Activity And Localization Of Maltodextrin Binding Site Mutants Of Glycogen Synthase In Saccharomyces Cerevisiae
Slade, Stephen J.
Davis, Keri D.
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Glycogen is a glucose polymer formed by the enzyme glycogen synthase and is used in many organisms to store chemical energy. Saccharomyces cerevisiae (baker’s yeast) was used to study the activity and localization of glycogen synthase. Genes GSY1 and GSY2 encode glycogen synthase. GSY2 is responsible for the formation of Gsy2p, whose action accounts for ~90% of glycogen synthase activity; the remainder of total glycogen synthase activity stems from Gsy1p. Because glycogen synthase binds to glycogen, it can be used to determine glycogen localization. Glycogen synthase can appear in distinct patterns throughout the cell. Gsy2p has been shown to be regulated by phosphorylation. Phosphorylation of Gsy2p leads to inactivation of the enzyme, a decrease in glycogen storage, and a more localized pattern of glycogen synthase. Conversely, lowering the phosphorylation state of Gsy2p results in increased glycogen production and delocalization of glycogen synthase throughout the cell. Glucose-6-P (glucose-6-phosphate) activates glycogen synthase regardless of its phosphorylation state. We obtained a set of plasmids from a collaborator, encoding Gsy2p mutated at sites believed to be involved with maltodextrin binding. Maltodextrin is a chain of 20 or fewer dextrose molecules with α (1→4) glycosidic bonds. A protein sequence involved in maltodextrin binding likely would also bind to glycogen. Our task was to discover the localization pattern shown by the maltodextrin binding site mutants of glycogen synthase using a GFP tag on GSY2. The goal of this study was to determine the 16 effects of Gsy2p maltodextrin binding mutants on glycogen synthase activity, localization, and glycogen accumulation.
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