title: Identification and biochemical characterization of four wood-associated glucuronoxylan methyltransferases in Populus.
abstract: Wood is one of the promising bioenergy feedstocks for lignocellulosic biofuel production. Understanding how wood components are synthesized will help us design strategies for better utilization of wood for biofuel production. One of the major wood components is xylan, in which about 10% of xylosyl residues are substituted with glucuronic acid (GlcA) side chains. All the GlcA side chains of xylan in wood of Populus trichocarpa are methylated, which is different from Arabidopsis xylan in which about 60% of GlcA side chains are methylated. Genes responsible for methylation of GlcA side chains in Populus xylan have not been identified. Here, we report genetic and biochemical analyses of four DUF579 domain-containing proteins, PtrGXM1, PtrGXM2, PtrGXM3 and PtrGXM4, from Populus trichocarpa and their roles in GlcA methylation in xylan. The PtrGXM genes were found to be highly expressed in wood-forming cells and their encoded proteins were shown to be localized in the Golgi. When overexpressed in the Arabidopsis gxm1/2/3 triple mutant, PtrGXMs were able to partially complement the mutant phenotypes including defects in glucuronoxylan methyltransferase activity and GlcA methylation in xylan, indicating that PtrGXMs most likely function as glucuronoxylan methyltransferases. Direct evidence was provided by enzymatic analysis of recombinant PtrGXM proteins showing that they possessed a methyltransferase activity capable of transferring the methyl group onto GlcA-substituted xylooligomers. Kinetic analysis showed that PtrGXMs exhibited differential affinities toward the GlcA-substituted xylooligomer acceptor with PtrGXM3 and PtrGXM4 having 10 times higher K m values than PtrGXM1 and PtrGXM2. Together, these findings indicate that PtrGXMs are methyltransferases mediating GlcA methylation in Populus xylan during wood formation.
authors: Department of Plant Biology, University of Georgia, Athens, Georgia, United States of America.
authors: Youxi
authors: Y
authors: Yuan
authors: Department of Pharmaceutical and Biomedical Sciences, University of Georgia, Athens, Georgia, United States of America.
authors: Quincy
authors: Q
authors: Teng
authors: Department of Plant Biology, University of Georgia, Athens, Georgia, United States of America.
authors: Ruiqin
authors: R
authors: Zhong
authors: Department of Plant Biology, University of Georgia, Athens, Georgia, United States of America.
authors: Zheng-Hua
authors: ZH
authors: Ye
mesh_terms: D002473:Cell Wall / Q000201:enzymology / Q000378:metabolism; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D016695:Glycosyltransferases / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D010641:Phenotype; D010802:Phylogeny; D032107:Populus / Q000201:enzymology* / Q000235:genetics*; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000378:metabolism; D014934:Wood / Q000201:enzymology / Q000378:metabolism; D014990:Xylans / Q000737:chemistry*
chemical_list: D011994:Recombinant Proteins; D014990:Xylans; C038910:glucuronoxylan; D008780:Methyltransferases; D016695:Glycosyltransferases