4- O -methylation of glucuronic acid in Arabidopsis glucuronoxylan is catalyzed by a domain of unknown function family 579 protein

Abstract

The hemicellulose 4-O-methyl glucuronoxylan is one of the principle components present in the secondary cell walls of eudicotyledonous plants. However, the biochemical mechanisms leading to the formation of this polysaccharide and the effects of modulating its structure on the physical properties of the cell wall are poorly understood. We have identified and functionally characterized an Arabidopsis glucuronoxylan methyltransferase (GXMT) that catalyzes 4-O-methylation of the glucuronic acid substituents of this polysaccharide. AtGXMT1, which was previously classified as a domain of unknown function (DUF) 579 protein, specifically transfers the methyl group from S-adenosyl-L-methionine to O-4 of α-D-glucopyranosyluronic acid residues that are linked to O-2 of the xylan backbone. Biochemical characterization of the recombinant enzyme indicates that GXMT1 is localized in the Golgi apparatus and requires Co 2+ for optimal activity in vitro. Plants lacking GXMT1 synthesize glucuronoxylan in which the degree of 4-O-methylation is reduced by 75%. This result is correlated to a change in lignin monomer composition and an increase in glucuronoxylan release during hydrothermal treatment of secondary cell walls. We propose that the DUF579 proteins constitute a previously undescribed family of cation-dependent, polysaccharide-specific O-methyl-transferases. This knowledge provides new opportunities to selectively manipulate polysaccharide O-methylation and extends the portfolio of structural targets that can be modified either alone or in combination to modulate biopolymer interactions in the plant cell wall.recalcitrance | biosynthesis

Reduced Wall Acetylation Proteins Play Vital and Distinct Roles in Cell Wall O-Acetylation in Arabidopsis

“…In the triple mutant, the ratio was closer to 2:1. These observations suggest that lack of Ac in Xylp carrying GlcA impacts the methylation of GlcA, which is known to take place at the polymer level (Lee et al, 2012;Urbanowicz et al, 2012). The observation of reduced methylation of GlcA in the rwa1 rwa3 rwa4 mutant is consistent with results of Lee et al (2011) reporting increased GlcA level in their rwa1 rwa2 rwa3 rwa4 mutant.…”

Plant Cell Wall Polysaccharides: Structure and Biosynthesis

“…A GX methyltransferase (GXMT), encoding a protein containing a domain of unknown function (DUF) 579, specifically transfers a methyl group from S-adenosylmethionine to the O-4 position of GlcA side chains of GX (Lee et al 2012;Urbanowicz et al 2012). The Trichome Birefringence-like (TBL) family member TBL29 contains a DUF231 domain and has been shown to transfer O-acetyl groups onto both O-2 and O-3 positions of xylose residues in the xylan backbone, hence affecting the solubility and digestibility of GX in plants mutated in these genes (At3g55990, At3g06550) (Manabe et al 2013;Yuan et al 2013;Xiong et al 2013).…”

Cell wall integrity

“…5 Most plant cell wall polymers, except cellulose and some glucans, can be substituted with acetyl groups 6 and HG and glucuronoxylan can also be methyl-esterified. 7,8 The modern view of the plant cell wall is of a dynamic extracellular complex that responds to external or internal cellular signals and forms a continuum with the plasma membrane and cytoskeleton. 9 In addition to well established cell wall functions in maintaining and determining cell shape, 10,11 resisting internal turgor pressure, 12 controlling and directing cell and plant growth, 13 contributing to plant morphology 14 and regulating diffusion through the apoplast, the cell wall also has recently revealed signaling functions.…”