A Single Xyloglucan Xylosyltransferase Is Sufficient for Generation of the XXXG Xylosylation Pattern of Xyloglucan

Multiprotein Complexes of Plant Glycosyltransferases Involved in Their Function and Trafficking

Plant cells utilize protein oligomerization for their functions in numerous important cellular processes. Protein-protein interactions are necessary to stabilize, optimize, and activate enzymes, as well as localize proteins to specific organelles and membranes. Glycosyltransferases-enzymes that attach sugars to polysaccharides, proteins, lipids, and RNA-across multiple plant biosynthetic processes have been demonstrated to interact with one another. The mechanisms behind these interactions are still unknown, but recent research has highlighted extensive examples of protein-protein interactions, specifically in the plant cell wall hemicellulose and pectin biosynthesis that takes place in the Golgi apparatus. In this review, we will discuss what is known so far about the interactions among Golgi-localized glycosyltransferases that are important for their functioning, trafficking, as well as structural aspects.

New insights on β-glycan synthases using in vitro GT-array (i-GT-ray) platform

Cellulose and hemicellulose are the major structural β-glycan polysaccharides in cell walls of land plants. They are characterized by a backbone of β-(1,3)- and/or β-(1,4)-linked sugars such as glucose, mannose, or xylose. The backbones of these polymers are produced by processive glycosyltransferases (GTs) called synthases having multiple transmembrane domains anchoring them to the membrane. Thus, they are among the most difficult membrane proteins to test in vitro and to purify. Recently, we developed an in vitro GT-array (i-GTray) platform and showed that non-processive type II membrane GTs could be produced via cell-free system in a soluble and active form and tested in this platform. To determine whether i-GT-ray platform is adequate for the production and testing of β-glycan synthases, we tested five synthases involved in cellulose, xyloglucan, (gluco)mannan, and β-(1,3)(1,4)-mixed-linkage glucan synthesis. Our results revealed unsuspected features of these enzymes. For example, all these synthases could be produced in a soluble and active form and are active in the absence of detergent or membrane lipids, and none of them required a primer for initiation of synthesis. All synthases produced ethanol-insoluble products that were susceptible to the appropriate hydrolases (i.e., cellulase, lichenase, mannanase). Using this platform, we showed that AtCslC4 and AtXXT1 interact directly to form an active xyloglucan synthase that produced xylosylated cello-oligosaccharides (up to three xylosyl residues) when supplied with UDP-Glc and UDP-Xyl. i-GTray platform represents a simple and powerful functional genomics tool for discovery of new insights of synthase activities and can be adapted to other enzymes.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2021-2022

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates is a well-established technique and this review is the 12th update of the original article published in 1999 and brings coverage of the literature to the end of 2022. As with previous review, this review also includes a few papers that describe methods appropriate to analysis by MALDI, such as sample preparation, even though the ionization method is not MALDI. The review follows the same format as previous reviews. It is divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of computer software for structural identification. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other general areas such as medicine, industrial processes, natural products and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. MALDI is still an ideal technique for carbohydrate analysis, particularly in its ability to produce single ions from each analyte and advancements in the technique and range of applications show little sign of diminishing.

The Arabidopsis xylosyltransferases, XXT3, XXT4, and XXT5, are essential to complete the fully xylosylated glucan backbone XXXG-type structure of xyloglucans

Although most xyloglucans (XyGs) biosynthesis enzymes have been identified, the molecular mechanism that defines XyG branching patterns is unclear. Four out of five XyG xylosyltransferases (XXT1, XXT2, XXT4, and XXT5) are known to add the xylosyl residue from UDP-xylose onto a glucan backbone chain; however, the function of XXT3 has yet to be demonstrated. Single xxt3 and triple xxt3xxt4xxt5 mutant Arabidopsis (Arabidopsis thaliana) plants were generated using CRISPR-Cas9 technology to determine the specific function of XXT3. Combined biochemical, bioinformatic, and morphological data conclusively established for the first time that XXT3, together with XXT4 and XXT5, adds xylosyl residue specifically at the third glucose in the glucan chain to synthesize XXXG-type XyGs. We propose that the specificity of XXT3, XXT4, and XXT5 is directed toward the prior synthesis of the acceptor substrate by the other two enzymes, XXT1 and XXT2. We also conclude that XXT5 plays a dominant role in the synthesis of XXXG-type XyGs, while XXT3 and XXT4 complementarily contribute their activities in a tissue-specific manner. The newly generated xxt3xxt4xxt5 mutant produces only XXGG-type XyGs, which further helps to understand the impact of structurally deficient polysaccharides on plant cell wall organization, growth, and development.

Broad Specific Xyloglucan:Xyloglucosyl Transferases Are Formidable Players in the Re-Modelling of Plant Cell Wall Structures

Plant xyloglucan:xyloglucosyl transferases, known as xyloglucan endo-transglycosylases (XETs) are the key players that underlie plant cell wall dynamics and mechanics. These fundamental roles are central for the assembly and modifications of cell walls during embryogenesis, vegetative and reproductive growth, and adaptations to living environments under biotic and abiotic (environmental) stresses. XET enzymes (EC 2.4.1.207) have the β-sandwich architecture and the β-jelly-roll topology, and are classified in the glycoside hydrolase family 16 based on their evolutionary history. XET enzymes catalyse transglycosylation reactions with xyloglucan (XG)-derived and other than XG-derived donors and acceptors, and this poly-specificity originates from the structural plasticity and evolutionary diversification that has evolved through expansion and duplication. In phyletic groups, XETs form the gene families that are differentially expressed in organs and tissues in time- and space-dependent manners, and in response to environmental conditions. Here, we examine higher plant XET enzymes and dissect how their exclusively carbohydrate-linked transglycosylation catalytic function inter-connects complex plant cell wall components. Further, we discuss progress in technologies that advance the knowledge of plant cell walls and how this knowledge defines the roles of XETs. We construe that the broad specificity of the plant XETs underscores their roles in continuous cell wall restructuring and re-modelling.

Xyloglucan Biosynthesis: From Genes to Proteins and Their Functions

The plant's recalcitrant cell wall is composed of numerous polysaccharides, including cellulose, hemicellulose, and pectin. The most abundant hemicellulose in dicot cell walls is xyloglucan, which consists of a β-(1- > 4) glucan backbone with α-(1- > 6) xylosylation producing an XXGG or XXXG pattern. Xylose residues of xyloglucan are branched further with different patterns of arabinose, fucose, galactose, and acetylation that varies between species. Although xyloglucan research in other species lag behind Arabidopsis thaliana, significant advances have been made into the agriculturally relevant species Oryza sativa and Solanum lycopersicum, which can be considered model organisms for XXGG type xyloglucan. In this review, we will present what is currently known about xyloglucan biosynthesis in A. thaliana, O. sativa, and S. lycopersicum and discuss the recent advances in the characterization of the glycosyltransferases involved in this complex process and their organization in the Golgi.