Cotranslational proteolysis dominates glutathione homeostasis to support proper growth and development

Two related families of metal transferases, ZNG1 and ZNG2, are involved in acclimation to poor Zn nutrition in Arabidopsis

Metal homeostasis has evolved to tightly modulate the availability of metals within the cell, avoiding cytotoxic interactions due to excess and protein inactivity due to deficiency. Even in the presence of homeostatic processes, however, low bioavailability of these essential metal nutrients in soils can negatively impact crop health and yield. While research has largely focused on how plants assimilate metals, acclimation to metal-limited environments requires a suite of strategies that are not necessarily involved in metal transport across membranes. The identification of these mechanisms provides a new opportunity to improve metal-use efficiency and develop plant foodstuffs with increased concentrations of bioavailable metal nutrients. Here, we investigate the function of two distinct subfamilies of the nucleotide-dependent metallochaperones (NMCs), named ZNG1 and ZNG2, that are found in plants, using Arabidopsis thaliana as a reference organism. AtZNG1 (AT1G26520) is an ortholog of human and fungal ZNG1, and like its previously characterized eukaryotic relatives, localizes to the cytosol and physically interacts with methionine aminopeptidase type I (AtMAP1A). Analysis of AtZNG1, AtMAP1A, AtMAP2A, and AtMAP2B transgenic mutants are consistent with the role of Arabidopsis ZNG1 as a Zn transferase for AtMAP1A, as previously described in yeast and zebrafish. Structural modeling reveals a flexible cysteine-rich loop that we hypothesize enables direct transfer of Zn from AtZNG1 to AtMAP1A during GTP hydrolysis. Based on proteomics and transcriptomics, loss of this ancient and conserved mechanism has pleiotropic consequences impacting the expression of hundreds of genes, including those involved in photosynthesis and vesicle transport. Members of the plant-specific family of NMCs, ZNG2A1 (AT1G80480) and ZNG2A2 (AT1G15730), are also required during Zn deficiency, but their target protein(s) remain to be discovered. RNA-seq analyses reveal wide-ranging impacts across the cell when the genes encoding these plastid-localized NMCs are disrupted.

Enhancement of root sulfur metabolic pathway by overexpression of OAS-TL3 to increase total soybean seed protein content

Sulfur is essential for plant growth, and the uptake of sulfate by plant roots is the primary source of plant sulfur. Previous studies have shown that the OAS-TL gene is a key enzyme in the sulfur metabolic pathway and regulates cysteine (Cys) synthase production. However, the interaction mechanism of the glycine max OAS-TL3 Cys synthase (OAS-TL3) gene on soybean root morphology construction and seed protein accumulation is unclear. This study shows that mutant M18 has better root growth and development, higher seed protein content, and higher methionine (Met) content in sulfur-containing amino acids than wild-type JN18. By transcriptome sequencing, the differentially expressed OAS-TL3 gene was targeted in the mutant M18 root line. The relative expression of the OAS-TL3 gene in roots, stems, and leaves during the seedling, flowering, and bulking stages of the OAS-TL3 gene overexpression lines is higher than that of the recipient material. Compared to the recipient material JN74, the enzymatic activities, Cys, and GSH contents of OAS-TL are higher in the sulfur metabolic pathway of seedling roots. The receptor material JN74 is exogenously applied with different concentrations of reduced glutathione. The results demonstrate a positive correlation between reduced glutathione on total root length, projected area, surface area, root volume, total root tip number, total bifurcation number, and total crossing number. The Met and total protein contents of sulfur-containing amino acids in soybean seeds of the OAS-TL3 gene overexpression lines are higher than those of the recipient material JN74, while the gene-edited lines show the opposite results. In conclusion, the OAS-TL3 gene positively regulates soybean root growth, root activity, and the content of Met in the seeds through the OAS-TL-Cys-GSH pathway. It breaks the limitation of other amino acids and facilitates the increase of total seed protein content.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01348-y.

