Insight into multi-site mechanisms of glycosyl transfer in (1-->4)beta-D-glycans provided by the cereal mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Enzymes in 3D: Synthesis, remodelling, and hydrolysis of cell wall (1,3;1,4)-β-glucans

Recent breakthroughs in structural biology have provided valuable new insights into enzymes involved in plant cell wall metabolism. More specifically, the molecular mechanism of synthesis of (1,3;1,4)-β-glucans, which are widespread in cell walls of commercially important cereals and grasses, has been the topic of debate and intense research activity for decades. However, an inability to purify these integral membrane enzymes or apply transgenic approaches without interpretative problems associated with pleiotropic effects has presented barriers to attempts to define their synthetic mechanisms. Following the demonstration that some members of the CslF sub-family of GT2 family enzymes mediate (1,3;1,4)-β-glucan synthesis, the expression of the corresponding genes in a heterologous system that is free of background complications has now been achieved. Biochemical analyses of the (1,3;1,4)-β-glucan synthesized in vitro, combined with 3-dimensional (3D) cryogenic-electron microscopy and AlphaFold protein structure predictions, have demonstrated how a single CslF6 enzyme, without exogenous primers, can incorporate both (1,3)- and (1,4)-β-linkages into the nascent polysaccharide chain. Similarly, 3D structures of xyloglucan endo-transglycosylases and (1,3;1,4)-β-glucan endo- and exohydrolases have allowed the mechanisms of (1,3;1,4)-β-glucan modification and degradation to be defined. X-ray crystallography and multi-scale modeling of a broad specificity GH3 β-glucan exohydrolase recently revealed a previously unknown and remarkable molecular mechanism with reactant trajectories through which a polysaccharide exohydrolase can act with a processive action pattern. The availability of high-quality protein 3D structural predictions should prove invaluable for defining structures, dynamics, and functions of other enzymes involved in plant cell wall metabolism in the immediate future.

Genetic Approaches to Increase Arabinoxylan and β-Glucan Content in Wheat

Wheat is one of the three staple crops feeding the world. The demand for wheat is ever increasing as a relatively good source of protein, energy, nutrients, and dietary fiber (DF) when consumed as wholemeal. Arabinoxylan and β-glucan are the major hemicelluloses in the cell walls and dietary fiber in wheat grains. The amount and structure of DF varies between grain tissues. Reducing post-prandial glycemic response as well as intestinal transit time and contribution to increased fecal bulk are only a few benefits of DF consumption. Dietary fiber is fermented in the colon and stimulates growth of beneficial bacteria producing SCFA, considered responsible for a wide range of health benefits, including reducing the risk of heart disease and colon cancer. The recommended daily intake of 25-30 g is met by only few individuals. Cereals cover nearly 40% of fiber in the Western diet. Therefore, wheat is a good target for improving dietary fiber content, as it would increase the fiber intake and simultaneously impact the health of many people. This review reflects the current status of the research on genetics of the two major dietary fiber components, as well as breeding approaches used to improve their quantity and quality in wheat grain.

The maize mixed-linkage (1->3),(1->4)-beta-D-glucan polysaccharide is synthesized at the golgi membrane

With the exception of cellulose and callose, the cell wall polysaccharides are synthesized in Golgi membranes, packaged into vesicles, and exported to the plasma membrane where they are integrated into the microfibrillar structure. Consistent with this paradigm, several published reports have shown that the maize (Zea mays) mixed-linkage (1-->3),(1-->4)-beta-D-glucan, a polysaccharide that among angiosperms is unique to the grasses and related Poales species, is synthesized in vitro with isolated maize coleoptile Golgi membranes and the nucleotide-sugar substrate, UDP-glucose. However, a recent study reported the inability to detect the beta-glucan immunocytochemically at the Golgi, resulting in a hypothesis that the mixed-linkage beta-glucan oligomers may be initiated at the Golgi but are polymerized at the plasma membrane surface. Here, we demonstrate that (1-->3),(1-->4)-beta-D-glucans are detected immunocytochemically at the Golgi of the developing maize coleoptiles. Further, when maize seedlings at the third-leaf stage were pulse labeled with [(14)C]O(2) and Golgi membranes were isolated from elongating cells at the base of the developing leaves, (1-->3),(1-->4)-beta-D-glucans of an average molecular mass of 250 kD and higher were detected in isolated Golgi membranes. When the pulse was followed by a chase period, the labeled polysaccharides of the Golgi membrane diminished with subsequent transfer to the cell wall. (1-->3),(1-->4)-beta-D-Glucans of at least 250 kD were isolated from cell walls, but much larger aggregates were also detected, indicating a potential for intermolecular interactions with glucuronoarabinoxylans or intermolecular grafting in muro.

