In humans, the majority of ingested glucose is stored as glycogen in either the liver or skeletal muscle. Glycogen accumulation therefore plays a central role in the maintenance of blood glucose homeostasis. Despite much effort, some basic questions regarding the regulation of glycogen storage remain. The proposed project uses the yeast, Saccharomyces cerevisiae, as a model system in which to address questions relating to the control of glycogen accumulation. The enzymes of glycogen metabolism are highly conserved between yeast and mammals. In yeast, as in higher eukaryotes, the main control point in glycogen synthesis is at the level of glycogen synthase, which catalyzes bulk glycogen synthesis. There are two glycogen synthases in yeast, Gsy1p and Gsy2p, with Gsy2p accounting for approximately 90% of total enzyme activity. A number of different signal transduction pathways have been shown to lead to phosphorylation and inactivation of glycogen synthase. Our long-term goals are to determine precisely how each of these pathways act to regulate glycogen synthesis and to arrive at an overall picture of the control of glycogen storage. This proposal moves us towards our goal by addressing two recent findings. We have established that (i) the subcellular localization of Gsy2p is regulated by phosphorylation and (ii) despite contributing only 10% of total glycogen synthase activity Gsy1p can play an important role in determining glycogen levels.
The specific aims to be pursued are: (i) To test the hypothesis that the subcellular localization of glycogen synthase is an important determinant of glycogen storage. We propose that translocation of glycogen synthase from the bulk cytoplasm to discrete structures, visible with the light microscope, represents a novel means of control of enzyme activity. (ii) To test the hypothesis that the two isoforms of yeast glycogen synthase are not functionally interchangeable. We propose that the regulatory properties of Gsy1p and Gsy2p differ. Native glycogen synthase is oligomeric and mixed oligomers of Gsy1p and Gsy2p can form. We propose that one function of Gsy1p is to regulate Gsy2p. We expect that some of our results may be directly applicable to our understanding of the control of mammalian glycogen synthase whilst others may lead to enhanced understanding of signal transduction processes in general. The proposed work will also expose undergraduate students to basic research. Classical genetic approaches and the most modern techniques of molecular biology are readily implemented in the yeast system, providing excellent learning opportunities. Public Health Relevance: Glycogen is a storage form of glucose and the synthesis of glycogen plays an important role in controlling the concentration of glucose in the blood. The proposed study uses a model organism, baker's yeast, to define mechanisms responsible for the regulation of glycogen accumulation. The enzymes responsible for glycogen synthesis are highly conserved between yeast and mammals, implying that our findings will have relevance to human physiology. ? ? ?
