In Vitro Studies of Enzymatic Properties of Starch Synthases and Interactions Between Starch Synthase I and Starch Branching Enzymes from Rice

The present study was conducted to characterize the functions of the major starch synthase (SS) isozymes SSI, SSIIa, and SSIIIa in rice endosperm and their functional interaction with starch branching enzyme (BE), by using their purified recombinant proteins. All the SS isozymes had similarly significant activities toward branched glucans such as amylopecin and glycogen whereas they scarcely showed activities toward maltohexaose. In vitro studies indicate that SSI mainly attacked A and B chains with degree of polymerization (DP) of 6 and 7 in their external segments and elongated them to DP8. It is likely that SSIIa and SSIIIa produced wider ranges of intermediate chains and long chains, respectively. This study also revealed that without addition of exogenous primer, the glucan synthesis of SSI in the presence of ≧0.3 M citrate was accelerated by the addition of any of the rice BE isozymes- BEI, BEIIa, or BEIIb, whereas no such interaction occurred between SSIIa or SSIIIa with any of the BEs. The SSI-BE unprimed glucan synthesis absolutely required citrate. The interaction between SSI and BE was established by stimulation of SSI activity with BE and by activation of the BE activity by SSI.