N-terminal modifications, the associated processing machinery, and their evolution in plastid-containing organisms

The N-terminus is a frequent site of protein modifications. Referring primarily to knowledge gained from land plants, here we review the modifications that change protein N-terminal residues and provide updated information about the associated machinery, including that in Archaeplastida. These N-terminal modifications include many proteolytic events as well as small group additions such as acylation or arginylation and oxidation. Compared with that of the mitochondrion, the plastid-dedicated N-terminal modification landscape is far more complex. In parallel, we extend this review to plastid-containing Chromalveolata including Stramenopiles, Apicomplexa, and Rhizaria. We report a well-conserved machinery, especially in the plastid. Consideration of the two most abundant proteins on Earth-Rubisco and actin-reveals the complexity of N-terminal modification processes. The progressive gene transfer from the plastid to the nuclear genome during evolution is exemplified by the N-terminus modification machinery, which appears to be one of the latest to have been transferred to the nuclear genome together with crucial major photosynthetic landmarks. This is evidenced by the greater number of plastid genes in Paulinellidae and red algae, the most recent and fossil recipients of primary endosymbiosis.

System-wide analyses reveal essential roles of N-terminal protein modification in bacterial membrane integrity

The removal of the N-terminal formyl group on nascent proteins by peptide deformylase (PDF) is the most prevalent protein modification in bacteria. PDF is a critical target of antibiotic development; however, its role in bacterial physiology remains a long-standing question. This work used the time-resolved analyses of the Escherichia coli translatome and proteome to investigate the consequences of PDF inhibition. Loss of PDF activity rapidly induces cellular stress responses, especially those associated with protein misfolding and membrane defects, followed by a global down-regulation of metabolic pathways. Rapid membrane hyperpolarization and impaired membrane integrity were observed shortly after PDF inhibition, suggesting that the plasma membrane disruption is the most immediate and primary consequence of formyl group retention on nascent proteins. This work resolves the physiological function of a ubiquitous protein modification and uncovers its crucial role in maintaining the structure and function of the bacterial membrane.

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PDF inhibition induces membrane defects and metabolic imbalance

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Deformylation is involved in nascent protein folding

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Membrane is the earliest and primary target of N-formylation on nascent proteins

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PDF activity is essential for redox homeostasis in bacteria

Glutathione regulates transcriptional activation of iron transporters via S-nitrosylation of bHLH factors to modulate subcellular iron homoeostasis

Glutathione (GSH) is known to regulate iron (Fe) deficiency response in plants but its involvement in modulating subcellular Fe homoeostasis remains elusive. In this study, we report that the GSH-deficient mutants, cad2-1 and pad2-1 displayed increased sensitivity to Fe deficiency with significant downregulation of the vacuolar Fe exporters, AtNRAMP3 and AtNRAMP4, and the chloroplast Fe importer, AtPIC1. Moreover, the pad2-1 mutant accumulated higher Fe levels in vacuoles but lower Fe levels in chloroplasts compared to wild type (Columbia ecotype [Col-0]) under Fe limited conditions. Exogenous GSH treatment enhanced chloroplast Fe contents in Col-0 but failed to do so in the nramp3nramp4 double mutants demonstrating that GSH plays a role in modulating subcellular Fe homoeostasis. Pharmacological experiments, mutant analysis, and promoter assays revealed that this regulation involves the transcriptional activation of Fe transporter genes by a GSH-S-nitrosoglutathione (GSNO) module. The Fe responsive bHLH transcription factors (TFs), AtbHLH29, AtbHLH38, and AtbHLH101 were found to interact with the promoters of these genes, which were, in turn, activated via S-nitrosylation (SNO). Taken together, the present study highlights the role of the GSH-GSNO module in regulating subcellular Fe homoeostasis by transcriptional activation of the Fe transporters AtNRAMP3, AtNRAMP4, and AtPIC1 via SNO of bHLH TFs during Fe deficiency.

N-Acetylation of secreted proteins is widespread in Apicomplexa and independent of acetyl-CoA ER-transporter AT1

Acetyl-CoA participates in post-translational modification of proteins, central carbon and lipid metabolism in several cell compartments. In mammals, the acetyl-CoA transporter 1 (AT1) facilitates the flux of cytosolic acetyl-CoA into the endoplasmic reticulum (ER), enabling the acetylation of proteins of the secretory pathway, in concert with dedicated acetyltransferases including NAT8. However, the implication of the ER acetyl-CoA pool in acetylation of ER-transiting proteins in Apicomplexa is unknown. We identify homologues of AT1 and NAT8 in Toxoplasma gondii and Plasmodium berghei. Proteome-wide analyses revealed widespread N-terminal acetylation marks of secreted proteins in both parasites. Such acetylation profile of N-terminally processed proteins was never observed so far in any other organisms. AT1 deletion resulted in a considerable reduction of parasite fitness. In P. berghei, AT1 is important for growth of asexual blood stages and production of female gametocytes and male gametocytogenesis impaling its requirement for transmission. In the absence of AT1, the lysine and N-terminal acetylation sites remained globally unaltered, suggesting an uncoupling between the role of AT1 in development and active acetylation occurring along the secretory pathway.

