Combining various strategies to increase the coverage of the plant cell wall glycoproteome

LSPpred Suite: Tools for Leaderless Secretory Protein Prediction in Plants

Plant proteins that are secreted without a classical signal peptide leader sequence are termed leaderless secretory proteins (LSPs) and are implicated in both plant development and (a)biotic stress responses. In plant proteomics experimental workflows, identification of LSPs is hindered by the possibility of contamination from other subcellar compartments upon purification of the secretome. Applying machine learning algorithms to predict LSPs in plants is also challenging due to the rarity of experimentally validated examples for training purposes. This work attempts to address this issue by establishing criteria for identifying potential plant LSPs based on experimental observations and training random forest classifiers on the putative datasets. The resultant plant protein database LSPDB and bioinformatic prediction tools LSPpred and SPLpred are available at lsppred.lspdb.org. The LSPpred and SPLpred modules are internally validated on the training dataset, with false positives controlled at 5%, and are also able to classify the limited number of established plant LSPs (SPLpred (3/4, LSPpred 4/4). Until such time as a larger set of bona fide (independently experimentally validated) LSPs is established using imaging technologies (light/fluorescence/electron microscopy) to confirm sub-cellular location, these tools represent a bridging method for predicting and identifying plant putative LSPs for subsequent experimental validation.

N-glycoproteins in Plant Cell Walls: A Survey

Cell walls are an extracellular compartment specific to plant cells, which are not found in animal cells. Their composition varies between cell types, plant species, and physiological states. They are composed of a great diversity of polymers, i.e., polysaccharides, proteins, and lignins. Cell wall proteins (CWPs) are major players involved in the plasticity of cell walls which support cell growth and differentiation, as well as adaptation to environmental changes. In order to reach the extracellular space, CWPs are transported through the secretory pathway where they may undergo post-translational modifications, including N-glycosylations on the Asn residues in specific motifs (Asn-X-Ser/Thr-X, with X≠Pro). This review aims at providing a survey of the present knowledge related to cell wall N-glycoproteins with (i) an overview of the experimental workflows, (ii) a selection of relevant articles dedicated to N-glycoproteomics, (iii) a description of the diversity of N-glycans, and (iv) a focus on the importance of N-glycans for CWP structure and/or function.

Antithrombotic and antihemolytic effects of Lagerstroemia speciosa (Lythraceae) aqueous extract

Introduction: Aqueous extract of Lagerstroemia speciosa (EALS) (Lythraceae) is widely used to treat diabetes. This plant has been shown an in vitro thrombolytic activity that indicates its potential to prevent the formation of blood clots in vivo. Thus, this study was undertaken to evaluate the antithrombotic and antihemolytic effects of EALS. Methods: Rats of both sexes (200 ± 5 g) were divided into five groups of six animals. Each group received orally distilled water, EALS (250, 500, 1000 mg/kg), and acetylsalicylic acid (100 mg/kg) for five days. After treatment, the FeCl3-induced arterial thrombus formation method was used to determine occlusion time. A coagulometer was used to detect activated partial thromboplastin time (aPTT) and prothrombin time (PT). Rabbit blood was used to determine clot lysis activity in vitro and antihemolytic activity using the 2,2-azobis hydrochloride (2-methylpropionamidine) (AAPH) method. Results: EALS increased the occlusion time in a dose-dependent manner. At the dose of 1000 mg/kg, EALS increased the occlusion time significantly, from 4.59 ± 2.45 minutes to 15.52 ± 2.38 minutes (P<0.01). At high concentrations (1-4 mg/mL), EALS showed a significant increase in aPPT and PT (P<0.05). Streptokinase and EALS (4 mg/mL) induced significant clot lysis with percentage values of 78.48 ± 2.2 % and 49.5 ± 1.53 %, respectively (P<0.001). EALS inhibited AAPH-induced hemolysis. Conclusion: EALS exhibited antithrombotic and antihemolytic activities. The antithrombotic property of the plant could be attributed to its anticoagulant and thrombolytic activities. Regular consumption of L. speciosa leaves may prevent or treat thrombotic diseases.

A lipid viewpoint on the plant endoplasmic reticulum stress response

Organisms, including humans, seem to be constantly exposed to various changes, which often have undesirable effects, referred to as stress. To keep up with these changes, eukaryotic cells may have evolved a number of relevant cellular processes, such as the endoplasmic reticulum (ER) stress response. Owing to presumably intimate links between human diseases and the ER function, the ER stress response has been extensively investigated in various organisms for a few decades. Based on these studies, we now have a picture of the molecular mechanisms of the ER stress response, one of which, the unfolded protein response (UPR), is highly conserved among yeasts, mammals, higher plants, and green algae. In this review, we attempt to highlight the plant UPR from the perspective of lipids, especially membrane phospholipids. Phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) are the most abundant membrane phospholipids in eukaryotic cells. The ratio of PtdCho to PtdEtn and the unsaturation of fatty acyl tails in both phospholipids may be critical factors for the UPR, but the pathways responsible for PtdCho and PtdEtn biosynthesis are distinct in animals and plants. We discuss the plant UPR in comparison with the system in yeasts and animals in the context of membrane phospholipids.

Lewis A Glycans Are Present on Proteins Involved in Cell Wall Biosynthesis and Appear Evolutionarily Conserved Among Natural Arabidopsis thaliana Accessions

N-glycosylation is a highly abundant protein modification present in all domains of life. Terminal sugar residues on complex-type N-glycans mediate various crucial biological processes in mammals such as cell-cell recognition or protein-ligand interactions. In plants, the Lewis A trisaccharide constitutes the only known outer-chain elongation of complex N-glycans. Lewis A containing complex N-glycans appear evolutionary conserved, having been identified in all plant species analyzed so far. Despite their ubiquitous occurrence, the biological function of this complex N-glycan modification is currently unknown. Here, we report the identification of Lewis A bearing glycoproteins from three different plant species: Arabidopsis thaliana, Nicotiana benthamiana, and Oryza sativa. Affinity purification via the JIM84 antibody, directed against Lewis A structures on complex plant N-glycans, was used to enrich Lewis A bearing glycoproteins, which were subsequently identified via nano-LC-MS. Selected identified proteins were recombinantly expressed and the presence of Lewis A confirmed via immunoblotting and site-specific N-glycan analysis. While the proteins identified in O. sativa are associated with diverse functions, proteins from A. thaliana and N. benthamiana are mainly involved in cell wall biosynthesis. However, a Lewis A-deficient mutant line of A. thaliana showed no change in abundance of cell wall constituents such as cellulose or lignin. Furthermore, we investigated the presence of Lewis A structures in selected accessions from the 1001 genome database containing amino acid variations in the enzymes required for Lewis A biosynthesis. Besides one relict line showing no detectable levels of Lewis A, the modification was present in all other tested accessions. The data provided here comprises the so far first attempt at identifying Lewis A bearing glycoproteins across different species and will help to shed more light on the role of Lewis A structures in plants.

