Ida Tamami, Crofts Naoko, Miura Satoko, Matsushima Ryo, Fujita Naoko
1 Laboratory of Plant Physiology, Department of Biological Production, Faculty of Bioresource Science, Akita Prefectural University.
2 Institute of Plant Science and Resources, Okayama University.
J Appl Glycosci (1999). 2022 May 25;69(2):23-33. doi: 10.5458/jag.jag.JAG-2021_0015. eCollection 2022.
Amylopectin, which consists of highly branched glucose polymers, is a major component of starch. Biochemical processes that regulate the elongation of glucose polymers and the generation and removal of glucose branches are essential for determining the properties of starch. Starch synthases (SSs) and branching enzyme (BE) mainly form complexes consisting of SSI, SSIIa, and BEIIb during endosperm development. Loss of BEIIb in rice is complemented by BEIIa, but the compensatory effects differ depending on the presence or absence of inactive BEIIb. To better understand these compensatory mechanisms, + double mutant, which possessed truncated inactive SSIIa and inactive BEIIb, were analyzed. Soluble proteins separated by gel filtration chromatography showed that SSIIa and BEIIb proteins in the wild-type exhibited a broad range of elution patterns and only small amounts were detected in high molecular mass fractions. In contrast, most of truncated inactive SSIIa and inactive BEIIb from + were found in high molecular mass fractions, and the SSI-SSIIa-BEIIb trimeric protein complex found in the wild-type was likely absent in +. Those SSIIa and BEIIb proteins in high molecular mass fractions in + were also identified by mass spectrometry. Parental single mutant had negligible amounts of SSIIa suggesting that the truncated inactive SSIIa was recruited to high-molecular mass complexes in the presence of inactive BEIIb in + double mutant. In addition, SSIVb might be involved in the formation of alternative protein complexes with < 300 kDa in +.
支链淀粉由高度分支的葡萄糖聚合物组成，是淀粉的主要成分。调节葡萄糖聚合物伸长以及葡萄糖分支生成和去除的生化过程对于确定淀粉的性质至关重要。在胚乳发育过程中，淀粉合酶（SSs）和分支酶（BE）主要形成由SSI、SSIIa和BEIIb组成的复合物。水稻中BEIIb的缺失由BEIIa补偿，但补偿效果因无活性BEIIb的存在与否而有所不同。为了更好地理解这些补偿机制，对具有截短的无活性SSIIa和无活性BEIIb的+双突变体进行了分析。凝胶过滤色谱分离的可溶性蛋白质显示，野生型中的SSIIa和BEIIb蛋白呈现出广泛的洗脱模式，且在高分子量组分中仅检测到少量。相比之下，来自+的大多数截短的无活性SSIIa和无活性BEIIb存在于高分子量组分中，野生型中发现的SSI-SSIIa-BEIIb三聚体蛋白复合物在+中可能不存在。+中高分子量组分中的那些SSIIa和BEIIb蛋白也通过质谱鉴定。亲本单突变体中SSIIa的量可忽略不计，这表明在+双突变体中，截短的无活性SSIIa在无活性BEIIb存在的情况下被招募到高分子量复合物中。此外，SSIVb可能参与了+中形成分子量小于300 kDa的替代蛋白复合物。