Inflammation and Wasting of Skeletal Muscles in Kras-p53-Mutant Mice with Intraepithelial Neoplasia and Pancreatic Cancer-When Does Cachexia Start?

Skeletal muscle wasting critically impairs the survival and quality of life in patients with pancreatic ductal adenocarcinoma (PDAC). To identify the local factors initiating muscle wasting, we studied inflammation, fiber cross-sectional area (CSA), composition, amino acid metabolism and capillarization, as well as the integrity of neuromuscular junctions (NMJ, pre-/postsynaptic co-staining) and mitochondria (electron microscopy) in the hindlimb muscle of LSL-KrasG12D/+; LSL-TrP53R172H/+; Pdx1-Cre mice with intraepithelial-neoplasia (PanIN) 1-3 and PDAC, compared to wild-type mice (WT). Significant decreases in fiber CSA occurred with PDAC but not with PanIN 1-3, compared to WT: These were found in the gastrocnemius (type 2x: −20.0%) and soleus (type 2a: −21.0%, type 1: −14.2%) muscle with accentuation in the male soleus (type 2a: −24.8%, type 1: −17.4%) and female gastrocnemius muscle (−29.6%). Significantly higher densities of endomysial CD68+ and cyclooxygenase-2+ (COX2+) cells were detected in mice with PDAC, compared to WT mice. Surprisingly, CD68+ and COX2+ cell densities were also higher in mice with PanIN 1-3 in both muscles. Significant positive correlations existed between muscular and hepatic CD68+ or COX2+ cell densities. Moreover, in the gastrocnemius muscle, suppressor-of-cytokine-3 (SOCS3) expressions was upregulated >2.7-fold with PanIN 1A-3 and PDAC. The intracellular pools of proteinogenic amino acids and glutathione significantly increased with PanIN 1A-3 compared to WT. Capillarization, NMJ, and mitochondrial ultrastructure remained unchanged with PanIN or PDAC. In conclusion, the onset of fiber atrophy coincides with the manifestation of PDAC and high-grade local (and hepatic) inflammatory infiltration without compromised microcirculation, innervation or mitochondria. Surprisingly, muscular and hepatic inflammation, SOCS3 upregulation and (proteolytic) increases in free amino acids and glutathione were already detectable in mice with precancerous PanINs. Studies of initial local triggers and defense mechanisms regarding cachexia are warranted for targeted anti-inflammatory prevention.

Protein Phosphorylation Response to Abiotic Stress in Plants

Plants are an important part of nature because as photoautotrophs, they provide a nutrient source for many other living organisms. Due to their sessile nature, to overcome both biotic and abiotic stresses, plants have developed intricate mechanisms for perception of and reaction to these stresses, both on an external level (perception) and on an internal level (reaction). Specific proteins found within cells play crucial roles in stress mitigation by enhancing cellular processes that facilitate the plants survival during the unfavorable conditions. Well before plants are able to synthesize nascent proteins in response to stress, proteins which already exist in the cell can be subjected to an array of posttranslation modifications (PTMs) that permit a rapid response. These activated proteins can, in turn, aid in further stress responses. Different PTMs have different functions in growth and development of plants. Protein phosphorylation, a reversible form of modification has been well elucidated, and its role in signaling cascades is well documented. In this mini-review, we discuss the integration of protein phosphorylation with other components of abiotic stress-responsive pathways including phytohormones and ion homeostasis. Overall, this review demonstrates the high interconnectivity of the stress response system in plants and how readily plants are able to toggle between various signaling pathways in order to survive harsh conditions. Most notably, fluctuations of the cytosolic calcium levels seem to be a linking component of the various signaling pathways.