Identification and Characterization of Glycoproteins and Their Responsive Patterns upon Ethylene Stimulation in the Rubber Latex

Natural rubber is an important industrial material, which is obtained from the only commercially cultivated rubber tree, Hevea brasiliensis. In rubber latex production, ethylene has been extensively used as a stimulant. Recent research showed that post-translational modifications (PTMs) of latex proteins, such as phosphorylation, glycosylation and ubiquitination, are crucial in natural rubber biosynthesis. In this study, comparative proteomics was performed to identify the glycosylated proteins in rubber latex treated with ethylene for different days. Combined with Pro-Q Glycoprotein gel staining and mass spectrometry techniques, we provided the first visual profiling of glycoproteomics of rubber latex and finally identified 144 glycosylated protein species, including 65 differentially accumulated proteins (DAPs) after treating with ethylene for three and/or five days. Gene Ontology (GO) functional annotation showed that these ethylene-responsive glycoproteins are mainly involved in cell parts, membrane components and metabolism. Pathway analysis demonstrated that these glycosylated rubber latex proteins are mainly involved in carbohydrate metabolism, energy metabolism, degradation function and cellular processes in rubber latex metabolism. Protein-protein interaction analysis revealed that these DAPs are mainly centered on acetyl-CoA acetyltransferase and hydroxymethylglutaryl-CoA synthase (HMGS) in the mevalonate pathway for natural rubber biosynthesis. In our glycoproteomics, three protein isoforms of HMGS2 were identified from rubber latex, and only one HMGS2 isoform was sharply increased in rubber latex by ethylene treatment for five days. Furthermore, the HbHMGS2 gene was over-expressed in a model rubber-producing grass Taraxacum Kok-saghyz and rubber content in the roots of transgenic rubber grass was significantly increased over that in the wild type plant, indicating HMGS2 is the key component for natural rubber production.

Cell Wall Proteins Play Critical Roles in Plant Adaptation to Phosphorus Deficiency

Phosphorus is one of the mineral nutrient elements essential for plant growth and development. Low phosphate (Pi) availability in soils adversely affects crop production. To cope with low P stress, remodeling of root morphology and architecture is generally observed in plants, which must be accompanied by root cell wall modifications. It has been documented that cell wall proteins (CWPs) play critical roles in shaping cell walls, transmitting signals, and protecting cells against environmental stresses. However, understanding of the functions of CWPs involved in plant adaptation to P deficiency remains fragmentary. The aim of this review was to summarize advances in identification and functional characterization of CWPs in responses to P deficiency, and to highlight the critical roles of CWPs in mediating root growth, P reutilization, and mobilization in plants.

Global and site-specific analysis of protein glycosylation in complex biological systems with Mass Spectrometry

Protein glycosylation is ubiquitous in biological systems and plays essential roles in many cellular events. Global and site-specific analysis of glycoproteins in complex biological samples can advance our understanding of glycoprotein functions and cellular activities. However, it is extraordinarily challenging because of the low abundance of many glycoproteins and the heterogeneity of glycan structures. The emergence of mass spectrometry (MS)-based proteomics has provided us an excellent opportunity to comprehensively study proteins and their modifications, including glycosylation. In this review, we first summarize major methods for glycopeptide/glycoprotein enrichment, followed by the chemical and enzymatic methods to generate a mass tag for glycosylation site identification. We next discuss the systematic and quantitative analysis of glycoprotein dynamics. Reversible protein glycosylation is dynamic, and systematic study of glycoprotein dynamics helps us gain insight into glycoprotein functions. The last part of this review focuses on the applications of MS-based proteomics to study glycoproteins in different biological systems, including yeasts, plants, mice, human cells, and clinical samples. Intact glycopeptide analysis is also included in this section. Because of the importance of glycoproteins in complex biological systems, the field of glycoproteomics will continue to grow in the next decade. Innovative and effective MS-based methods will exponentially advance glycoscience, and enable us to identify glycoproteins as effective biomarkers for disease detection and drug targets for disease treatment. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 9999: XX-XX, 2019.

Subcellular locations of potential cell wall proteins in plants: predictors, databases and cross-referencing

The cell wall is the most striking feature that distinguishes plant cells from animal cells. It plays an essential role in cell shape, stability, growth and protection. Despite being present in small amounts, cell wall proteins (CWPs) are crucial components of the cell wall. The cell wall proteome generally consists of sensu stricto CWPs, apoplast proteins and extracellular secreted proteins. Currently, there is a need for the bioinformatics analysis of a tremendous number of protein sequences that have been generated from genomic, transcriptomic and proteomics research. Compared with intracellular proteins, the location prediction of CWPs is challenging because many aspects of these proteins have not been experimentally characterized, and there are no CWP-trained, specific predictors available. By introducing the biological relevance (particularly molecular aspects) of the cell wall and CWPs, we critically evaluated the accuracy of 16 state-of-the-art predictors and classical predictors for the prediction of CWPs using an independent database of Arabidopsis and rice proteins. All experimentally verified CWPs and non-CWPs were retrieved from the UniProt Knowledgebase. Based on the evaluation, we currently recommend the predictors mGOASVM, HybridGO-Loc and FUEL-mLoc for CWPs. Furthermore, we outlined the public databases that can be used to cross-reference the subcellular location of CWPs. We illustrate a flowchart of the subcellular location prediction and a cross-reference of possible CWPs. Finally, we discuss challenges and perspectives in the bioinformatics analysis of CWPs. It is hoped that this article will provide practical guidance regarding CWPs for nonspecialists and provide insight for bioinformatics experts to develop computational tools for CWPs.

The Secretome and N-Glycosylation Profiles of the Charophycean Green Alga, Penium margaritaceum, Resemble Those of Embryophytes

The secretome can be defined as the population of proteins that are secreted into the extracellular environment. Many proteins that are secreted by eukaryotes are N-glycosylated. However, there are striking differences in the diversity and conservation of N-glycosylation patterns between taxa. For example, the secretome and N-glycosylation structures differ between land plants and chlorophyte green algae, but it is not clear when this divergence took place during plant evolution. A potentially valuable system to study this issue is provided by the charophycean green algae (CGA), which is the immediate ancestors of land plants. In this study, we used lectin affinity chromatography (LAC) coupled with mass spectrometry to characterize the secretome including secreted N-glycoproteins of Penium margaritaceum, which is a member of the CGA. The identified secreted proteins and N-glycans were compared to those known from the chlorophyte green alga Chlamydomonas reinhardtii and the model land plant, Arabidopsis thaliana, to establish their evolutionary context. Our approach allowed the identification of cell wall proteins and proteins modified with N-glycans that are identical to those of embryophytes, which suggests that the P. margaritaceum secretome is more closely related to those of land plants than to those of chlorophytes. The results of this study support the hypothesis that many of the proteins associated with plant cell wall modification as well as other extracellular processes evolved prior to the colonization of terrestrial habitats.

