Factors affecting degree of polymerization of GBSS1 in Oryza sativa L. and their roles in increasing its enzymatic activity
Derui Liu, Xiuling Cai*

National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, Shanghai 200032.
*Corresponding Author：Tel: (+8621) 5492 4078; Fax: (+8621) 5492 4015; E-mail:  xlcai@sibs.ac.cn  

Abstract 
GBSS1（Granule-bound starch synthase1） controls amylose synthesis in all cereals. Previous reports on its regulation have been primarily focused on its transcriptional and post-transcriptional levels. In this study, we established that GBSS1 from Oryza sativa L. (OsGBSS1) is capable of forming oligomers in the rice endosperm and the polymerized OsGBSS1 exhibits much higher enzymatic activity. We found that there’s a monomer-oligomer transition equilibrium for OsGBSS1 in the endosperm. The redox potential was found to be a key factor affecting the oligomer percentage as well as its activity. Interestingly, as the direct donor of glucose, ADPG (Adenosine diphosphate glucose) impacted OsGBSS1 polymerization in a concentration-dependent manner. OsGBSS1 polymerization was built on its phosphorylation modification, which was strongly enhanced by MAPK (Mitogen-activated protein kinase) and ATP (Adenosine triphosphate) treatment; and vice versa. Since OsGBSS1 is incapable of forming complexes with other amylopectin synthetic enzymes, it largely affects the eating quality of rice, which is mainly determined by the ratio of amylose and amylopectin. Our results shed a new light on the regulation of the protein or proteome level of OsGBSS1; and provide a novel approach towards improving the rice eating quality.

Key Words: OsGBSS1, polymerization, enzyme activity, eating quality, rice