The Arabidopsis Nα -acetyltransferase NAA60 locates to the plasma membrane and is vital for the high salt stress response

In humans and plants, N-terminal acetylation plays a central role in protein homeostasis, affects 80% of proteins in the cytoplasm and is catalyzed by five ribosome-associated N-acetyltransferases (NatA-E). Humans also possess a Golgi-associated NatF (HsNAA60) that is essential for Golgi integrity. Remarkably, NAA60 is absent in fungi and has not been identified in plants. Here we identify and characterize the first plasma membrane-anchored post-translationally acting N-acetyltransferase AtNAA60 in the reference plant Arabidopsis thaliana by the combined application of reverse genetics, global proteomics, live-cell imaging, microscale thermophoresis, circular dichroism spectroscopy, nano-differential scanning fluorometry, intrinsic tryptophan fluorescence and X-ray crystallography. We demonstrate that AtNAA60, like HsNAA60, is membrane-localized in vivo by an α-helical membrane anchor at its C-terminus, but in contrast to HsNAA60, AtNAA60 localizes to the plasma membrane. The AtNAA60 crystal structure provides insights into substrate-binding, the broad substrate specificity and the catalytic mechanism probed by structure-based mutagenesis. Characterization of the NAA60 loss-of-function mutants (naa60-1 and naa60-2) uncovers a plasma membrane-localized substrate of AtNAA60 and the importance of NAA60 during high salt stress. Our findings provide evidence for the plant-specific evolution of a plasma membrane-anchored N-acetyltransferase that is vital for adaptation to stress.

Cyclic GMP-AMP synthase promotes the inflammatory and autophagy responses in Huntington disease

Huntington disease (HD) is a genetic disorder caused by glutamine-expansion in the huntingtin (mHTT) protein, which affects motor, psychiatric, and cognitive function, but the mechanisms remain unclear. mHTT is known to induce DNA damage and affect autophagy, both associated with inflammatory responses, but what mediates all these were unknown. Here we report that cGAS, a DNA damage sensor, is highly upregulated in the striatum of a mouse model and HD human patient’s tissue. We found ribosomes, which make proteins, are robustly accumulated on the cGAS mRNA in HD cells. cGAS depletion decreases—and cGAS expression increases—both inflammatory and autophagy responses in HD striatal cells. Thus, cGAS is a therapeutic target for HD. Blocking cGAS will prevent/slow down HD symptoms.

Huntington disease (HD) is caused by an expansion mutation of the N-terminal polyglutamine of huntingtin (mHTT). mHTT is ubiquitously present, but it induces noticeable damage to the brain’s striatum, thereby affecting motor, psychiatric, and cognitive functions. The striatal damage and progression of HD are associated with the inflammatory response; however, the underlying molecular mechanisms remain unclear. Here, we report that cGMP-AMP synthase (cGAS), a DNA sensor, is a critical regulator of inflammatory and autophagy responses in HD. Ribosome profiling revealed that the cGAS mRNA has high ribosome occupancy at exon 1 and codon-specific pauses at positions 171 (CCG) and 172 (CGT) in HD striatal cells. Moreover, the protein levels and activity of cGAS (based on the phosphorylated STING and phosphorylated TBK1 levels), and the expression and ribosome occupancy of cGAS-dependent inflammatory genes (Ccl5 and Cxcl10) are increased in HD striatum. Depletion of cGAS diminishes cGAS activity and decreases the expression of inflammatory genes while suppressing the up-regulation of autophagy in HD cells. In contrast, reinstating cGAS in cGAS-depleted HD cells activates cGAS activity and promotes inflammatory and autophagy responses. Ribosome profiling also revealed that LC3A and LC3B, the two major autophagy initiators, show altered ribosome occupancy in HD cells. We also detected the presence of numerous micronuclei, which are known to induce cGAS, in the cytoplasm of neurons derived from human HD embryonic stem cells. Collectively, our results indicate that cGAS is up-regulated in HD and mediates inflammatory and autophagy responses. Thus, targeting the cGAS pathway may offer therapeutic benefits in HD.