Mass Spectrometry Approaches to Glycomic and Glycoproteomic Analyses

Glycomic and glycoproteomic analyses involve the characterization of oligosaccharides (glycans) conjugated to proteins. Glycans are produced through a complicated nontemplate driven process involving the competition of enzymes that extend the nascent chain. The large diversity of structures, the variations in polarity of the individual saccharide residues, and the poor ionization efficiencies of glycans all conspire to make the analysis arguably much more difficult than any other biopolymer. Furthermore, the large number of glycoforms associated with a specific protein site makes it more difficult to characterize than any post-translational modification. Nonetheless, there have been significant progress, and advanced separation and mass spectrometry methods have been at its center and the main reason for the progress. While glycomic and glycoproteomic analyses are still typically available only through highly specialized laboratories, new software and workflow is making it more accessible. This review focuses on the role of mass spectrometry and separation methods in advancing glycomic and glycoproteomic analyses. It describes the current state of the field and progress toward making it more available to the larger scientific community.

A light-sensitive mutation in Arabidopsis LEW3 reveals the important role of N-glycosylation in root growth and development

Plant roots have the potential capacity to grow almost indefinitely if meristematic and lateral branching is sustained. In a genetic screen we identified an Arabidopsis mutant showing limited root growth (lrg1) due to defects in cell division and elongation in the root meristem. Positional cloning determined that lrg1 affects an alpha-1,2-mannosyltransferase gene, LEW3, involved in protein N-glycosylation. The lrg1 mutation causes a synonymous substitution that alters the correct splicing of the fourth intron in LEW3, causing a mix of wild-type and truncated protein. LRG1 RNA missplicing in roots and short root phenotypes in lrg1 are light-intensity dependent. This mutation disrupts a GC-base pair in a three-base-pair stem with a four-nucleotide loop, which seems to be necessary for correct LEW3 RNA splicing. We found that the lrg1 short root phenotype correlates with high levels of reactive oxygen species and low pH in the apoplast. Proteomic analyses of N-glycosylated proteins identified GLU23/PYK10 and PRX34 as N-glycosylation targets of LRG1 activity. The lrg1 mutation reduces the positive interaction between Arabidopsis and Serendipita indica. A prx34 mutant showed a significant reduction in root growth, which is additive to lrg1. Taken together our work highlights the important role of N-glycosylation in root growth and development.

Towards a better understanding of protein changes in common bean under drought: A case study of N-glycoproteins

Drought is one of the major abiotic stress conditions limiting crop growth and productivity. Glycosylation of proteins is very important post-translational modification that is involved in many physiological functions and biological pathways. To understand the involvement of N-glycoproteins in the mechanism of drought response in leaves of common bean, a proteomic approach using lectin affinity chromatography, SDS-PAGE and LC-MS/MS was applied. Quantification of N-glycoproteins was performed using MaxQuant with a label free quantification approach. Thirty five glycoproteins were changed in abundance in leaves of common bean under drought. The majority of these proteins were classified into functional groups that include cell wall processes, defence/stress related proteins and proteins related to proteolysis. Beta-glucosidase showed the highest increase in abundance among proteins involved in cell wall metabolism, suggesting its role in cell wall modification under drought stress. These results fit with the general concept of the stress response in plants and suggest that drought stress might affect biochemical metabolism in the cell wall. The structures of N-glycans were determined manually from spectra, where structures of high mannose, complex and hybrid types of N-glycans were found. The present study provided an insight into the glycoproteins related to drought stress in common bean at the proteome level, which is important for further understanding of molecular mechanisms of drought response in this important legume.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Proline Hydroxylation in Cell Wall Proteins: Is It Yet Possible to Define Rules?

Cell wall proteins (CWPs) play critical and dynamic roles in plant cell walls by contributing to developmental processes and response to environmental cues. Since the CWPs go through the secretion pathway, most of them undergo post-translational modifications (PTMs) which can modify their biological activity. Glycosylation is one of the major PTMs of CWPs and refers to N-glycosylation, O-glycosylation and glypiation. Each of these PTMs occurs in different amino acid contexts which are not all well defined. This article deals with the hydroxylation of Pro residues which is a prerequisite for O-glycosylation of CWPs on hydroxyproline (Hyp) residues. The location of Hyp residues is well described in several structural CWPs, but yet rarely described in other CWPs. In this article, it is studied in detail in five Arabidopsis thaliana proteins using mass spectrometry data: one of them (At4g38770, AtPRP4) is a structural CWP containing 32.5% of Pro residues arranged in typical motifs, the others are either rich (27-28%, At1g31580 and At2g10940) or poor (6-8%, At1g09750 and At3g08030) in Pro residues. The known rules of Pro hydroxylation allowed a good prediction of Hyp location in AtPRP4. However, they could not be applied to the other proteins whatever their Pro content. In addition, variability of the Pro hydroxylation patterns was observed within some amino acid motifs in all the proteins and new patterns of Pro hydroxylation are described. Altogether, this work shows that Hyp residues are present in more protein families than initially described, and that Pro hydroxylation patterns could be different in each of them. This work paves the way for completing the existing Pro hydroxylation code.

Osmotic stress is accompanied by protein glycation in Arabidopsis thaliana

Among the environmental alterations accompanying oncoming climate changes, drought is the most important factor influencing crop plant productivity. In plants, water deficit ultimately results in the development of oxidative stress and accumulation of osmolytes (e.g. amino acids and carbohydrates) in all tissues. Up-regulation of sugar biosynthesis in parallel to the increasing overproduction of reactive oxygen species (ROS) might enhance protein glycation, i.e. interaction of carbonyl compounds, reducing sugars and α-dicarbonyls with lysyl and arginyl side-chains yielding early (Amadori and Heyns compounds) and advanced glycation end-products (AGEs). Although the constitutive plant protein glycation patterns were characterized recently, the effects of environmental stress on AGE formation are unknown so far. To fill this gap, we present here a comprehensive in-depth study of the changes in Arabidopsis thaliana advanced glycated proteome related to osmotic stress. A 3 d application of osmotic stress revealed 31 stress-specifically and 12 differentially AGE-modified proteins, representing altogether 56 advanced glycation sites. Based on proteomic and metabolomic results, in combination with biochemical, enzymatic and gene expression analysis, we propose monosaccharide autoxidation as the main stress-related glycation mechanism, and glyoxal as the major glycation agent in plants subjected to drought.

