![]() ![]() Gid1, Gid4, and Gid5 had previously been identified as Vid30, Vid24, and Vid28, respectively. Among those genes, GID3 and GID6 were shown to encode the ubiquitin-conjugating enzyme Ubc8 and the deubiquitinating enzyme Ubp14 ( Schüle et al., 2000 Regelmann et al., 2003). In addition to the previously identified GID1/VID30, GID2/RMD5, and GID3 genes, a genome-wide screen for FBPase stabilization upon glucose shift allowed the discovery of six further so-called glucose induced degradation deficient ( GID) genes termed GID4 to GID9 ( Regelmann et al., 2003). Degradation of FBPase blocks gluconeogenesis and prevents an otherwise ongoing futile cycle of ATP hydrolysis ( Schork et al., 1994a, b, 1995 Wolf, 2004). Similar mechanisms are described for PEPCK, cytosolic malate dehydrogenase, and isocitrate lyase ( Holzer, 1976 Hämmerle et al., 1998). Subsequently, FBPase undergoes rapid degradation ( Holzer, 1989). Catabolite inactivation encompasses repression of the FBP1 gene, decrease of the enzyme activity upon phosphorylation, and allosteric inhibition by fructose-2,6-bisphosphate and AMP ( Mazon et al., 1982 von Herrath and Holzer, 1988). After glucose addition, catabolite inactivation and degradation of FBPase occur ( Gancedo, 1971 Holzer, 1976 Marcus et al., 1988 Hämmerle et al., 1998 Wolf, 2004). FBPase is expressed when yeast cells are growing in media without a fermentable carbon source (growth on ethanol or acetate). These events include gene expression and repression, alterations in the stability of certain mRNAs, and posttranslational modification of many gene products ( Gancedo, 1998 Vaulont et al., 2000). In the yeast Saccharomyces cerevisiae, major regulation events occur when cells previously grown in a glucose-deprived medium are supplied with this sugar. Dysregulation of these antagonistic pathways in humans leads to type 2 diabetes ( Wahren and Ekberg, 2007). In gluconeogenesis, these steps are circumvented by pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK), fructose-1,6-bisphosphatase (FBPase), and glucose-6-phosphatase. 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. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |