Multiprotein complexes are key determinants of Golgi apparatus structure and its capacity for intracellular transport and glycoprotein modification. Three complexes that have previously been partially characterized include (a) the Golgi transport complex (GTC), identified in an in vitro membrane transport assay, (b) the ldlCp complex, identified in analyses of CHO cell mutants with defects in Golgi-associated glycosylation reactions, and (c) the mammalian Sec34 complex, identified by homology to yeast Sec34p, implicated in vesicular transport. We show that these three complexes are identical and rename them the conserved oligomeric Golgi (COG) complex. The COG complex comprises four previously characterized proteins (Cog1/ldlBp, Cog2/ldlCp, Cog3/Sec34, and Cog5/GTC-90), three homologues of yeast Sec34/35 complex subunits (Cog4, -6, and -8), and a previously unidentified Golgi-associated protein (Cog7). EM of ldlB and ldlC mutants established that COG is required for normal Golgi morphology. “Deep etch” EM of purified COG revealed an ∼37-nm-long structure comprised of two similarly sized globular domains connected by smaller extensions. Consideration of biochemical and genetic data for mammalian COG and its yeast homologue suggests a model for the subunit distribution within this complex, which plays critical roles in Golgi structure and function.

▪ Abstract The SNARE superfamily has become, since its discovery approximately a decade ago, the most intensively studied element of the protein machinery involved in intracellular trafficking. Intracellular membrane fusion in eukaryotes requires SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor) proteins that form complexes bridging the two membranes. Although common themes have emerged from structural and functional studies of SNAREs and other components of the eukaryotic membrane fusion machinery, there is still much to learn about how the assembly and activity of this machinery is choreographed in living cells.

The conserved oligomeric Golgi (COG) complex is a soluble hetero-octamer associated with the cytoplasmic surface of the Golgi. Mammalian somatic cell mutants lacking the Cog1 (ldlB) or Cog2 (ldlC) subunits exhibit pleiotropic defects in Golgi-associated glycoprotein and glycolipid processing that suggest COG is involved in the localization, transport, and/or function of multiple Golgi processing proteins. We have identified a set of COG-sensitive, integral membrane Golgi proteins called GEARs (mannosidase II, GOS-28, GS15, GPP130, CASP, giantin, and golgin-84) whose abundances were reduced in the mutant cells and, in some cases, increased in COG-overexpressing cells. In the mutants, some GEARs were abnormally localized in the endoplasmic reticulum and were degraded by proteasomes. The distributions of the GEARs were altered by small interfering RNA depletion of ϵ-COP in wild-type cells under conditions in which COG-insensitive proteins were unaffected. Furthermore, synthetic phenotypes arose in mutants deficient in both ϵ-COP and either Cog1 or Cog2. COG and COPI may work in concert to ensure the proper retention or retrieval of a subset of proteins in the Golgi, and COG helps prevent the endoplasmic reticulum accumulation and degradation of some GEARs.

We describe a new Type II congenital disorder of glycosylation (CDG-II) caused by mutations in the conserved oligomeric Golgi (COG) complex gene, COG8. The patient has severe psychomotor retardation, seizures, failure to thrive and intolerance to wheat and dairy products. Analysis of serum transferrin and total serum N-glycans showed normal addition of one sialic acid, but severe deficiency in subsequent sialylation of mostly normal N-glycans. Patient fibroblasts were deficient in sialylation of both N- and O-glycans, and also showed slower brefeldin A (BFA)-induced disruption of the Golgi matrix, reminiscent of COG7-deficient cells. Patient fibroblasts completely lacked COG8 protein and had reduced levels and/or mislocalization of several other COG proteins. The patient had two COG8 mutations which severely truncated the protein and destabilized the COG complex. The first, IVS3 + 1G > A, altered the conserved splicing site of intron 3, and the second deleted two nucleotides (1687–1688 del TT) in exon 5, truncating the last 47 amino acids. Lentiviral-mediated complementation with normal COG8 corrected mislocalization of other COG proteins, normalized sialylation and restored normal BFA-induced Golgi disruption. We propose to call this new disorder CDG-IIh or CDG-II/COG8.