Cao H, Imparl-Radosevich J, Guan H, Keeling P L, James M G, Myers A M
Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011, USA.
Plant Physiol. 1999 May;120(1):205-16. doi: 10.1104/pp.120.1.205.
This study identified the complement of soluble starch synthases (SSs) present in developing maize (Zea mays) endosperm. The product of the du1 gene, DU1, was shown to be one of the two major soluble SSs. The C-terminal 450 residues of DU1 comprise eight sequence blocks conserved in 28 known or predicted glucan synthases. This region of DU1 was expressed in Escherichia coli and shown to possess SS activity. DU1-specific antisera detected a soluble endosperm protein of more than 200 kD that was lacking in du1- mutants. These antisera eliminated 20% to 30% of the soluble SS activity from kernel extracts. Antiserum against the isozyme zSSI eliminated approximately 60% of the total soluble SS, and immunodepletion of du1- mutant extracts with this antiserum nearly eliminated SS activity. Two soluble SS activities were identified by electrophoretic fractionation, each of which correlated specifically with zSSI or DU1. Thus, DU1 and zSSI accounted for the great majority of soluble SS activity present in developing endosperm. The relative activity of the two isozymes did not change significantly during the starch biosynthetic period. DU1 and zSSI may be interdependent, because mutant extracts lacking DU1 exhibited a significant stimulation of the remaining SS activity.
本研究鉴定了发育中的玉米（Zea mays）胚乳中可溶性淀粉合酶（SSs）的组成。du1基因的产物DU1被证明是两种主要的可溶性SSs之一。DU1的C末端450个残基包含28种已知或预测的葡聚糖合酶中保守的8个序列块。DU1的这一区域在大肠杆菌中表达，并显示具有SS活性。DU1特异性抗血清检测到一种分子量超过200 kD的可溶性胚乳蛋白，该蛋白在du1突变体中不存在。这些抗血清消除了籽粒提取物中20%至30%的可溶性SS活性。针对同工酶zSSI的抗血清消除了约60%的总可溶性SS，用这种抗血清对du1突变体提取物进行免疫去除几乎消除了SS活性。通过电泳分离鉴定出两种可溶性SS活性，每种活性都与zSSI或DU1特异性相关。因此，DU1和zSSI占发育中的胚乳中可溶性SS活性的绝大部分。在淀粉生物合成期间，这两种同工酶的相对活性没有显著变化。DU1和zSSI可能相互依赖，因为缺乏DU1的突变体提取物表现出对剩余SS活性的显著刺激。