Arabidopsis Regenerating Protoplast: A Powerful Model System for Combining the Proteomics of Cell Wall Proteins and the Visualization of Cell Wall Dynamics

The development of a range of sub-proteomic approaches to the plant cell wall has identified many of the cell wall proteins. However, it remains difficult to elucidate the precise biological role of each protein and the cell wall dynamics driven by their actions. The plant protoplast provides an excellent means not only for characterizing cell wall proteins, but also for visualizing the dynamics of cell wall regeneration, during which cell wall proteins are secreted. It therefore offers a unique opportunity to investigate the de novo construction process of the cell wall. This review deals with sub-proteomic approaches to the plant cell wall through the use of protoplasts, a methodology that will provide the basis for further exploration of cell wall proteins and cell wall dynamics.

Hippophae rhamnoides N-glycoproteome analysis: a small step towards sea buckthorn proteome mining

Hippophae rhamnoides is a hardy shrub capable of growing under extreme environmental conditions namely, high salt, drought and cold. Its ability to grow under extreme conditions and its wide application in pharmaceutical and nutraceutical industry calls for its in-depth analysis. N-glycoproteome mining by con A affinity chromatography from seedling was attempted. The glycoproteome was resolved on first and second dimension gel electrophoresis. A total of 48 spots were detected and 10 non-redundant proteins were identified by MALDI-TOF/TOF. Arabidopsis thaliana protein disulfide isomerase-like 1-4 (ATPDIL1-4) electron transporter, protein disulphide isomerase, calreticulin 1 (CRT1), glycosyl hydrolase family 38 (GH 38) protein, phantastica, maturase k, Arabidopsis trithorax related protein 6 (ATXR 6), cysteine protease inhibitor were identified out of which ATXR 6, phantastica and putative ATPDIL1-4 electron transporter are novel glycoproteins. Calcium binding protein CRT1 was validated for its calcium binding by stains all staining. GO analysis showed involvement of GH 38 and ATXR 6 in glycan and lysine degradation pathways. This is to our knowledge the first report of glycoproteome analysis for any Elaeagnaceae member.

Extraction and Characterization of Extracellular Proteins and Their Post-Translational Modifications from Arabidopsis thaliana Suspension Cell Cultures and Seedlings: A Critical Review

Proteins secreted by plant cells into the extracellular space, consisting of the cell wall, apoplastic fluid, and rhizosphere, play crucial roles during development, nutrient acquisition, and stress acclimation. However, isolating the full range of secreted proteins has proven difficult, and new strategies are constantly evolving to increase the number of proteins that can be detected and identified. In addition, the dynamic nature of the extracellular proteome presents the further challenge of identifying and characterizing the post-translational modifications (PTMs) of secreted proteins, particularly glycosylation and phosphorylation. Such PTMs are common and important regulatory modifications of proteins, playing a key role in many biological processes. This review explores the most recent methods in isolating and characterizing the plant extracellular proteome with a focus on the model plant Arabidopsis thaliana, highlighting the current challenges yet to be overcome. Moreover, the crucial role of protein PTMs in cell wall signalling, development, and plant responses to biotic and abiotic stress is discussed.

Understanding the Remodelling of Cell Walls during Brachypodium distachyon Grain Development through a Sub-Cellular Quantitative Proteomic Approach

Brachypodiumdistachyon is a suitable plant model for studying temperate cereal crops, such as wheat, barley or rice, and helpful in the study of the grain cell wall. Indeed, the most abundant hemicelluloses that are in the B. distachyon cell wall of grain are (1-3)(1-4)-β-glucans and arabinoxylans, in a ratio similar to those of cereals such as barley or oat. Conversely, these cell walls contain few pectins and xyloglucans. Cell walls play an important role in grain physiology. The modifications of cell wall polysaccharides that occur during grain development and filling are key in the determination of the size and weight of the cereal grains. The mechanisms required for cell wall assembly and remodelling are poorly understood, especially in cereals. To provide a better understanding of these processes, we purified the cell wall at three developmental stages of the B. distachyon grain. The proteins were then extracted, and a quantitative and comparative LC-MS/MS analysis was performed to investigate the protein profile changes during grain development. Over 466 cell wall proteins (CWPs) were identified and classified according to their predicted functions. This work highlights the different proteome profiles that we could relate to the main phases of grain development and to the reorganization of cell wall polysaccharides that occurs during these different developmental stages. These results provide a good springboard to pursue functional validation to better understand the role of CWPs in the assembly and remodelling of the grain cell wall of cereals.

Post-translational modifications of plant cell wall proteins and peptides: A survey from a proteomics point of view

Plant cell wall proteins (CWPs) and peptides are important players in cell walls contributing to their assembly and their remodeling during development and in response to environmental constraints. Since the rise of proteomics technologies at the beginning of the 2000's, the knowledge of CWPs has greatly increased leading to the discovery of new CWP families and to the description of the cell wall proteomes of different organs of many plants. Conversely, cell wall peptidomics data are still lacking. In addition to the identification of CWPs and peptides by mass spectrometry (MS) and bioinformatics, proteomics has allowed to describe their post-translational modifications (PTMs). At present, the best known PTMs consist in proteolytic cleavage, N-glycosylation, hydroxylation of P residues into hydroxyproline residues (O), O-glycosylation and glypiation. In this review, the methods allowing the capture of the modified proteins based on the specific properties of their PTMs as well as the MS technologies used for their characterization are briefly described. A focus is done on proteolytic cleavage leading to protein maturation or release of signaling peptides and on O-glycosylation. Some new technologies, like top-down proteomics and terminomics, are described. They aim at a finer description of proteoforms resulting from PTMs or degradation mechanisms. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Plant protein glycosylation

Protein glycosylation is an essential co- and post-translational modification of secretory and membrane proteins in all eukaryotes. The initial steps of N-glycosylation and N-glycan processing are highly conserved between plants, mammals and yeast. In contrast, late N-glycan maturation steps in the Golgi differ significantly in plants giving rise to complex N-glycans with β1,2-linked xylose, core α1,3-linked fucose and Lewis A-type structures. While the essential role of N-glycan modifications on distinct mammalian glycoproteins is already well documented, we have only begun to decipher the biological function of this ubiquitous protein modification in different plant species. In this review, I focus on the biosynthesis and function of different protein N-linked glycans in plants. Special emphasis is given on glycan-mediated quality control processes in the ER and on the biological role of characteristic complex N-glycan structures.