The Scope, Functions, and Dynamics of Posttranslational Protein Modifications

Assessing posttranslational modification (PTM) patterns within protein molecules and reading their functional implications present grand challenges for plant biology. We combine four perspectives on PTMs and their roles by considering five classes of PTMs as examples of the broader context of PTMs. These include modifications of the N terminus, glycosylation, phosphorylation, oxidation, and N-terminal and protein modifiers linked to protein degradation. We consider the spatial distribution of PTMs, the subcellular distribution of modifying enzymes, and their targets throughout the cell, and we outline the complexity of compartmentation in understanding of PTM function. We also consider PTMs temporally in the context of the lifetime of a protein molecule and the need for different PTMs for assembly, localization, function, and degradation. Finally, we consider the combined action of PTMs on the same proteins, their interactions, and the challenge ahead of integrating PTMs into an understanding of protein function in plants.

MetAP1 and MetAP2 drive cell selectivity for a potent anti-cancer agent in synergy, by controlling glutathione redox state

Fumagillin and its derivatives are therapeutically useful because they can decrease cancer progression. The specific molecular target of fumagillin is methionine aminopeptidase 2 (MetAP2), one of the two MetAPs present in the cytosol. MetAPs catalyze N-terminal methionine excision (NME), an essential pathway of cotranslational protein maturation. To date, it remains unclear the respective contribution of MetAP1 and MetAP2 to the NME process in vivo and why MetAP2 inhibition causes cell cycle arrest only in a subset of cells. Here, we performed a global characterization of the N-terminal methionine excision pathway and the inhibition of MetAP2 by fumagillin in a number of lines, including cancer cell lines. Large-scale N-terminus profiling in cells responsive and unresponsive to fumagillin treatment revealed that both MetAPs were required in vivo for M[VT]X-targets and, possibly, for lower-level M[G]X-targets. Interestingly, we found that the responsiveness of the cell lines to fumagillin was correlated with the ability of the cells to modulate their glutathione homeostasis. Indeed, alterations to glutathione status were observed in fumagillin-sensitive cells but not in cells unresponsive to this agent. Proteo-transcriptomic analyses revealed that both MetAP1 and MetAP2 accumulated in a cell-specific manner and that cell sensitivity to fumagillin was related to the levels of these MetAPs, particularly MetAP1. We suggest that MetAP1 levels could be routinely checked in several types of tumor and used as a prognostic marker for predicting the response to treatments inhibiting MetAP2.

Stress-triggered redox signalling: what's in pROSpect?

Reactive oxygen species (ROS) have a profound influence on almost every aspect of plant biology. Here, we emphasize the fundamental, intimate relationships between light-driven reductant formation, ROS, and oxidative stress, together with compartment-specific differences in redox buffering and the perspectives for their analysis. Calculations of approximate H2 O2 concentrations in the peroxisomes are provided, and based on the likely values in other locations such as chloroplasts, we conclude that much of the H2 O2 detected in conventional in vitro assays is likely to be extracellular. Within the context of scant information on ROS perception mechanisms, we consider current knowledge, including possible parallels with emerging information on oxygen sensing. Although ROS can sometimes be signals for cell death, we consider that an equally important role is to transmit information from metabolism to allow appropriate cellular responses to developmental and environmental changes. Our discussion speculates on novel sensing mechanisms by which this could happen and how ROS could be counted by the cell, possibly as a means of monitoring metabolic flux. Throughout, we place emphasis on the positive effects of ROS, predicting that in the coming decades they will increasingly be defined as hallmarks of viability within a changing and challenging environment.

The intriguing realm of protein biogenesis: Facing the green co-translational protein maturation networks

The ribosome is the cell's protein-making factory, a huge protein-RNA complex, that is essential to life. Determining the high-resolution structures of the stable "core" of this factory was among the major breakthroughs of the past decades, and was awarded the Nobel Prize in 2009. Now that the mysteries of the ribosome appear to be more traceable, detailed understanding of the mechanisms that regulate protein synthesis includes not only the well-known steps of initiation, elongation, and termination but also the less comprehended features of the co-translational events associated with the maturation of the nascent chains. The ribosome is a platform for co-translational events affecting the nascent polypeptide, including protein modifications, folding, targeting to various cellular compartments for integration into membrane or translocation, and proteolysis. These events are orchestrated by ribosome-associated protein biogenesis factors (RPBs), a group of a dozen or more factors that act as the "welcoming committee" for the nascent chain as it emerges from the ribosome. In plants these factors have evolved to fit the specificity of different cellular compartments: cytoplasm, mitochondria and chloroplast. This review focuses on the current state of knowledge of these factors and their interaction around the exit tunnel of dedicated ribosomes. Particular attention has been accorded to the plant system, highlighting the similarities and differences with other organisms.

Increased glutathione contributes to stress tolerance and global translational changes in Arabidopsis

Although glutathione is well known for its reactive oxygen species (ROS) scavenging function and plays a protective role in biotic stress, its regulatory function in abiotic stress still remains to be elucidated. Our previous study showed that exogenously applied reduced glutathione (GSH) could improve abiotic stress tolerance in Arabidopsis. Here, we report that endogenously increased GSH also conferred tolerance to drought and salt stress in Arabidopsis. Moreover, both exogenous and endogenous GSH delayed senescence and flowering time. Polysomal profiling results showed that global translation was enhanced after GSH treatment and by the induced increase of GSH level by salt stress. By performing transcriptomic analyses of steady-state and polysome-bound mRNAs in GSH-treated plants, we reveal that GSH has a substantial impact on translation. Translational changes induced by GSH treatment target numerous hormones and stress signaling molecules, which might contribute to the enhanced stress tolerance in GSH-treated plants. Our translatome analysis also revealed that abscisic acid (ABA), auxin and jasmonic acid (JA) biosynthesis, as well as signaling genes, were activated during GSH treatment, which has not been reported in previously published transcriptomic data. Together, our data suggest that the increased glutathione level results in stress tolerance and global translational changes.

N-terminal protein modifications: Bringing back into play the ribosome

N-terminal protein modifications correspond to the first modifications which in principle any protein may undergo, before translation is completed by the ribosome. This class of essential modifications can have different nature or function and be catalyzed by a variety of dedicated enzymes. Here, we review the current state of the major N-terminal co-translational modifications, with a particular emphasis to their catalysts, which belong to metalloprotease and acyltransferase clans. The earliest of these modifications corresponds to the N-terminal methionine excision, an ubiquitous and essential process leading to the removal of the first methionine. N-alpha acetylation occurs also in all Kingdoms although its extent appears to be significantly increased in higher eukaryotes. Finally, N-myristoylation is a crucial pathway existing only in eukaryotes. Recent studies dealing on how some of these co-translational modifiers might work in close vicinity of the ribosome is starting to provide new information on when these modifications exactly take place on the elongating nascent chain and the interplay with other ribosome biogenesis factors taking in charge the nascent chains. Here a comprehensive overview of the recent advances in the field of N-terminal protein modifications is given.

Two N-terminal acetyltransferases antagonistically regulate the stability of a nod-like receptor in Arabidopsis

Nod-like receptors (NLRs) serve as immune receptors in plants and animals. The stability of NLRs is tightly regulated, though its mechanism is not well understood. Here, we show the crucial impact of N-terminal acetylation on the turnover of one plant NLR, Suppressor of NPR1, Constitutive 1 (SNC1), in Arabidopsis thaliana. Genetic and biochemical analyses of SNC1 uncovered its multilayered regulation by different N-terminal acetyltransferase (Nat) complexes. SNC1 exhibits a few distinct N-terminal isoforms generated through alternative initiation and N-terminal acetylation. Its first Met is acetylated by N-terminal acetyltransferase complex A (NatA), while the second Met is acetylated by N-terminal acetyltransferase complex B (NatB). Unexpectedly, the NatA-mediated acetylation serves as a degradation signal, while NatB-mediated acetylation stabilizes the NLR protein, thus revealing antagonistic N-terminal acetylation of a single protein substrate. Moreover, NatA also contributes to the turnover of another NLR, RESISTANCE TO P. syringae pv maculicola 1. The intricate regulation of protein stability by Nats is speculated to provide flexibility for the target protein in maintaining its homeostasis.

