GUX1 and GUX2 glucuronyltransferases decorate distinct domains of glucuronoxylan with different substitution patterns

Abstract

SUMMARYXylan comprises up to one-third of plant cell walls, and it influences the properties and processing of biomass. Glucuronoxylan in Arabidopsis is characterized by a linear b-(1,4)-linked backbone of xylosyl residues substituted by glucuronic acid and 4-O-methylglucuronic acid (collectively termed [Me]GlcA). The role of these substitutions remains unclear. GUX1 (glucuronic acid substitution of xylan 1) and GUX2, recently identified as glucuronyltransferases, are both required for substitution of the xylan backbone with [Me]GlcA. Here, we demonstrate clear differences in the pattern of [Me]GlcA substitution generated by each of these glucuronyltransferases. GUX1 decorates xylan with a preference for addition of [Me]GlcA at evenly spaced xylosyl residues. Intervals of eight or 10 residues dominate, but larger intervals are observed. GUX2, in contrast, produces more tightly clustered decorations with most frequent spacing of five, six or seven xylosyl residues, with no preference for odd or even spacing. Moreover, each of these GUX transferases substitutes a distinct domain of secondary cell wall xylan, which we call the major and minor domains. These major and minor xylan domains were not separable from each other by size or charge, a finding that suggests that they are tightly associated. The presence of both differently [Me]GlcA decorated domains may produce a xylan molecule that is heterogeneous in its properties. We speculate that the major and minor domains of xylan may be specialised, such as for interaction with cellulose or lignin. These findings have substantial implications for our understanding of xylan synthesis and structure, and for models of the molecular architecture of the lignocellulosic matrix of plant cell walls.

Regular Motifs in Xylan Modulate Molecular Flexibility and Interactions with Cellulose Surfaces

“…; Shimizu et al, 1978), sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtuse; Yamasaki et al, 2011), and suggested for spruce (Jacobs et al, 2001). The presence of major domains with even distribution of the mGlcA units along the backbone and minor domains with uneven glucuronation was already reported in glucuronoxylan from Arabidopsis (Arabidopsis thaliana), which was ascribed to the action of two distinct glucuronyltransferases (Bromley et al, 2013). This structural information reveals that the intramolecular glycosylation pattern in AGX is more complex than reported previously (Busse-Wicher et al, 2016b).…”

“…This demonstrates the occurrence of a more complex and controlled molecular regularity in softwood xylans than hitherto reported. Bromley et al (2013) already proposed the presence of molecular domains with different uronic acid spacing for glucuronoxylan in Arabidopsis, which was attributed to the action of two different glucuronyltransferases. The presence of distinct domains with even and clustered placement of glycosyl decorations may be directed by the 3D spatial arrangement of the glucuronyltransferase and arabinofuranosyltransferase catalytic units with respect to the nascent xylan chain.…”

“…1;St John et al, 2011;Bromley et al, 2013). Interestingly, two main aldouronic acid oligosaccharides (P 6 U m and P 7 U m ) can be observed in AGX-A (Fig.…”

“…A regular distribution of glucuronic acids was reported previously in the xylans extracted from softwoods, whereas the pattern in hardwoods seemed to be irregular (Jacobs et al, 2001). Recent studies, however, have shown that glycosidic and acetyl decorations are preferably evenly spaced in specific domains of the xylan backbone (Bromley et al, 2013;Busse-Wicher et al, 2014;Chong et al, 2014), which could sterically allow favorable interactions with the hydrophilic surfaces of cellulose microfibrils (BusseWicher et al, 2016a(BusseWicher et al, , 2016b. Moreover, backbone substitution influences the solubility of hemicelluloses and their macroscopic properties Sternemalm et al, 2008;Pitkänen et al, 2009;Escalante et al, 2012;Bosmans et al, 2014;Littunen et al, 2015).…”

“…Spatial and temporal heterogeneity in the composition and molecular structure of plant cell wall polysaccharides may be directed by biosynthetic processes in order to modulate their molecular diversity, adaptability, and biological function (Burton et al, 2010); however, the biological causes and effects of these fine structural variations are unknown. We are starting to understand how the molecular structure of hemicelluloses regulates the association with cellulose microfibrils (Bromley et al, 2013;Busse-Wicher et al, 2014, 2016a, 2016b and the occurrence of covalent linkages with lignins in lignin-carbohydrate complexes (Lawoko et al, 2005). The presence of intermolecular forces and the interconnected supramolecular architecture in lignocellulosic biomass are responsible for its recalcitrance against conversion into valuable biofuels and platform chemicals (Mortimer et al, 2010;Ding et al, 2012;Silveira et al, 2013).…”