Exploring the evolution of (1,3;1,4)-beta-D-glucans in plant cell walls: comparative genomics can help!

A key distinguishing feature of the grasses is that their cell walls contain (1,3;1,4)-beta-D-glucans, which are distributed almost exclusively within the Poaceae. The identification of genes that mediate in (1,3;1,4)-beta-D-glucan biosynthesis has been possible through relatively recent genome sequencing programmes and comparative genomics techniques. The evolution of a single new gene appears to have been sufficient for the first synthesis of (1,3;1,4)-beta-D-glucans and there is compelling evidence that existing hydrolytic enzymes were adapted for the specific hydrolysis of the polysaccharide during wall turnover or degradation. Manipulation of the expression levels of genes involved in (1,3;1,4)-beta-D-glucan synthesis is likely to provide opportunities to enhance the value of grasses and cereals in commercial applications such as human nutrition and biofuel production.

Type I arabinogalactan contains beta-D-Galp-(1-->3)-beta-D-Galp structural elements

Arabinogalactan type I from potato was partially degraded by endo-galactanase from Aspergillus niger. High-performance anion-exchange chromatography revealed that several of the oligomeric degradation products eluted as double peaks. To investigate the nature of these products, the digest was fractionated by Bio-Gel P2 chromatography. The pool that contained tetramers was treated with a beta-D-Galp-(1-->4)-specific galactosidase from Bifidobacterium adolescentis to obtain a dimer with deviating linkage type, which was further purified by BioGel P2 chromatography. By obtaining all (1)H and (13)C chemical shifts and the presence of intra residual scalar coupling (HMBC) it could be concluded that the dimer contained a beta-(1-->3)-linkage instead of the expected beta-(1-->4)-linkage. Using the same NMR techniques as for the dimer, it was found that the pool of tetramers consisted of the following two galactose tetramers: beta-Galp-(1-->4)-beta-Galp-(1-->4)-beta-Galp-(1-->4)-alpha/beta-Galp-OH and beta-Galp-(1-->4)-beta-Galp-(1-->4)-beta-Galp-(1-->3)-alpha/beta-Galp-OH. The fact that the deviating beta-(1-->3)-linked galactose was found at the reducing end of the dimer showed that this deviating linkage is present within the backbone. The beta-(1-->3)-galactosyl interruption appeared to be a common structural feature of type I arabinogalactans with a frequency ranging from approximately 1 in 160 (potato, soy, citrus) to 1 in 250 (onion).

Sucrose metabolism: regulatory mechanisms and pivotal roles in sugar sensing and plant development

Sucrose cleavage is vital to multicellular plants, not only for the allocation of crucial carbon resources but also for the initiation of hexose-based sugar signals in importing structures. Only the invertase and reversible sucrose synthase reactions catalyze known paths of sucrose breakdown in vivo. The regulation of these reactions and its consequences has therefore become a central issue in plant carbon metabolism. Primary mechanisms for this regulation involve the capacity of invertases to alter sugar signals by producing glucose rather than UDPglucose, and thus also two-fold more hexoses than are produced by sucrose synthase. In addition, vacuolar sites of cleavage by invertases could allow temporal control via compartmentalization. In addition, members of the gene families encoding either invertases or sucrose synthases respond at transcriptional and posttranscriptional levels to diverse environmental signals, including endogenous changes that reflect their own action (e.g. hexoses and hexose-responsive hormone systems such as abscisic acid [ABA] signaling). At the enzyme level, sucrose synthases can be regulated by rapid changes in sub-cellular localization, phosphorylation, and carefully modulated protein turnover. In addition to transcriptional control, invertase action can also be regulated at the enzyme level by highly localized inhibitor proteins and by a system that has the potential to initiate and terminate invertase activity in vacuoles. The extent, path, and site of sucrose metabolism are thus highly responsive to both internal and external environmental signals and can, in turn, dramatically alter development and stress acclimation.

