Abstract：
Grass xylan consists of a linear chain of β-1,4-linked xylosyl residues that often form domains substituted only with either arabinofuranose (Araf) or glucuronic acid (GlcA)/methylglucuronic acid (MeGlcA) residues, and it lacks the unique reducing end tetrasaccharide sequence found in dicot xylan. The mechanism of how grass xylan backbone elongation is initiated and how its distinctive substitution pattern is determined remains elusive. Here, we performed biochemical characterization of rice xylan biosynthetic enzymes, including xylan synthases, glucuronyltransferases and methyltransferases. Activity assays of rice xylan synthases demonstrated that they required short xylooligomers as acceptors for their activities. While rice xylan glucuronyltransferases effectively glucuronidated unsubstituted xylohexaose acceptors, they transferred little GlcA residues onto (Araf)-substituted xylohexaoses and rice xylan 3-O-arabinosyltransferase could not arabinosylate GlcA-substituted xylohexaoses, indicating that their intrinsic biochemical properties may contribute to the distinctive substitution patterns of rice xylan. In addition, we found that rice xylan methyltransferase exhibited a low substrate binding affinity, which may explain the partial GlcA methylation in rice xylan. Furthermore, immunolocalization of xylan in xylem cells of both rice and Arabidopsis showed that it was deposited together with cellulose in secondary walls without forming xylan-rich nanodomains. Together, our findings provide new insights into the biochemical mechanisms underlying xylan backbone elongation and substitutions in grass species.
摘要：
草木聚糖由β-1,4-连接的木糖基残基的线性链组成，该残基通常形成仅被阿拉伯呋喃糖（Araf）或（甲基）葡糖醛酸[（Me）GlcA]残基取代的结构域，并且缺乏独特的还原末端四糖序列在双子木聚糖中发现。草木聚糖主链延伸的启动机制以及如何确定其独特的取代模式仍然难以捉摸。这里,我们进行了水稻木聚糖生物合成酶的生化表征，包括木聚糖合酶,葡糖醛酸转移酶和甲基转移酶。水稻木聚糖合酶的活性测定表明，它们需要短的木聚糖低聚物作为其活性的受体。虽然稻木聚糖葡糖醛酸基转移酶有效地将未取代的木二己糖受体葡糖醛酸化，他们将少量的GlcA残基转移到Araf取代的木糖六糖上，而水稻木聚糖3-O-阿拉伯糖基转移酶不能阿拉伯糖基化GlcA取代的木糖六糖，表明它们的内在生化特性可能有助于水稻木聚糖的独特取代模式。此外,我们发现水稻木聚糖甲基转移酶表现出低的底物结合亲和力，这可以解释水稻木聚糖中GlcA的部分甲基化。此外，木聚糖在水稻和拟南芥木质部细胞中的免疫定位表明，它与纤维素一起沉积在次生壁中，而没有形成富含木聚糖的纳米结构域。一起,我们的发现为草类木聚糖主链延伸和取代的生化机制提供了新的见解。
