for this conformational exchange. 15N relaxation parameters were also recorded in dodecylphosphocholine (DPC) micelles and DMSO. Similar chemical exchange around the fatty acid was found in DPC but not in DMSO, which demonstrates that this phenomenon only occurs in micellar media. Consequently, it may either reflect the disorder observed in our structures determined in SDS or originate from an interaction of the lipopeptide with the detergent, which would be qualitatively similar with an anionic (SDS) or a zwitterionic (DPC) detergent. These structural and dynamics results on surfactin are the first NMR characterization of a lipopeptide incorporated in micelles. Moreover, they provide a model of surfactin determined in a more biomimetic environment than an organic solvent, which could be useful for understanding the molecular mechanism of its biological activity.

1H, 13C and 15N resonance assignment of the oxidized form (Cys(67)-Cys (70)) of the N-terminal domain of PilB from Neisseria meningitidis.

We report the nearly complete 1H, 13C and 15N resonance assignments of the oxidized form (Cys(67)-Cys(70)) of the N-terminal domain of PilB from Neisseria meningitidis. Secondary structure determination using CSI method and TALOS leads mainly to the prediction of 7 alpha-helical and 5 beta-sheet parts.

Glutathionylation of chloroplast thioredoxin f is a redox signaling mechanism in plants.

Thioredoxin f (TRXf) is a key factor in the redox regulation of chloroplastic carbon fixation enzymes, whereas glutathione is an important thiol buffer whose status is modulated by stress conditions. Here, we report specific glutathionylation of TRXf. A conserved cysteine is present in the TRXf primary sequence, in addition to its two active-site cysteines. The additional cysteine becomes glutathionylated when TRXf is exposed to oxidized glutathione or to reduced glutathione plus oxidants. No other chloroplastic TRX, from either Arabidopsis or Chlamydomonas, is glutathionylated under these conditions. Glutathionylation decreases the ability of TRXf to be reduced by ferredoxin-thioredoxin reductase and results in impaired light activation of target enzymes in a reconstituted thylakoid system. Although several mammalian proteins undergoing glutathionylation have already been identified, TRXf is among the first plant proteins found to undergo this posttranslational modification. This report suggests that a crosstalk between the TRX and glutathione systems mediates a previously uncharacterized form of redox signaling in plants in stress conditions.

Crystal structure and solution NMR dynamics of a D (type II) peroxiredoxin glutaredoxin and thioredoxin dependent: a new insight into the peroxiredoxin oligomerism.

Peroxiredoxins (Prxs) constitute a family of thiol peroxidases that reduce hydrogen peroxide, peroxinitrite, and hydroperoxides using a strictly conserved cysteine. Very abundant in all organisms, Prxs are produced as diverse isoforms characterized by different catalytic mechanisms and various thiol-containing reducing agents. The oligomeric state of Prxs and the link with their functionality is a subject of intensive research. We present here a combined X-ray and nuclear magnetic resonance (NMR) study of a plant Prx that belongs to the D-Prx (type II) subfamily. The Populus trichocarpa Prx is the first Prx shown to be regenerated in vitro by both the glutaredoxin and thioredoxin systems. The crystal structure and solution NMR provide evidence that the reduced protein is a specific noncovalent homodimer both in the crystal and in solution. The dimer interface is roughly perpendicular to the plane of the central beta sheet and differs from the interface of A- and B-Prx dimers, where proteins associate in the plane parallel to the beta sheet. The homodimer interface involves residues strongly conserved in the D (type II) Prxs, suggesting that all Prxs of this family can homodimerize. The study provides a new insight into the Prx oligomerism and the basis for protein-protein and enzyme-substrate interaction studies by NMR.