An update on post-translational modifications of hydroxyproline-rich glycoproteins: toward a model highlighting their contribution to plant cell wall architecture

Plant cell walls are composite structures mainly composed of polysaccharides, also containing a large set of proteins involved in diverse functions such as growth, environmental sensing, signaling, and defense. Research on cell wall proteins (CWPs) is a challenging field since present knowledge of their role into the structure and function of cell walls is very incomplete. Among CWPs, hydroxyproline (Hyp)-rich O-glycoproteins (HRGPs) were classified into three categories: (i) moderately glycosylated extensins (EXTs) able to form covalent scaffolds; (ii) hyperglycosylated arabinogalactan proteins (AGPs); and (iii) Hyp/proline (Pro)-Rich proteins (H/PRPs) that may be non-, weakly- or highly-glycosylated. In this review, we provide a description of the main features of their post-translational modifications (PTMs), biosynthesis, structure, and function. We propose a new model integrating HRGPs and their partners in cell walls. Altogether, they could form a continuous glyco-network with non-cellulosic polysaccharides via covalent bonds or non-covalent interactions, thus strongly contributing to cell wall architecture.Fil: Hijazi, May. Centre National de la Recherche Scientifique; Francia. Université de Toulouse; FranciaFil: Velásquez, Silvia Melina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; ArgentinaFil: Jamet, Elisabeth. Centre National de la Recherche Scientifique; Francia. Université de Toulouse; FranciaFil: Estevez, Jose Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; ArgentinaFil: Albenne, Cécile. Centre National de la Recherche Scientifique; Francia. Université de Toulouse; Franci

An Arabidopsis thaliana arabinogalactan-protein (AGP31) and several cationic AGP fragments catalyse the boron bridging of rhamnogalacturonan-II

Rhamnogalacturonan-II (RG-II) is a complex pectic domain in plant primary cell walls. In vivo, most RG-II domains are covalently dimerised via borate diester bridges, essential for correct cell-wall assembly, but the dimerisation of pure RG-II monomers by boric acid in vitro is extremely slow. Cationic ‘chaperones’ can promote dimerisation, probably by overcoming the mutual repulsion between neighbouring anionic RG-II molecules. Highly effective artificial chaperones include Pb(2+) and polyhistidine, but the proposed natural chaperones remained elusive. We have now tested cationic peptide fragments of several Arabidopsis thaliana arabinogalactan-proteins (AGPs) as candidates. Fragments of AGP17, 18, 19 and 31 were effective, typically at ∼25 µg/ml (9–19 µM), promoting the boron bridging of 16–20 µM monomeric RG-II at pH 4.8 in vitro. Native AGP31 glycoprotein was also effective, and hexahistidine was moderately so. All chaperones tested interacted reversibly with RG-II and were not consumed during the reaction; thus they acted catalytically, and may constitute the first reported boron-acting enzyme activity, an RG-II borate diesterase. Many of the peptide chaperones became less effective catalysts at higher concentration, which we interpret as due to the formation of RG-II–peptide complexes with a net positive charge, as mutually repulsive as negatively charged pure RG-II molecules. The four unique AGPs studied here may serve an enzymic role in the living plant cell, acting on RG-II within Golgi cisternae and/or in the apoplast after secretion. In this way, RG-II and specific AGPs may contribute to cell-wall assembly and hence plant cell expansion and development

LptM promotes oxidative maturation of the lipopolysaccharide translocon by substrate binding mimicry

Insertion of lipopolysaccharide (LPS) into the bacterial outer membrane (OM) is mediated by a druggable OM translocon consisting of a β-barrel membrane protein, LptD, and a lipoprotein, LptE. The β-barrel assembly machinery (BAM) assembles LptD together with LptE at the OM. In the enterobacterium Escherichia coli, formation of two native disulfide bonds in LptD controls translocon activation. Here we report the discovery of LptM (formerly YifL), a lipoprotein conserved in Enterobacteriaceae, that assembles together with LptD and LptE at the BAM complex. LptM stabilizes a conformation of LptD that can efficiently acquire native disulfide bonds, whereas its inactivation makes disulfide bond isomerization by DsbC become essential for viability. Our structural prediction and biochemical analyses indicate that LptM binds to sites in both LptD and LptE that are proposed to coordinate LPS insertion into the OM. These results suggest that, by mimicking LPS binding, LptM facilitates oxidative maturation of LptD, thereby activating the LPS translocon

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

Ingénierie rationnelle de l'amylosaccharase de Neisseria polysaccharea (de la structure tri-dimensionnelle aux bases moléculaires de la catalyse)

