The enzyme UDP-glucose dehydrogenase (UGD) competes with sucrose-phosphate synthase for the common photosynthesis product UDP-glucose. Sucrose-phosphate synthase is part of a pathway for the export of sucrose from source leaves to neighboring cells or the phloem. UGD is a central enzyme in a pathway for many nucleotide sugars used in local cell wall biosynthesis. Here, we identify a highly conserved phosphorylation site in UGD which is readily phosphorylated by MAP-kinase 3 in Arabidopsis. Phosphorylation occurs at a surface-exposed extra loop in all plant UGDs that is absent in UGDs from bacteria or animals. Phosphorylated sucrose-phosphate synthase is shifted to an inactive form which we did not measure for phosphorylated UGD. Plant UGDs have an extra loop which is phosphorylated by AtMPK3. Phosphorylation is not causing a reduction of UGD activity as found for the competitor enzymes and thus sets a preference for maintaining UDP-sugars at a constant level to prioritize cell wall biosynthesis.

中文翻译：

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拟南芥 MAP-激酶 3 磷酸化 UDP-葡萄糖脱氢酶：为细胞壁生物合成提供 UDP-糖的关键酶


UDP-葡萄糖脱氢酶 (UGD) 与蔗糖磷酸合酶竞争常见的光合作用产物 UDP-葡萄糖。蔗糖磷酸合酶是蔗糖从源叶输出到邻近细胞或韧皮部的途径的一部分。 UGD 是局部细胞壁生物合成中使用的许多核苷酸糖途径中的中心酶。在这里，我们鉴定了 UGD 中高度保守的磷酸化位点，该位点很容易被拟南芥中的 MAP 激酶 3 磷酸化。磷酸化发生在所有植物 UGD 中表面暴露的额外环处，而细菌或动物 UGD 中不存在磷酸化。磷酸化蔗糖磷酸合酶转变为无活性形式，我们没有测量磷酸化 UGD。植物 UGD 有一个额外的环，可被 AtMPK3 磷酸化。磷酸化不会像竞争酶那样导致 UGD 活性降低，因此倾向于将 UDP 糖维持在恒定水平，以优先考虑细胞壁生物合成。