36002:

Understanding the functions and interactions of multiple forms of maize starch synthase and branching enzyme.

Authors:	Guan, Hanping(A)Li, Ping(A)McKean, Angela(A)Imparl-Radosevich, Jennifer(A)Nichols, Debra(A)Gao, Zhong(A)Sun, Jingdong(A)Keeling, Peter(A)
Affiliations:	(A): ExSeed Genetics L. L. C. (B): Agronomy Department, Interdepartment Major of Plant Physiology, Iowa State University
Presenter:	Guan, Hanping , hpguan@iastate.edu

Multiple forms of Starch synthase (SS) and branching enzyme (BE) have been identified from maize endosperm. Difficulties in purifying these enzymes from developing maize endosperm have hampered the efforts to understand their functions in starch synthesis. Expression of these enzymes in E. coli has provided a useful tool to study these enzymes. First, bacterial expression has allowed us to purify and characterize these enzymes. Using the purified recombinant enzyme, we have obtained the evidence that maize SSI can catalyze the de novo synthesis of alpha 1,4 -glucans. This also allows us to study the catalytic mechanism of SS by chemical modification and mutagenesis. Secondly, bacterial expression facilitates the study on the specificity and interactions of different isoforms of SS and BE. Starch biosynthesis in higher plants and glycogen biosynthesis in E. coli have similar reactions which use ADPGlc as a substrate. This similarity allows us to use plant SS and BE to complement the functions of glycogen synthase (GS) and glycogen branching enzyme (GBE) in E. coli deficient in GS and GBE. Co-expression of the glgC, maize SS and BE produced branched polysaccharides. Distinct properties of plant SS and BE make it possible to synthesize polysaccharides with distinct structure and functionalities. While maize SSI preferentially synthesized short chains (degree of polymerization [dp] 6 - 15), SSIIa and SSIIb preferentially transferred long chains (dp > 24) and intermediate chains (dp 16 - 24) respectively. This study provides the evidence that both SS, BE and their interactions play a major role in determining the structure of amylopectin.