COG Complex‐Mediated Recycling of Golgi Glycosyltransferases is Essential for Normal Protein Glycosylation

Abstract

Defects in conserved oligomeric Golgi (COG) complex result in multiple deficiencies in protein glycosylation. On the other hand, acute knock-down (KD) of Cog3p (COG3 KD) causes accumulation of intra-Golgi COG complex-dependent (CCD) vesicles. Here, we analyzed cellular phenotypes at different stages of COG3 KD to uncover the molecular link between COG function and glycosylation disorders. For the first time, we demonstrated that medial-Golgi enzymes are transiently relocated into CCD vesicles in COG3 KD cells. As a result, Golgi modifications of both plasma membrane (CD44) and lysosomal (Lamp2) glycoproteins are distorted. Localization of these proteins is not altered, indicating that the COG complex is not required for anterograde trafficking and accurate sorting. COG7 KD and double COG3/COG7 KD caused similar defects with respect to both Golgi traffic and glycosylation, suggesting that the entire COG complex orchestrates recycling of medialGolgi-resident proteins. COG complex-dependent docking of isolated CCD vesicles was reconstituted in vitro, supporting their role as functional trafficking intermediates. Altogether, the data suggest that constantly cycling medial-Golgi enzymes are transported from distal compartments in CCD vesicles. Dysfunction of COG complex leads to separation of glycosyltransferases from anterograde cargo molecules passing along secretory pathway, thus affecting normal protein glycosylation.

Plasma N-Glycan Profiling by Mass Spectrometry for Congenital Disorders of Glycosylation Type II

“…The fourth COG7-CDG patient showed a typical percentage (1.2%). The increased occurrence of oligomannosidic glycans has been explained by mislocalization of the 1,2-Nacetylglucosaminyltransferase 1 and mannosidase II enzymes in the medial Golgi (32 ). Mutations in the a2 subunit of the vacuolar H ϩ -ATPase (ATP6V0A2) (33,34 ) cause impaired Golgi trafficking via altering the intracellular pH gradient and produce glycosylation abnormalities at several levels.…”

Rab33b and Rab6 are Functionally Overlapping Regulators of Golgi Homeostasis and Trafficking

“…HeLa cells were lysed in 2% SDS, followed by standard SDS-PAGE (∼12% acrylamide) and western blotting as described previously (51). Antibodies and dilutions used for immunoblotting were anti-ZW10 (affinity purified, 1:100), anti-Rab6 (1:1000), anti-COG3 (1:1000), anti-Rab33b (D5,1:500) and anti-α-tubulin (1:2000).…”

Mutations inCog7affect Golgi structure, meiotic cytokinesis and sperm development duringDrosophilaspermatogenesis

“…Mutations in the genes encoding human COG1, COG4-COG8 have been associated with congenital disorders of glycosylation (CDG) (Foulquier et al, 2006Foulquier et al, 2007Kranz et al, 2007;Lübbehusen et al, 2010;Ng et al, 2007;Paesold-Burda et al, 2009;Reynders et al, 2009;Spaapen et al, 2005;Steet and Kornfeld, 2006;Wu et al, 2004) indicating a role for COG in the transport and/or stability of Golgi glycosylation enzymes. Indeed studies in both yeast and mammalian cells have suggested that COG complex might function as a vesicle-tethering factor in intra-Golgi retrograde COPI transport (Ungar et al, 2002), thus regulating the distribution and the stability of Golgi resident proteins (Oka et al, 2004;Shestakova et al, 2006;Suvorova et al, 2001;Suvorova et al, 2002;Walter et al, 1998). A set of Golgi proteins called GEARs including giantin matrix proteins and glycosyltransferases/ glycosidases were shown to mislocalize and to be abnormally degraded in Chinese hamster ovary (CHO) cells mutant for either Cog1 or Cog2 subunits (Oka et al, 2004).…”