Distinct Functions of STARCH SYNTHASE 4 Domains in Starch Granule Formation
Distinct Functions of STARCH SYNTHASE 4 Domains in Starch Granule Formation
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DOI：
10.1104/pp.17.01008
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发表时间：
2018-01-01
期刊：
影响因子：
7.4
通讯作者：
Zeeman, Samuel C.
中科院分区：
文献类型：
--
作者：
Lu, Kuan-Jen;Pfister, Barbara;Zeeman, Samuel C.
拟南芥（Arabidopsis thaliana）叶片叶绿体中正常淀粉粒的形成需要淀粉合成酶4（SS 4）。在缺乏SS 4的植物中，叶绿体通常只产生一个圆形颗粒，而不是多个透镜状颗粒。SS 4决定颗粒数量和形态的机制尚不清楚。SS 4的N-末端区域在SS同种型中是独特的，并且包含几个长卷曲螺旋基序，通常涉及蛋白质-蛋白质相互作用。C-末端区域包含催化葡糖基转移酶结构域，其在植物SS和细菌糖原合酶（GS）同种型中广泛保守。我们研究了具体的作用的N-和C-末端区域的SS 4通过表达截短版本的SS 4和N-末端区域的SS 4和GS之间的融合在拟南芥ss 4突变体。单独表达SS 4的N-末端区域不改变SS 4突变体表型。单独表达SS 4的C-末端区域增加颗粒起始，但不能挽救其异常的圆形形态。自启动GS从根癌农杆菌的表达也增加了圆形颗粒的数量。值得注意的是，SS 4的N-末端区域与A.根癌农杆菌GS恢复了野生型样透镜状淀粉颗粒的发育。有趣的是，单独或融合到GS的SS 4的N-末端区域赋予了类似于全长SS 4蛋白的片状亚叶绿体定位。综合考虑，这些数据表明，虽然SS 4的葡糖基转移酶活性对于颗粒起始是重要的，但SS 4的N-末端部分通过适当地定位该活性来建立正确的颗粒形态。
The formation of normal starch granules in Arabidopsis (Arabidopsis thaliana) leaf chloroplasts requires STARCH SYNTHASE 4 (SS4). In plants lacking SS4, chloroplasts typically produce only one round granule rather than multiple lenticular granules. The mechanisms by which SS4 determines granule number and morphology are not understood. The N-terminal region of SS4 is unique among SS isoforms and contains several long coiled-coil motifs, typically implicated in protein-protein interactions. The C-terminal region contains the catalytic glucosyltransferase domains, which are widely conserved in plant SS and bacterial glycogen synthase (GS) isoforms. We investigated the specific roles of the N-and C-terminal regions of SS4 by expressing truncated versions of SS4 and a fusion between the N-terminal region of SS4 and GS in the Arabidopsis ss4 mutant. Expression of the N-terminal region of SS4 alone did not alter the ss4 mutant phenotype. Expression of the C-terminal region of SS4 alone increased granule initiation but did not rescue their aberrant round morphology. Expression of a self-priming GS from Agrobacterium tumefaciens also increased the number of round granules. Remarkably, fusion of the N-terminal region of SS4 to A. tumefaciens GS restored the development of wild-type-like lenticular starch granules. Interestingly, the N-terminal region of SS4 alone or when fused to GS conferred a patchy subchloroplastic localization similar to that of the full-length SS4 protein. Considered together, these data suggest that, while the glucosyltransferase activity of SS4 is important for granule initiation, the N-terminal part of SS4 serves to establish the correct granule morphology by properly localizing this activity.