Engaging challenges in glycoproteomics: recent advances in MS-based glycopeptide analysis

The proteomic analysis of glycosylation is uniquely challenging. The numerous and varied biological roles of protein-linked glycans have fueled a tremendous demand for technologies that enable rapid, in-depth structural examination of glycosylated proteins in complex biological systems. In turn, this demand has driven many innovations in wide ranging fields of bioanalytical science. This review will summarize key developments in glycoprotein separation and enrichment, glycoprotein proteolysis strategies, glycopeptide separation and enrichment, the role of mass measurement accuracy in glycopeptide detection, glycopeptide ion dissociation methods for MS/MS, and informatic tools for glycoproteomic analysis. In aggregate, this selection of topics serves to encapsulate the present status of MS-based analytical technologies for engaging the challenges of glycoproteomic analysis.

Ascorbate deficiency influences the leaf cell wall glycoproteome in Arabidopsis thaliana

The cell wall forms the first line of interaction between the plant and the external environment. Based on the observation that ascorbate-deficient vtc mutants of Arabidopsis thaliana have increased cell wall peroxidase activity, the cell wall glycoproteome of vtc2-2 was investigated. Glycoproteins were purified from fully expanded leaves by Concanavalin A affinity chromatography and analysed by liquid chromatography quadrupole time-of-flight mass spectrometry. This procedure identified 63 proteins with predicted glycosylation sites and cell wall localization. Of these, 11 proteins were differentially expressed between vtc2-2 and wild type. In particular, PRX33/34 were identified as contributing to increased peroxidase activity in response to ascorbate deficiency. This is the same peroxidase previously shown to contribute to hydrogen peroxide generation and pathogen resistance. Three fasciclin-like arabinogalactan proteins (FLA1, 2 and 8) had lower abundance in vtc2-2. Inspection of published microarray data shows that these also have lower gene expression in vtc1 and vtc2-1 and are decreased in expression by pathogen challenge and oxidative stresses. Ascorbate deficiency therefore impacts expression of cell wall proteins involved in pathogen responses and these presumably contribute to the increased resistance of vtc mutants to biotrophic pathogens.

Identifying the ionically bound cell wall and intracellular glycoside hydrolases in late growth stage Arabidopsis stems: implications for the genetic engineering of bioenergy crops

Identifying the cell wall-ionically bound glycoside hydrolases (GHs) in Arabidopsis stems is important for understanding the regulation of cell wall integrity. For cell wall proteomics studies, the preparation of clean cell wall fractions is a challenge since cell walls constitute an open compartment, which is more likely to contain a mixture of intracellular and extracellular proteins due to cell leakage at the late growth stage. Here, we utilize a CaCl2-extraction procedure to isolate non-structural proteins from Arabidopsis whole stems, followed by the in-solution and in-gel digestion methods coupled with Nano-LC-MS/MS, bioinformatics and literature analyses. This has led to the identification of 75 proteins identified using the in-solution method and 236 proteins identified by the in-gel method, among which about 10% of proteins predicted to be secreted. Together, eight cell wall proteins, namely AT1G75040, AT5G26000, AT3G57260, AT4G21650, AT3G52960, AT3G49120, AT5G49360, and AT3G14067, were identified by the in-solution method; among them, three were the GHs (AT5G26000, myrosinase 1, GH1; AT3G57260, β-1,3-glucanase 2, GH17; AT5G49360, bifunctional XYL 1/α-L-arabinofuranosidase, GH3). Moreover, four more GHs: AT4G30270 (xyloglucan endotransferase, GH16), AT1G68560 (bifunctional α-l-arabinofuranosidase/XYL, GH31), AT1G12240 (invertase, GH32) and AT2G28470 (β-galactosidase 8, GH35), were identified by the in-gel solution method only. Notably, more than half of above identified GHs are xylan- or hemicellulose-modifying enzymes, and will likely have an impact on cellulose accessibility, which is a critical factor for downstream enzymatic hydrolysis of plant tissues for biofuels production. The implications of these cell wall proteins identified at the late growth stage for the genetic engineering of bioenergy crops are discussed.

Characterization of protein N-glycosylation by tandem mass spectrometry using complementary fragmentation techniques

The analysis of post-translational modifications (PTMs) by proteomics is regarded as a technically challenging undertaking. While in recent years approaches to examine and quantify protein phosphorylation have greatly improved, the analysis of many protein modifications, such as glycosylation, are still regarded as problematic. Limitations in the standard proteomics workflow, such as use of suboptimal peptide fragmentation methods, can significantly prevent the identification of glycopeptides. The current generation of tandem mass spectrometers has made available a variety of fragmentation options, many of which are becoming standard features on these instruments. We have used three common fragmentation techniques, namely CID, HCD, and ETD, to analyze a glycopeptide and highlight how an integrated fragmentation approach can be used to identify the modified residue and characterize the N-glycan on a peptide.

Arabinogalactan protein 31 (AGP31), a putative network-forming protein in Arabidopsis thaliana cell walls?

BACKGROUND AND AIMS

Arabinogalactan protein 31 (AGP31) is a remarkable plant cell-wall protein displaying a multi-domain organization unique in Arabidopsis thaliana: it comprises a predicted signal peptide (SP), a short AGP domain of seven amino acids, a His-stretch, a Pro-rich domain and a PAC (PRP-AGP containing Cys) domain. AGP31 displays different O-glycosylation patterns with arabinogalactans on the AGP domain and Hyp-O-Gal/Ara-rich motifs on the Pro-rich domain. AGP31 has been identified as an abundant protein in cell walls of etiolated hypocotyls, but its function has not been investigated thus far. Literature data suggest that AGP31 may interact with cell-wall components. The purpose of the present study was to identify AGP31 partners to gain new insight into its function in cell walls.

METHODS

Nitrocellulose membranes were prepared by spotting different polysaccharides, which were either obtained commercially or extracted from cell walls of Arabidopsis thaliana and Brachypodium distachyon. After validation of the arrays, in vitro interaction assays were carried out by probing the membranes with purified native AGP31 or recombinant PAC-V5-6xHis. In addition, dynamic light scattering (DLS) analyses were carried out on an AGP31 purified fraction.

KEY RESULTS

It was demonstrated that AGP31 interacts through its PAC domain with galactans that are branches of rhamnogalacturonan I. This is the first experimental evidence that a PAC domain, also found as an entire protein or a domain of AGP31 homologues, can bind carbohydrates. AGP31 was also found to bind methylesterified polygalacturonic acid, possibly through its His-stretch. Finally, AGP31 was able to interact with itself in vitro through its PAC domain. DLS data showed that AGP31 forms aggregates in solution, corroborating the hypothesis of an auto-assembly.

CONCLUSIONS

These results allow the proposal of a model of interactions of AGP31 with different cell-wall components, in which AGP31 participates in complex supra-molecular scaffolds. Such scaffolds could contribute to the strengthening of cell walls of quickly growing organs such as etiolated hypocotyls.

Plant Cell Wall Proteins: A Large Body of Data, but What about Runaways?

Plant cell wall proteomics has been a very dynamic field of research for about fifteen years. A full range of strategies has been proposed to increase the number of identified proteins and to characterize their post-translational modifications. The protocols are still improving to enlarge the coverage of cell wall proteomes. Comparisons between these proteomes have been done based on various working strategies or different physiological stages. In this review, two points are highlighted. The first point is related to data analysis with an overview of the cell wall proteomes already described. A large body of data is now available with the description of cell wall proteomes of seventeen plant species. CWP contents exhibit particularities in relation to the major differences in cell wall composition and structure between these plants and between plant organs. The second point is related to methodology and concerns the present limitations of the coverage of cell wall proteomes. Because of the variety of cell wall structures and of the diversity of protein/polysaccharide and protein/protein interactions in cell walls, some CWPs can be missing either because they are washed out during the purification of cell walls or because they are covalently linked to cell wall components.

A comparative study of lectin affinity based plant N-glycoproteome profiling using tomato fruit as a model

Lectin affinity chromatography (LAC) can provide a valuable front-end enrichment strategy for the study of N-glycoproteins and has been used to characterize a broad range eukaryotic N-glycoproteomes. Moreover, studies with mammalian systems have suggested that the use of multiple lectins with different affinities can be particularly effective. A multi-lectin approach has also been reported to provide a significant benefit for the analysis of plant N-glycoproteins; however, it has yet to be determined whether certain lectins, or combinations of lectins are optimal for plant N-glycoproteome profiling; or whether specific lectins show preferential association with particular N-glycosylation sites or N-glycan structures. We describe here a comparative study of three mannose-binding lectins, concanavalin A, snowdrop lectin, and lentil lectin, to profile the N-glycoproteome of mature green stage tomato (Solanum lycopersicum) fruit pericarp. Through coupling lectin affinity chromatography with a shotgun proteomics strategy, we identified 448 putative N-glycoproteins, whereas a parallel lectin affinity chromatography plus hydrophilic interaction chromatography analysis revealed 318 putative N-glycosylation sites on 230 N-glycoproteins, of which 100 overlapped with the shotgun analysis, as well as 17 N-glycan structures. The use of multiple lectins substantially increased N-glycoproteome coverage and although there were no discernible differences in the structures of N-glycans, or the charge, isoelectric point (pI) or hydrophobicity of the glycopeptides that differentially bound to each lectin, differences were observed in the amino acid frequency at the -1 and +1 subsites of the N-glycosylation sites. We also demonstrated an alternative and complementary in planta recombinant expression strategy, followed by affinity MS analysis, to identify the putative N-glycan structures of glycoproteins whose abundance is too low to be readily determined by a shotgun approach, and/or combined with deglycosylation for predicted deamidated sites, using a xyloglucan-specific endoglucanase inhibitor protein as an example.

Uncovering plant-pathogen crosstalk through apoplastic proteomic studies

Plant pathogens have evolved by developing different strategies to infect their host, which in turn have elaborated immune responses to counter the pathogen invasion. The apoplast, including the cell wall and extracellular space outside the plasma membrane, is one of the first compartments where pathogen-host interaction occurs. The plant cell wall is composed of a complex network of polysaccharides polymers and glycoproteins and serves as a natural physical barrier against pathogen invasion. The apoplastic fluid, circulating through the cell wall and intercellular spaces, provides a means for delivering molecules and facilitating intercellular communications. Some plant-pathogen interactions lead to plant cell wall degradation allowing pathogens to penetrate into the cells. In turn, the plant immune system recognizes microbial- or damage-associated molecular patterns (MAMPs or DAMPs) and initiates a set of basal immune responses, including the strengthening of the plant cell wall. The establishment of defense requires the regulation of a wide variety of proteins that are involved at different levels, from receptor perception of the pathogen via signaling mechanisms to the strengthening of the cell wall or degradation of the pathogen itself. A fine regulation of apoplastic proteins is therefore essential for rapid and effective pathogen perception and for maintaining cell wall integrity. This review aims to provide insight into analyses using proteomic approaches of the apoplast to highlight the modulation of the apoplastic protein patterns during pathogen infection and to unravel the key players involved in plant-pathogen interaction.

How to use 2D gel electrophoresis in plant proteomics

Two-dimensional electrophoresis has nurtured the birth of proteomics. It is however no longer the exclusive setup used in proteomics, with the development of shotgun proteomics techniques that appear more fancy and fashionable nowadays.Nevertheless, 2D gel-based proteomics still has valuable features, and sometimes unique ones, which make it often an attractive choice when a proteomics strategy must be selected. These features are detailed in this chapter, as is the rationale for selecting or not 2D gel-based proteomics as a proteomic strategy.

Biological significance of complex N-glycans in plants and their impact on plant physiology

Asparagine (N)-linked protein glycosylation is a ubiquitous co- and post-translational modification which can alter the biological function of proteins and consequently affects the development, growth, and physiology of organisms. Despite an increasing knowledge of N-glycan biosynthesis and processing, we still understand very little about the biological function of individual N-glycan structures in plants. In particular, the N-glycan-processing steps mediated by Golgi-resident enzymes create a structurally diverse set of protein-linked carbohydrate structures. Some of these complex N-glycan modifications like the presence of β1,2-xylose, core α1,3-fucose or the Lewis a-epitope are characteristic for plants and are evolutionary highly conserved. In mammals, complex N-glycans are involved in different cellular processes including molecular recognition and signaling events. In contrast, the complex N-glycan function is still largely unknown in plants. Here, in this short review, I focus on important recent developments and discuss their implications for future research in plant glycobiology and plant biotechnology.

Glycoproteome of elongating cotton fiber cells

Cotton ovule epidermal cell differentiation into long fibers primarily depends on wall-oriented processes such as loosening, elongation, remodeling, and maturation. Such processes are governed by cell wall bound structural proteins and interacting carbohydrate active enzymes. Glycosylation plays a major role in the structural, functional, and localization aspects of the cell wall and extracellular destined proteins. Elucidating the glycoproteome of fiber cells would reflect its wall composition as well as compartmental requirement, which must be system specific. Following complementary proteomic approaches, we have identified 334 unique proteins comprising structural and regulatory families. Glycopeptide-based enrichment followed by deglycosylation with PNGase F and A revealed 92 unique peptides containing 106 formerly N-linked glycosylated sites from 67 unique proteins. Our results showed that structural proteins like arabinogalactans and carbohydrate active enzymes were relatively more abundant and showed stage- and isoform-specific expression patterns in the differentiating fiber cell. Furthermore, our data also revealed the presence of heterogeneous and novel forms of structural and regulatory glycoproteins. Comparative analysis with other plant glycoproteomes highlighted the unique composition of the fiber glycoproteome. The present study provides the first insight into the identity, abundance, diversity, and composition of the glycoproteome within single celled cotton fibers. The elucidated composition also indirectly provides clues about unicellular compartmental requirements underlying single cell differentiation.

Exploring the N-glycosylation pathway in Chlamydomonas reinhardtii unravels novel complex structures

Chlamydomonas reinhardtii is a green unicellular eukaryotic model organism for studying relevant biological and biotechnological questions. The availability of genomic resources and the growing interest in C. reinhardtii as an emerging cell factory for the industrial production of biopharmaceuticals require an in-depth analysis of protein N-glycosylation in this organism. Accordingly, we used a comprehensive approach including genomic, glycomic, and glycoproteomic techniques to unravel the N-glycosylation pathway of C. reinhardtii. Using mass-spectrometry-based approaches, we found that both endogenous soluble and membrane-bound proteins carry predominantly oligomannosides ranging from Man-2 to Man-5. In addition, minor complex N-linked glycans were identified as being composed of partially 6-O-methylated Man-3 to Man-5 carrying one or two xylose residues. These findings were supported by results from a glycoproteomic approach that led to the identification of 86 glycoproteins. Here, a combination of in-source collision-induced dissodiation (CID) for glycan fragmentation followed by mass tag-triggered CID for peptide sequencing and PNGase F treatment of glycopeptides in the presence of (18)O-labeled water in conjunction with CID mass spectrometric analyses were employed. In conclusion, our data support the notion that the biosynthesis and maturation of N-linked glycans in the endoplasmic reticulum and Golgi apparatus occur via a GnT I-independent pathway yielding novel complex N-linked glycans that maturate differently from their counterparts in land plants.

The Arabidopsis wood model-the case for the inflorescence stem

Arabidopsis thaliana has successfully served as a model to discover genes and proteins that have roles in a wide range of plant traits, including wood-related traits, such as lignin, cellulose and hemicellulose biosynthesis, secondary growth regulation, and secondary cell wall synthesis. Both the radially thickened hypocotyl and the inflorescence stem (flower stalk) have been studied. In this review, we address lingering doubts regarding the utility of Arabidopsis as a model for wood development by highlighting studies that provide new biochemical and biophysical evidence that extend support for the Arabidopsis inflorescence stem as a model for wood development beyond what is currently thought. We describe different aspects of Arabidopsis that make it a highly versatile tool for the study of wood development. One would likely utilise the radially thickened hypocotyl because of its more fully developed vascular cambium for traits related specifically to secondary (i.e. cambial) growth. It is more productive to utilise the inflorescence stem for wood-like biophysical traits. Accession variation has been underexploited as a powerful method to discover genes governing wood-like traits. We discuss recent findings that survey the accession variation in Arabidopsis for biochemical and biophysical properties of various wood traits, such as microfibril angle, tensile strength and cellulose/hemicellulose content. Furthermore we discuss how larger-scale studies of this nature using plants grown in long days (as opposed to the current short-day paradigm) could accelerate gene discovery and our understanding of cell wall and wood development. We highlight some relatively unexplored areas of research relating to the secondary cell wall composition, architecture and biophysical properties of the inflorescence stem, and how these traits are relevant to wood formation. The Arabidopsis inflorescence stem has other characteristics, expressed genes and traits held in common with woody species that have not been widely characterised or discussed to date. We discuss how this conservation may indicate the more general potential for "true" woodiness in herbaceous species, in the context of so-called secondary woodiness.

Characterizing the link between glycosylation state and enzymatic activity of the endo-β1,4-glucanase KORRIGAN1 from Arabidopsis thaliana

Defects in N-glycosylation and N-glycan processing frequently cause alterations in plant cell wall architecture, including changes in the structure of cellulose, which is the most abundant plant polysaccharide. KORRIGAN1 (KOR1) is a glycoprotein enzyme with an essential function during cellulose biosynthesis in Arabidopsis thaliana. KOR1 is a membrane-anchored endo-β1,4-glucanase and contains eight potential N-glycosylation sites in its extracellular domain. Here, we expressed A. thaliana KOR1 as a soluble, enzymatically active protein in insect cells and analyzed its N-glycosylation state. Structural analysis revealed that all eight potential N-glycosylation sites are utilized. Individual elimination of evolutionarily conserved N-glycosylation sites did not abolish proper KOR1 folding, but mutations of Asn-216, Asn-324, Asn-345, and Asn-567 resulted in considerably lower enzymatic activity. In contrast, production of wild-type KOR1 in the presence of the class I α-mannosidase inhibitor kifunensine, which abolished the conversion of KOR1 N-glycans into complex structures, did not affect the activity of the enzyme. To address N-glycosylation site occupancy and N-glycan composition of KOR1 under more natural conditions, we expressed a chimeric KOR1-Fc-GFP fusion protein in leaves of Nicotiana benthamiana. Although Asn-108 and Asn-133 carried oligomannosidic N-linked oligosaccharides, the six other glycosylation sites were modified with complex N-glycans. Interestingly, the partially functional KOR1 G429R mutant encoded by the A. thaliana rsw2-1 allele displayed only oligomannosidic structures when expressed in N. benthamiana, indicating its retention in the endoplasmic reticulum. In summary, our data indicate that utilization of several N-glycosylation sites is important for KOR1 activity, whereas the structure of the attached N-glycans is not critical.

Phosphorylation of p27(KIP1) homologs KRP6 and 7 by SNF1-related protein kinase-1 links plant energy homeostasis and cell proliferation

SNF1-related protein kinase-1 (SnRK1), the plant kinase homolog of mammalian AMP-activated protein kinase (AMPK), is a sensor that maintains cellular energy homeostasis via control of anabolism/catabolism balance. AMPK-dependent phosphorylation of p27(KIP1) affects cell-cycle progression, autophagy and apoptosis. Here, we show that SnRK1 phosphorylates the Arabidopsis thaliana cyclin-dependent kinase inhibitor p27(KIP1) homologs AtKRP6 and AtKRP7, thus extending the role of this kinase to regulation of cell-cycle progression. AtKRP6 and 7 were phosphorylated in vitro by a recombinant activated catalytic subunit of SnRK1 (AtSnRK1α1). Tandem mass spectrometry and site-specific mutagenesis identified Thr152 and Thr151 as the phosphorylated residues on AtKRP6- and AtKRP7, respectively. AtSnRK1 physically interacts with AtKRP6 in the nucleus of transformed BY-2 tobacco protoplasts, but, in contrast to mammals, the AtKRP6 Thr152 phosphorylation state alone did not modify its nuclear localization. Using a heterologous yeast system, consisting of a cdc28 yeast mutant complemented by A. thaliana CDKA;1, cell proliferation was shown to be abolished by AtKRP6(WT) and by the non-phosphorylatable form AtKRP6(T152A) , but not by the phosphorylation-mimetic form AtKRP6(T152D). Moreover, A. thaliana SnRK1α1/KRP6 double over-expressor plants showed an attenuated AtKRP6-associated phenotype (strongly serrated leaves and inability to undergo callogenesis). Furthermore, this severe phenotype was not observed in AtKRP6(T152D) over-expressor plants. Overall, these results establish that the energy sensor AtSnRK1 plays a cardinal role in the control of cell proliferation in A. thaliana plants through inhibition of AtKRP6 biological function by phosphorylation.

How protein targeting to primary plastids via the endomembrane system could have evolved? A new hypothesis based on phylogenetic studies

Background

It is commonly assumed that a heterotrophic ancestor of the supergroup Archaeplastida/Plantae engulfed a cyanobacterium that was transformed into a primary plastid; however, it is still unclear how nuclear-encoded proteins initially were imported into the new organelle. Most proteins targeted to primary plastids carry a transit peptide and are transported post-translationally using Toc and Tic translocons. There are, however, several proteins with N-terminal signal peptides that are directed to higher plant plastids in vesicles derived from the endomembrane system (ES). The existence of these proteins inspired a hypothesis that all nuclear-encoded, plastid-targeted proteins initially carried signal peptides and were targeted to the ancestral primary plastid via the host ES.

Results

We present the first phylogenetic analyses of Arabidopsis thaliana α-carbonic anhydrase (CAH1), Oryza sativa nucleotide pyrophosphatase/phosphodiesterase (NPP1), and two O. sativa α-amylases (αAmy3, αAmy7), proteins that are directed to higher plant primary plastids via the ES. We also investigated protein disulfide isomerase (RB60) from the green alga Chlamydomonas reinhardtii because of its peculiar dual post- and co-translational targeting to both the plastid and ES. Our analyses show that these proteins all are of eukaryotic rather than cyanobacterial origin, and that their non-plastid homologs are equipped with signal peptides responsible for co-translational import into the host ES. Our results indicate that vesicular trafficking of proteins to primary plastids evolved long after the cyanobacterial endosymbiosis (possibly only in higher plants) to permit their glycosylation and/or transport to more than one cellular compartment.

Conclusions

The proteins we analyzed are not relics of ES-mediated protein targeting to the ancestral primary plastid. Available data indicate that Toc- and Tic-based translocation dominated protein import into primary plastids from the beginning. Only a handful of host proteins, which already were targeted through the ES, later were adapted to reach the plastid via the vesicular trafficking. They represent a derived class of higher plant plastid-targeted proteins with an unusual evolutionary history.

Reviewers

This article was reviewed by Prof. William Martin, Dr. Philippe Deschamps (nominated by Dr. Purificacion Lopez-Garcia) and Dr Simonetta Gribaldo.

Plant cell wall proteomics: the leadership of Arabidopsis thaliana

Plant cell wall proteins (CWPs) progressively emerged as crucial components of cell walls although present in minor amounts. Cell wall polysaccharides such as pectins, hemicelluloses, and cellulose represent more than 90% of primary cell wall mass, whereas hemicelluloses, cellulose, and lignins are the main components of lignified secondary walls. All these polymers provide mechanical properties to cell walls, participate in cell shape and prevent water loss in aerial organs. However, cell walls need to be modified and customized during plant development and in response to environmental cues, thus contributing to plant adaptation. CWPs play essential roles in all these physiological processes and particularly in the dynamics of cell walls, which requires organization and rearrangements of polysaccharides as well as cell-to-cell communication. In the last 10 years, plant cell wall proteomics has greatly contributed to a wider knowledge of CWPs. This update will deal with (i) a survey of plant cell wall proteomics studies with a focus on Arabidopsis thaliana; (ii) the main protein families identified and the still missing peptides; (iii) the persistent issue of the non-canonical CWPs; (iv) the present challenges to overcome technological bottlenecks; and (v) the perspectives beyond cell wall proteomics to understand CWP functions.

Cell wall proteomics of crops

Cell wall proteins play key roles in cell structure and metabolism, cell enlargement, signal transduction, responses to environmental stress, and many other physiological events. Agricultural crops are often used for investigating stress tolerance because cultivars with differing degrees of tolerance are available. Abiotic and biotic stress factors markedly influence the geographical distribution and yields of many crop species. Crop cell wall proteomics is of particular importance for improving crop productivity, particularly under unfavorable environmental conditions. To better understand the mechanisms underlying stress response in crops, cell wall proteomic analyses are being increasingly utilized. In this review, the methods of purification and purity assays of cell wall protein fractions from crops are described, and the results of protein identification using gel-based and gel-free proteomic techniques are presented. Furthermore, protein composition of the cell walls of rice, wheat, maize, and soybean are compared, and the role of cell wall proteins in crops under flooding and drought stress is discussed. This review will be useful for clarifying the role of the cell wall of crops in response to environmental stresses.

Myrosinases TGG1 and TGG2 from Arabidopsis thaliana contain exclusively oligomannosidic N-glycans

In all eukaryotes N-glycosylation is the most prevalent protein modification of secretory and membrane proteins. Although the N-glycosylation capacity and the individual steps of the N-glycan processing pathway have been well studied in the model plant Arabidopsis thaliana, little attention has been paid to the characterization of the glycosylation status of individual proteins. We report here the structural analysis of all N-glycans present on the endogenous thioglucoside glucohydrolases (myrosinases) TGG1 and TGG2 from A. thaliana. All nine glycosylation sites of TGG1 and all four glycosylation sites of TGG2 are occupied by oligomannosidic structures with Man₅GlcNAc₂ as the major glycoform. Analysis of the oligomannosidic isomers from wild-type plants and mannose trimming deficient mutants by liquid chromatography with porous graphitic carbon and mass spectrometry revealed that the N-glycans from both myrosinases are processed by Golgi-located α-mannosidases.