Topology of the maize mixed linkage (1->3),(1->4)-beta-d-glucan synthase at the Golgi membrane

Mixed-linkage (1-->3),(1-->4)-beta-d-glucan is a plant cell wall polysaccharide composed of cellotriosyl and cellotetraosyl units, with decreasingly smaller amounts of cellopentosyl, cellohexosyl, and higher cellodextrin units, each connected by single (1-->3)-beta-linkages. (1-->3),(1-->4)-beta-Glucan is synthesized in vitro with isolated maize (Zea mays) Golgi membranes and UDP-[(14)C]d-glucose. The (1-->3),(1-->4)-beta-glucan synthase is sensitive to proteinase K digestion, indicating that part of the catalytic domain is exposed to the cytoplasmic face of the Golgi membrane. The detergent [3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid] (CHAPS) also lowers (1-->3),(1-->4)-beta-glucan synthase activity. In each instance, the treatments selectively inhibit formation of the cellotriosyl units, whereas synthesis of the cellotetraosyl units is essentially unaffected. Synthesis of the cellotriosyl units is recovered when a CHAPS-soluble factor is permitted to associate with Golgi membranes at synthesis-enhancing CHAPS concentrations but lost if the CHAPS-soluble fraction is replaced by fresh CHAPS buffer. In contrast to other known Golgi-associated synthases, (1-->3),(1-->4)-beta-glucan synthase behaves as a topologic equivalent of cellulose synthase, where the substrate UDP-glucose is consumed at the cytosolic side of the Golgi membrane, and the glucan product is extruded through the membrane into the lumen. We propose that a cellulose synthase-like core catalytic domain of the (1-->3),(1-->4)-beta-glucan synthase synthesizes cellotetraosyl units and higher even-numbered oligomeric units and that a separate glycosyl transferase, sensitive to proteinase digestion and detergent extraction, associates with it to add the glucosyl residues that complete the cellotriosyl and higher odd-numbered units, and this association is necessary to drive polymer elongation.

Three-dimensional structures of the Mn and Mg dTDP complexes of the family GT-2 glycosyltransferase SpsA: a comparison with related NDP-sugar glycosyltransferases

The vast majority of glycosidic-bond synthesis in nature is performed by glycosyltransferases, which use activated glycosides as the sugar donor. Typically, the activated leaving group is a nucleoside phosphate, lipid phosphate or phosphate. The nucleotide-sugar-dependent glycosyltransferases fall into over 50 sequence-based families, with the largest and most widespread family of inverting transferases named family GT-2. Here, we present the three-dimensional crystal structure of SpsA, the first and currently the only structural representative from family GT-2, in complex with both Mn-dTDP and Mg-dTDP at a resolution of 2 A. These structures reveal how SpsA and related enzymes may display nucleotide plasticity and permit a comparison of the catalytic centre of this enzyme with those from related sequence families whose three-dimensional structures have recently been determined. Family GT-2 enzymes, together with enzymes from families 7, 13 and 43, appear to form a clan of related structures with identical catalytic apparatus and reaction mechanism.

Beta-D-glycan synthases and the CesA gene family: lessons to be learned from the mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Cellulose synthase genes (CesAs) encode a broad range of processive glycosyltransferases that synthesize (1-->4)beta-D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spanning domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher plants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi-gene families from two grass species, Oryza sativa and Zea mays, with Arabidopsis thaliana and other dicotyledonous species reveals that the CesA genes cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more aptly termed a 'class-specific region' (CSR). Several motifs containing cysteine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserved in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthesis of non-cellulosic polymers with (1-->4)beta-linked backbones, including chitin, heparan, and hyaluronan. These analyses re-open the question whether all the CesA genes encode cellulose synthases or whether some of the sub-class members may encode other non-cellulosic (1-->4)beta-glycan synthases in plants. For example, the mixed-linkage (1-->3)(1-->4)beta-D-glucan synthase is found specifically in grasses and possesses many features more similar to those of cellulose synthase than to those of other beta-linked cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta-glucan synthases not only provide special insight into the mechanisms of (1-->4)beta-glycan synthesis but may also uncover the genes that encode the synthases themselves.