In glycolysis, these steps include the phosphorylation of glucose by hexokinase, the phosphorylation of fructose-6-phosphate by phosphofructokinase, and the synthesis of pyruvate and ATP from phosphenolpyruvate by pyruvate kinase. Regulation of both pathways occurs at three steps catalyzed by different reciprocally acting enzymes.
Its consumption via glycolysis and its regeneration via gluconeogenesis are central pathways of carbohydrate metabolism in many organisms. Glucose also has a pivotal role in cellular regulation. Glucose is the main carbon source of many cells, providing energy and building blocks for a variety of essential cellular components. Our study uncovers a new type of ubiquitin ligase complex composed of novel subunits involved in carbohydrate metabolism and identifies Gid4/Vid24 as a major regulator of this E3. We also show that an additional gluconeogenic enzyme, phosphoenolpyruvate carboxykinase, is subject to Gid complex-dependent degradation. This suggests that Gid4/Vid24 initiates fructose-1,6-bisphosphatase polyubiquitination by the Gid complex and its subsequent elimination by the proteasome. Forcing abnormal expression of Gid4/Vid24 in gluconeogenic cells leads to fructose-1,6-bisphosphatase degradation. A seventh protein, Gid4/Vid24, occurs upon glucose addition to gluconeogenic cells and is afterwards eliminated. Six Gid proteins are present in gluconeogenic cells. In addition, we show that a mutation in the degenerated RING domain of Gid2/Rmd5 abolishes fructose-1,6-bisphosphatase polyubiquitination and elimination in vivo. In an in vitro assay, heterologous expression of GST-Gid2 leads to polyubiquitination of proteins. One of the subunits, Gid2/Rmd5, contains a degenerated RING finger domain.
Seven glucose induced degradation deficient (Gid)-proteins found previously in a genomic screen were shown to form a complex that binds FBPase. We have previously shown that the switch from gluconeogenesis to glycolysis is associated with ubiquitin-proteasome linked elimination of the key enzyme fructose-1,6-bisphosphatase. Glucose-dependent regulation of carbon metabolism is a subject of intensive studies.
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