Zhong Ruiqin, Teng Quincy, Lee Chanhui, Ye Zheng-Hua
Department of Plant Biology; University of Georgia; Athens, GA USA.
Department of Pharmaceutical and Biomedical Sciences; University of Georgia; Athens, GA USA.
Plant Signal Behav. 2014;9(2):e27933. doi: 10.4161/psb.27933. Epub 2014 Feb 12.
Arabidopsis xylan consists of a linear chain of β-1,4-linked D-xylosyl residues, about 10% of which are substituted with single residues of α-D-glucuronic acid (GlcA) or 4-O-methyl-α-D-glucuronic acid (MeGlcA) at O-2. In addition, about 60% of xylosyl residues are acetylated at O-2 and/or O-3. Previous studies have identified a number of genes responsible for elongation of the xylan backbone, addition of the GlcA substituents, and methylation of the GlcA residues. Yuan et al. (2013) have recently reported that the 2-O- and 3-O-monoacetylation of xylosyl residues in Arabidopsis xylan requires a DUF231 domain-containing protein, ESKIMO1 (ESK1), and proposed that ESK1 and its homologs are putative acetyltransferases responsible for xylan acetylation. It was noticed that the (1)H nuclear magnetic resonance (NMR) spectra of the acetylated xylan from the esk1 mutant and the wild-type Arabidopsis exhibited a prominent proton signal peak at 5.42 ppm in addition to resonances corresponding to known acetylated structural groups of xylan. Here, we performed detailed structural investigation of wild-type Arabidopsis acetylated xylan using 2-dimensional (1)H- (1)H and (1)H- (13)C NMR spectroscopy and found that the signal peak at 5.42 ppm in the (1)H NMR spectrum was attributed to GlcA residues substituted at O-2 with α-D-galactose (Gal), indicating the presence of Gal-GlcA disaccharide side chains in Arabidopsis xylan. This finding was further supported by analysis of endoxylanase-digested xylan using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Our study demonstrates that Arabidopsis xylan contains Gal-GlcA disaccharide side chains in addition to GlcA, MeGlcA, and acetyl substitutions.
拟南芥木聚糖由β-1,4-连接的D-木糖基残基线性链组成，其中约10%在O-2位被单个α-D-葡萄糖醛酸（GlcA）或4-O-甲基-α-D-葡萄糖醛酸（MeGlcA）残基取代。此外，约60%的木糖基残基在O-2和/或O-3位被乙酰化。先前的研究已经鉴定出许多负责木聚糖主链延伸、GlcA取代基添加以及GlcA残基甲基化的基因。袁等人（2013年）最近报道，拟南芥木聚糖中木糖基残基的2-O-和3-O-单乙酰化需要一种含有DUF231结构域的蛋白质，即爱斯基摩1（ESK1），并提出ESK1及其同源物是负责木聚糖乙酰化的假定乙酰转移酶。值得注意的是，来自esk1突变体和野生型拟南芥的乙酰化木聚糖的（1）H核磁共振（NMR）谱除了对应于木聚糖已知乙酰化结构基团的共振外，在5.42 ppm处还显示出一个突出的质子信号峰。在这里，我们使用二维（1）H-（1）H和（1）H-（13）C NMR光谱对野生型拟南芥乙酰化木聚糖进行了详细的结构研究，发现（1）H NMR谱中5.42 ppm处的信号峰归因于在O-2位被α-D-半乳糖（Gal）取代的GlcA残基，这表明拟南芥木聚糖中存在Gal-GlcA二糖侧链。使用基质辅助激光解吸电离飞行时间质谱对木聚糖内切酶消化的木聚糖进行分析进一步支持了这一发现。我们的研究表明，拟南芥木聚糖除了含有GlcA、MeGlcA和乙酰取代外，还含有Gal-GlcA二糖侧链。