A method for proteomic analysis of equine subchondral bone and epiphyseal cartilage

Proteomic analyses of cartilage and, to a lesser extent, of bone have long been impaired because of technical challenges related to their structure and biochemical properties. We have developed a unified method based on phenol extraction, 2DE, silver staining, and subsequent LC-MS/MS. This method proved to be efficient to characterize the proteome of equine cartilage and bone samples collected in vivo. Since proteins from several cellular compartments could be recovered, our procedure is mainly suitable for in situ molecular physiology studies focused on the cellular content of chondrocytes, osteoblasts, and osteoclasts as well as that of the extracellular matrix, with the exception of proteoglycans. Our method alleviates some drawbacks of cell culture that can mask physiological differences, as well as reduced reproducibility due to fractionation. Proteomic comparative studies between cartilage and bone samples from healthy and affected animals were thus achieved successfully. This achievement will contribute to increasing knowledge on the molecular mechanisms underlying the physiopathology of numerous osteoarticular diseases in horses and in humans.

Hibiscus chlorotic ringspot virus coat protein upregulates sulfur metabolism genes for enhanced pathogen defense

In both Hibiscus chlorotic ringspot virus (HCRSV)-infected and HCRSV coat protein (CP) agroinfiltrated plant leaves, we showed that sulfur metabolism pathway related genes-namely, sulfite oxidase (SO), sulfite reductase, and adenosine 5'-phosphosulfate kinase-were upregulated. It led us to examine a plausible relationship between sulfur-enhanced resistance (SED) and HCRSV infection. We broadened an established method to include different concentrations of sulfur (0S, 1S, 2S, and 3S) to correlate them to symptom development of HCRSV-infected plants. We treated plants with glutathione and its inhibitor to verify the SED effect. Disease resistance was induced through elevated glutathione contents during HCRSV infection. The upregulation of SO was related to suppression of symptom development induced by sulfur treatment. In this study, we established that HCRSV-CP interacts with SO which, in turn, triggers SED and leads to enhanced plant resistance. Thus, we have discovered a new function of SO in the SED pathway. This is the first report to demonstrate that the interaction of a viral protein and host protein trigger SED in plants. It will be interesting if such interaction applies generally to other host-pathogen interactions that will lead to enhanced pathogen defense.

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.

Seed germination and vigor

Germination vigor is driven by the ability of the plant embryo, embedded within the seed, to resume its metabolic activity in a coordinated and sequential manner. Studies using "-omics" approaches support the finding that a main contributor of seed germination success is the quality of the messenger RNAs stored during embryo maturation on the mother plant. In addition, proteostasis and DNA integrity play a major role in the germination phenotype. Because of its pivotal role in cell metabolism and its close relationships with hormone signaling pathways regulating seed germination, the sulfur amino acid metabolism pathway represents a key biochemical determinant of the commitment of the seed to initiate its development toward germination. This review highlights that germination vigor depends on multiple biochemical and molecular variables. Their characterization is expected to deliver new markers of seed quality that can be used in breeding programs and/or in biotechnological approaches to improve crop yields.

Molecular Biology, Biochemistry and Cellular Physiology of Cysteine Metabolism in Arabidopsis thaliana

Cysteine is one of the most versatile molecules in biology, taking over such different functions as catalysis, structure, regulation and electron transport during evolution. Research on Arabidopsis has contributed decisively to the understanding of cysteine synthesis and its role in the assimilatory pathways of S, N and C in plants. The multimeric cysteine synthase complex is present in the cytosol, plastids and mitochondria and forms the centre of a unique metabolic sensing and signaling system. Its association is reversible, rendering the first enzyme of cysteine synthesis active and the second one inactive, and vice-versa. Complex formation is triggered by the reaction intermediates of cysteine synthesis in response to supply and demand and gives rise to regulation of genes of sulfur metabolism to adjust cellular sulfur homeostasis. Combinations of biochemistry, forward and reverse genetics, structural- and cell-biology approaches using Arabidopsis have revealed new enzyme functions and the unique pattern of spatial distribution of cysteine metabolism in plant cells. These findings place the synthesis of cysteine in the centre of the network of primary metabolism.

