The Golgi apparatus is an important relay station in the secretory pathway as it plays a pivotal role in targeting proteins and lipids to distinct post-Golgi compartments (1
). During transit through the Golgi apparatus, most of the newly synthesized secretory and membrane-bound proteins undergo major modifications, mainly involving different types of glycosylation. One of these, N
-glycosylation, is a complex series of reactions, starting in the endoplasmic reticulum (ER) with the formation of a Glc3
-PP-Dolichol structure, which is subsequently transferred onto newly synthesized proteins (2
). Upon exit from the ER and arrival in the Golgi, these unprocessed N
-glycan chains are trimmed and modified to give rise to distinct types of complex N
-glycan structures. Most of the other types of glycosylation, like the formation of O
- and C
-glycans, are restricted to either the ER or the Golgi apparatus. To maintain the directional flow of transport, essential for these modifications, the Golgi apparatus is built out of stacked cisternae, laterally interlinked via fenestrated continuities. The differential distribution of proteins and enzymes over these stacks provides the Golgi complex with a cis
). A tightly regulated organization of transport is required in order to mediate cargo transit as well as to maintain the cis
organization. The exact mechanism of this transit is still not clarified, though it will most likely be a combination of the vesicular transport model, which implicates fixed cisternae with vesicles transporting cargo forward and recycling escaped proteins to earlier cisternae or the ER (4
) and the cisternal maturation model. In the latter model, the cisternae mature towards the trans
-side while vesicles constantly recycle enzymes back to earlier compartments (8
). More recently a two-phase membrane system was proposed in which small soluble cargo distributes equally across the stack and membrane proteins prefer certain cisternae only based on, for instance, the local lipid composition and length of the transmembrane domain (10
). Irrespective of the different models and its specificity and directionality, intra-Golgi transport requires a large number of proteins and protein complexes. These include COPI-coat proteins and associated small GTPases, like ARF1 mediating budding/fission, as well as ‘Soluble NSF attachment protein receptor’ family members that govern vesicle fusion. The targeting specificity is often organized through the concerted action of GTP-bound Rab proteins with so-called tethering complexes, which are soluble multimeric complexes, coupling incoming organelles with the fusion site. One example is the exocyst complex that acts as a tethering complex at the cell surface of budding yeast and in mammalian cells. The main tethering complexes in the early secretory pathway are the ‘Transport Protein Particle’ (TRAPP) I and II complexes which couple ER-derived vesicles to the ER-Golgi intermediate compartment and the conserved oligomeric Golgi (COG) complex which appears to function in intra-Golgi and/or Golgi-to-ER trafficking.
The COG complex is an octameric hetero-oligomer conserved from yeast to mammals and presents as a bi-lobed structure bridged through the COG1–COG8 interaction (11
). Deficiencies in single subunits lead to impaired functioning of the COG complexes, concomitant defects in Golgi morphology (13
) and affecting the correct functioning of the cell or entire organism. For instance, a deficiency of COG1 or COG2 in CHO cell lines showed a defective retrograde Golgi-to-ER transport, a subsequent disturbance of the Golgi morphology and a destabilization of several proteins, including glycosylation enzymes, thus causing glycosylation abnormalities. The importance of COG function in intra-Golgi transport and its secondary implications on glycosylation are furthermore underscored by the discovery of mutations in the genes encoding the COG1, COG7 and COG8 subunits that were linked to congenital disorders of glycosylation (CDG) (12
). CDG is a rare, recessive disorder first described in 1980 (20
). Since then, 22 subtypes have been described, which can be divided into two different groups: CDG type I and type II. CDG type I (CDG-I) is caused by defects in enzymes governing the synthesis and transfer of the oligosaccharide in the ER. On the other hand, defects leading to CDG-II have been shown to belong to different classes: enzymes responsible for the modifications of the N
-glycan chain in the Golgi apparatus, sugar transporters in the Golgi membrane and complexes essential for intra-Golgi and retrograde Golgi-to-ER transport such as the COG complex.
Here we describe a patient harbouring a heterozygous point mutation in the COG4 gene combined with a deletion on the maternal allele. Experiments performed on this patient's fibroblasts yielded similar defects albeit less severe as found in the cells of the previously described COG-deficient patients. Moreover, we present an updated overview of the different COG mutations identified thus far in which we attempt to correlate for the first time the respective clinical phenotypes with the severity in glycosylation and trafficking defects as well as with the Golgi integrity using transmission electron microscopy (TEM). Our analysis underscores the high importance of an intact COG complex in both intra-Golgi trafficking and the maintenance of the normal morphology of the Golgi apparatus. Furthermore, we provide novel insights in the steady-state localization of both full and partial complexes with implications on the action mechanism of the complex. With this study, the number of patients harbouring mutations in individual COG genes rises to ten, which is about one-third of the total number of CDG-II cases in which a mutation has been identified making COG mutations one of the most frequent causes of CDG-II. Furthermore, given the insights that the different individual studies have generated on COG complex formation and functioning, we are now at a point where a comparison of all mutant subunits along different criteria reveals more specific or even as yet unknown functions of not only the full complex, but also of different subunits and subcomplexes.