4-O-methylation of glucuronic acid in Arabidopsis glucuronoxylan is catalyzed by a domain of unknown function family 579 protein.
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.
Complex Carbohydrate Research Center and Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA 30602, USA.
Breeanna R
BR
Urbanowicz
Maria J
MJ
Peña
Supriya
S
Ratnaparkhe
Utku
U
Avci
Jason
J
Backe
Heather F
HF
Steet
Marcus
M
Foston
Hongjia
H
Li
Malcolm A
MA
O'Neill
Arthur J
AJ
Ragauskas
Alan G
AG
Darvill
Charles
C
Wyman
Harry J
HJ
Gilbert
William S
WS
York
D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002384:Catalysis; D002412:Cations / Q000378:metabolism; D002473:Cell Wall / Q000201:enzymology; D004987:Ethers / Q000378:metabolism; D020723:Glucuronic Acid / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D008031:Lignin / Q000378:metabolism; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016296:Mutagenesis / Q000502:physiology; D011134:Polysaccharides / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology; D014990:Xylans / Q000096:biosynthesis / Q000378:metabolism*
D029681:Arabidopsis Proteins; D002412:Cations; D004987:Ethers; D011134:Polysaccharides; D014990:Xylans; D020723:Glucuronic Acid; D008031:Lignin; C044967:4-O-methyl glucuronoxylan; C576671:GXMT1 protein, Arabidopsis; D008780:Methyltransferases