Lu Yan, Steichen Jon M, Yao Jian, Sharkey Thomas D
Department of Botany , University of Wisconsin, Madison, Wisconsin 53706, USA.
Plant Physiol. 2006 Nov;142(3):878-89. doi: 10.1104/pp.106.086850. Epub 2006 Sep 15.
Transitory starch of leaves is broken down hydrolytically, making maltose the predominant form of carbon exported from chloroplasts at night. Maltose metabolism in the cytoplasm of Escherichia coli requires amylomaltase (MalQ) and maltodextrin phosphorylase (MalP). Possible orthologs of MalQ and MalP in the cytosol of Arabidopsis (Arabidopsis thaliana) were proposed as disproportionating enzyme (DPE2, At2g40840) and alpha-glucan phosphorylase (AtPHS2, At3g46970). In this article, we measured the activities of recombinant DPE2 and AtPHS2 proteins with various substrates; we show that maltose and a highly branched, soluble heteroglycan (SHG) are excellent substrates for DPE2 and propose that a SHG is the in vivo substrate for DPE2 and AtPHS2. In E. coli, MalQ and MalP preferentially use smaller maltodextrins (G(3)-G(7)) and we suggest that MalQ and DPE2 have similar, but nonidentical, roles in maltose metabolism. To study this, we complemented a MalQ(-) E. coli strain with DPE2 and found that the rescue was not complete. To investigate the role of AtPHS2 in maltose metabolism, we characterized a T-DNA insertion line of the AtPHS2 gene. The nighttime maltose level increased 4 times in the Atphs2-1 mutant. The comparison of maltose metabolism in Arabidopsis with that in E. coli and the comparison of the maltose level in plants lacking DPE2 or AtPHS2 indicate that an alternative route to metabolize the glucan residues in SHG exists. Other plant species also contain SHG, DPE2, and alpha-glucan phosphorylase, so this pathway for maltose metabolism may be widespread among plants.
叶片中的暂存淀粉通过水解作用被分解，使得麦芽糖成为夜间从叶绿体输出的主要碳形式。大肠杆菌细胞质中的麦芽糖代谢需要淀粉麦芽糖酶（MalQ）和麦芽糖糊精磷酸化酶（MalP）。拟南芥（Arabidopsis thaliana）细胞质中MalQ和MalP可能的直系同源物被认为是歧化酶（DPE2，At2g40840）和α-葡聚糖磷酸化酶（AtPHS2，At3g46970）。在本文中，我们测定了重组DPE2和AtPHS2蛋白对各种底物的活性；我们发现麦芽糖和一种高度分支的可溶性杂聚糖（SHG）是DPE2的优良底物，并提出SHG是DPE2和AtPHS2在体内的底物。在大肠杆菌中，MalQ和MalP优先使用较小的麦芽糖糊精（G(3)-G(7)），我们认为MalQ和DPE2在麦芽糖代谢中具有相似但不完全相同的作用。为了研究这一点，我们用DPE2对MalQ(-)大肠杆菌菌株进行了互补，发现拯救并不完全。为了研究AtPHS2在麦芽糖代谢中的作用，我们对AtPHS2基因的一个T-DNA插入系进行了表征。Atphs2-1突变体中夜间麦芽糖水平增加了4倍。拟南芥与大肠杆菌中麦芽糖代谢的比较以及缺乏DPE2或AtPHS2的植物中麦芽糖水平的比较表明，存在一条代谢SHG中葡聚糖残基的替代途径。其他植物物种也含有SHG、DPE2和α-葡聚糖磷酸化酶，因此这条麦芽糖代谢途径可能在植物中广泛存在。
