The Conserved Oligomeric Complex (COG) complex is a retrograde tethering factor within the Golgi apparatus essential for proper localization of specific glycosylation enzymes. The COG complex is a peripheral membrane complex composed of eight proteins (Cogs1-8) arranged into two lobes: lobe A (Cogs1-4) and lobe B (Cogs5-8). Mutations or knockdowns of COG subunits result in the mislocalization or down-regulation of glycosylation enzymes, accumulation of vesicles, and disruption of specific Golgi SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex formation.
Using quantitative immunoprecipitation and confocal fluorescent microscopy, the membrane-associated COG complex is shown to be composed of at least two subcomplexes. The "lobe A subcomplex" is preferentially localized on the larger Golgi membranes, while the "lobe B subcomplex" is localized on both Golgi membranes and on smaller peripheral vesicles. We propose that both subcomplexes must join together to form a whole complex for the tethering function to occur.
A knockdown of any of the COG subunits specifically delays Brefeldin A-induced relocalization of medial- and trans-Golgi enzymes to the ER. These data are further complemented by a similar delay observed in a Sar1-T39N-induced relocalization of Golgi enzymes in COG knockdown cells. Furthermore, all COG knockdown cells demonstrate hypogalactosylation and increases in the number of terminal mannoses on the plasma membrane-localized glycoproteins and glycolipids observed via lectin-staining. MALDI-TOF of glycoproteins in COG depleted cells also demonstrates hyposialylation and hypogalactosylation and a decrease in the number of complex N-linked glycans.
We proposed that COG complex is regulating not only intra-Golgi trafficking of glycosyltransferases but also trafficking of other endogenous and exogenous proteins that are moving through the Golgi. We have tested this hypothesis by using a novel exogenous cytotoxin (Subtilase cytotoxin) known to traffic retrogradely from the plasma membrane to the ER. We demonstrated that any knockdown in the COG complex as well as knockdowns for Rab6 and COPI proteins (β-COP) inhibit the toxicity of Subtilase cytotoxin by blocking the intra-Golgi delivery of this protein.
We concluded that the COG complex is an essential component of trafficking machinery that operates for the retrograde traffic of both exogenous and endogenous proteins in the Golgi.
|School:||University of Arkansas for Medical Sciences|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 71/06, Dissertation Abstracts International|
|Keywords:||Brefeldin A, COG complex, Conserved oligomeric Golgi, Glycosylation, Golgi apparatus, Subtilase cytotoxin|
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