L'amylosaccharase de Neisseria polysaccharea (AS) est une transglucosidase de la famille 13 des glycoside-hydrolases qui utilise le saccharose pour synthétiser un homopolymère de type amylose et glucosyler très efficacement des accepteurs tels que le glycogène. L'obtention et l'analyse de complexes enzyme : substrat couplée à des expériences de mutagénèse dirigée, nous ont permis de montrer que la spécificité vis-à-vis du saccharose est conférée par des résidus du sous-site -1 (Asp144, Arg509, Tyr147 et Phe250). Les résidus du sous-site +1 (Asp394 et Arg446) assurent un positionnement correct des molécules acceptrices et sont responsables de la spécificité de synthèse mais aussi de rupture des liaisons a-1,4. En effet, nous avons révélé la capacité de l'AS à disproportionner efficacement les maltooligosaccharides de DP > 4, la présence d'un sous-site +4 fort (Arg415), associé à des sous-sites +1, +2 et +3 faibles (résidu Arg226 gênant), réduisant fortement l'accès au site actif des plus petites molécules. Une analyse approfondie des réactions catalysées en présence de saccharose seul nous a ensuite permis de démontrer que la formation du polymère résulte d'une élongation non-processive des maltooligosaccharides produits par l'AS, du côté de leur extrémité non-réductrice. Alors que les produits < DP4 sont accumulés du fait d'une fixation défavorable au niveau des sous-sites +1 à +3, la glucosylation des longues chaînes repose sur un arrimage performant au niveau de différents sites positionnés à la surface de l'enzyme. Enfin, un site secondaire, non catalytique, de fixation du saccharose a été révélé et pourrait être responsable du comportement cinétique atypique de l'AS, via de possibles modifications conformationnelles. Une étude de modélisation moléculaire a été initiée pour explorer l'éventualité d'une voie de passage pour le saccharose entre le site secondaire et le site actif. La modélisation a également permis de proposer une solution d'arrimage du glycogène, révélant une complémentarité de forme entre l'enzyme et cet accepteurTOULOUSE-INSA (315552106) / SudocSudocFranceF

Caractérisation structurale et fonctionnelle d'AGP31, une glycoprotéine atypique de la paroi chez Arabidopsis thaliana

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

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

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Phenotyping and cell wall polysaccharide composition dataset of five arabidopsis ecotypes grown at optimal or sub-optimal temperatures

International audienceThis article presents experimental data describing the morphology and the cell wall monosaccharide content of rosettes and flower stems of five Arabidopsis thaliana ecotypes grown at two contrasted temperatures. Besides, cell wall polysaccharides are reconstructed from data of monosaccharide quantification. The well-described and sequenced Columbia (Col) ecotype and four newly-described Pyrenees ecotypes (Duruflé et al., 2019) have been grown at two different temperatures (15 °C and 22 °C). For macrophenotyping, we provide dataset regarding (i) rosettes such as measurement of diameter and fresh mass as well as number of leaves just before bolting and (ii) floral stems at the first flower stage such as length, number of cauline leaves, mass and diameter at its base. Regarding cell wall composition, we provide data of quantification of seven monosaccharides and the reconstruction three polysaccharides. All these data are markers to differentiate both growth temperatures and the different ecotypes. They constitute a valuable resource for the community to study the adaptation of A. thaliana ecotypes to sub-optimal temperature growth conditions

Plant glycobiology: a sweet world of lectins, glycoproteins, glycolipids and glycans

Plants synthesize a wide variety of unique glycan structures which play essential roles during the life cycle of the plant. Being omnipresent throughout the plant kingdom, ranging from simple green algae to modern flowering plants, glycans contribute to many diverse processes. Glycans can function as structural components in the plant cell wall, assist in the folding of nascent proteins, act as signaling molecules in plant defense responses or (ER) stress pathways, or serve within the energy metabolism of a plant. In most cases, glycans are attached to other macromolecules to form so-called glycoconjugates (e.g. glycoproteins, proteoglycans and glycolipids), but they can also be present as free entities residing in the plant cell. Next to the broad, complex set of glycans, plants also evolved an elaborate collection of lectins or proteins with a lectin-like domain, which can recognize and bind to endogenous (plants-own) or exogenous (foreign) glycans. Though still poorly understood in plants, the dynamic interactions between lectins and carbohydrate structures are suggested to be involved in gene transcription, protein folding, protein transport, cell adhesion, signaling as well as defense responses. As such, a complex and largely undetermined glycan-interactome is established inside plant cells, between cells and their surrounding matrix, inside the extracellular matrix, and even between organisms. Studying the biological roles of plant glycans will enable to better understand plant development and physiology in order to fully exploit plants for food, feed and production of pharmaceutical proteins.In this Research Topic, we want to provide a platform for articles describing the latest research, perspectives and methodologies related to the fascinating world of plant glycobiology, with a focus on following subjects:1. Identification and characterization of plant glycans, their biosynthetic and degradation enzymes 2. Characterization of plant lectins and glycoproteins 3. Plant glycans in the plant’s energy metabolism 4. Role of plant glycans in plant defense signaling 5. Use of plant lectins in pest control 6. Plant lectins as new tools in human medicine 7. Glyco-engineering in plant

Bacterial machineries for the assembly of membrane-embedded β-barrel proteins

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