Starch-branching enzymes (SBEs) catalyze the formation of α(1→6) glycoside bonds in glucan polymers, thus, affecting the structure of amylopectin and starch granules. Two distinct classes of SBE are generally conserved in higher plants, although the specific role(s) of each isoform in determination of starch structure is not clearly understood. This study used a heterologous in vivo system to isolate the function of each of the three known SBE isoforms of maize (Zea mays) away from the other plant enzymes involved in starch biosynthesis. The ascomycete Brewer's yeast (Saccharomyces cerevisiae) was employed as the host species. All possible combinations of maize SBEs were expressed in the absence of the endogenous glucan-branching enzyme. Each maize SBE was functional in yeast cells, although SBEI had a significant effect only if SBEIIa and SBEIIb also were present. SBEI by itself did not support glucan accumulation, whereas SBEIIa and SBEIIb both functioned along with the native glycogen synthases (GSs) to produce significant quantities of α-glucan polymers. SBEIIa was phenotypically dominant to SBEIIb in terms of glucan structure. The specific branching enzyme present had a significant effect on the molecular weight of the product. From these data we suggest that SBEs and GSs work in a cyclically interdependent fashion, such that SBE action is needed for optimal GS activity; and GS, in turn, influences the further effects of SBE. Also, SBEIIa and SBEIIb appear to act before SBEI during polymer assembly in this heterologous system.
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