Interplay between N-terminal methionine excision and FtsH protease is essential for normal chloroplast development and function in Arabidopsis

N-terminal methionine excision (NME) is the earliest modification affecting most proteins. All compartments in which protein synthesis occurs contain dedicated NME machinery. Developmental defects induced in Arabidopsis thaliana by NME inhibition are accompanied by increased proteolysis. Although increasing evidence supports a connection between NME and protein degradation, the identity of the proteases involved remains unknown. Here we report that chloroplastic NME (cNME) acts upstream of the FtsH protease complex. Developmental defects and higher sensitivity to photoinhibition associated with the ftsh2 mutation were abolished when cNME was inhibited. Moreover, the accumulation of D1 and D2 proteins of the photosystem II reaction center was always dependent on the prior action of cNME. Under standard light conditions, inhibition of chloroplast translation induced accumulation of correctly NME-processed D1 and D2 in a ftsh2 background, implying that the latter is involved in protein quality control, and that correctly NME-processed D1 and D2 are turned over primarily by the thylakoid FtsH protease complex. By contrast, inhibition of cNME compromises the specific N-terminal recognition of D1 and D2 by the FtsH complex, whereas the unprocessed forms are recognized by other proteases. Our results highlight the tight functional interplay between NME and the FtsH protease complex in the chloroplast.

Analysis of the xylem sap proteome of Brassica oleracea reveals a high content in secreted proteins

Xylem plays a major role in plant development and is considered part of the apoplast. Here, we studied the proteome of Brassica oleracea cv Bartolo and compared it to the plant cell wall proteome of another Brassicaceae, the model plant Arabidopsis thaliana. B. oleracea was chosen because it is technically difficult to harvest enough A. thaliana xylem sap for proteomic analysis. We studied the whole proteome and an N-glycoproteome obtained after Concanavalin A affinity chromatography. Altogether, 189 proteins were identified by LC-MS/MS using Brassica EST and cDNA sequences. A predicted signal peptide was found in 164 proteins suggesting that most proteins of the xylem sap are secreted. Eighty-one proteins were identified in the N-glycoproteome, with 25 of them specific of this fraction, suggesting that they were concentrated during the chromatography step. All the protein families identified in this study were found in the cell wall proteomes. However, proteases and oxido-reductases were more numerous in the xylem sap proteome, whereas enzyme inhibitors were rare. The origin of xylem sap proteins is discussed. All the experimental data including the MS/MS data were made available in the WallProtDB cell wall proteomic database.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

Increased intracellular H₂O₂ availability preferentially drives glutathione accumulation in vacuoles and chloroplasts

One biochemical response to increased H₂O₂ availability is the accumulation of glutathione disulphide (GSSG), the disulphide form of the key redox buffer glutathione. It remains unclear how this potentially important oxidative stress response impacts on the different sub-cellular glutathione pools. We addressed this question by using two independent in situ glutathione labelling techniques in Arabidopsis wild type (Col-0) and the GSSG-accumulating cat2 mutant. A comparison of in situ labelling with monochlorobimane (MCB) and in vitro labelling with monobromobimane (MBB) revealed that, whereas in situ labelling of Col-0 leaf glutathione was complete within 2 h incubation, about 50% of leaf glutathione remained inaccessible to MCB in cat2. High-performance liquid chromatography (HPLC) and enzymatic assays showed that this correlated tightly with the glutathione redox state, pointing to significant in vivo pools of GSSG in cat2 that were unavailable for MCB labelling. Immunogold labelling of leaf sections to estimate sub-cellular glutathione distribution showed that the accumulated GSSG in cat2 was associated with only a minor increase in cytosolic glutathione but with a 3- and 10-fold increase in plastid and vacuolar pools, respectively. The data are used to estimate compartment-specific glutathione concentrations under optimal and oxidative stress conditions, and the implications for redox homeostasis and signalling are discussed.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

Sulfur assimilation in photosynthetic organisms: molecular functions and regulations of transporters and assimilatory enzymes

Sulfur is required for growth of all organisms and is present in a wide variety of metabolites having distinctive biological functions. Sulfur is cycled in ecosystems in nature where conversion of sulfate to organic sulfur compounds is primarily dependent on sulfate uptake and reduction pathways in photosynthetic organisms and microorganisms. In vascular plant species, transport proteins and enzymes in this pathway are functionally diversified to have distinct biochemical properties in specific cellular and subcellular compartments. Recent findings indicate regulatory processes of sulfate transport and metabolism are tightly connected through several modes of transcriptional and posttranscriptional mechanisms. This review provides up-to-date knowledge in functions and regulations of sulfur assimilation in plants and algae, focusing on sulfate transport systems and metabolic pathways for sulfate reduction and synthesis of downstream metabolites with diverse biological functions.