Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/7/2001
Publication Date: 3/7/2002
Citation: SEO, B., SCOTT, M.P., SINGLETARY, G., WANG, K., JAMES, M.G., MYERS, A. FUNCTIONAL INTERACTIONS BETWEEN HETEROLOGOUSLY EXPRESSED STARCH-BRANCHING ENZYMES OF MAIZE AND THE GLYCOGEN SYNTHASES OF BREWER'S YEAST. PLANT PHYSIOLOGY. 2002. v. 128. p. 1189-1199. Interpretive Summary: Starch is used in many different food and industrial products. An imporant goal of starch research is to be able to control the functional properties of starch so that it is better suited to each of these uses. This would benefit consumers in two ways. First, it would increase the quality of starch products. Second, it would reduce costs by eliminating the need chemical modifications that are currently used to improve the functional properties of starch. One way to improve the functional properties of starch is to manipulate the genes involved in starch production in plants. To do this, it is important to understand the specific roles of each of these genes. We have examined the roles of one class of genes, the starch branching enzyme (SBE) genes, by producing corn SBEs in yeast. Three SBEs from corn were produced singly and in all possible combinations. We then characterized the glycogen (a starch-like polymer) that accumulates in yeast cells. From these experiments, we learned how these three corn enzymes interact with each other and with the yeast enzymes that make glycogen. Scientists will use this information to decide how best to alter these genes in order to produce starch with descired functional properties.
Technical Abstract: Higher plants contain three conserved starch branching enzyme (BE) isoforms, although their specific roles in determination of starch structure are not well understood. In this study Saccharomyces cerevisiae was used as a heterologous host to express all maize BE combinations in the absence of endogenous glycogen branching enzyme. All three maize BEs were functional, although BEI showed significant activity only if BEIIa and BEIIb also were present in the host. BEI by itself was unable to support glucan accumulation, whereas BEIIa and BEIIb both were able to work with the native glucan synthases (GS) to produce glucose polymers. BEIIa was phenotypically dominant to BEIIb in terms of glucan structure. The specific BE present had a significant effect on the molecular weight of the product. From these data we suggest that the BEs and GSs work in a cyclically interdependent fashion, such that BE action is needed for optimal GS activity, and GS in turn influences the further effects of BE. We also suggest that BEIIa and BEIIb act prior to BEI.