Bridging the Gap between Glycosylation and Vesicle Traffic

Abstract

Glycosylation is recognized as a vitally important posttranslational modification. The structure of glycans that decorate proteins and lipids is largely dictated by biosynthetic reactions occurring in the Golgi apparatus. This biosynthesis relies on the relative distribution of glycosyltransferases and glycosidases, which is maintained by retrograde vesicle traffic between Golgi cisternae. Tethering of vesicles at the Golgi apparatus prior to fusion is regulated by Rab GTPases, coiled-coil tethers termed golgins and the multisubunit tethering complex known as the conserved oligomeric Golgi (COG) complex. In this review we discuss the mechanisms involved in vesicle tethering at the Golgi apparatus and highlight the importance of tethering in the context of glycan biosynthesis and a set of diseases known as congenital disorders of glycosylation.

Keywords: COG complex; Golgi apparatus; congenital disorders of glycosylation; glycan processing; vesicle tethering.

Figure 1

Different stages of N-glycan complexity. N-linked glycans are classified into oligomannose, hybrid and complex type glycans based on their structures. Hybrid and in particular complex N-glycans can contain more branches than shown by the addition of extra GlcNAc residues onto the mannoses already functionalized this way. In addition, a GlcNAc can also be added to the mannose linked to the chitobiose core. This can result in upto five branches on complex glycans. Every N-glycan consists of a core built up of a chitobiose core [two N-acetyl glucosamines (GlcNAc)] that links to an asparagine sidechain in an Asn-X-Ser/Thr sequence, followed by three mannoses that initiate two separate branches. The antennae of N-glycans then consist of either all mannose residues (oligomannose glycans, left structure), a combination of GlcNAc, galactose, sialic acid and fucose residues (complex glycans, right structure) or a mixture of these with one branch being complex, the other oligomannose (middle structure).

Figure 2

Glycan processing and complexity. N-linked glycans are processed from oligomannose to complex as they traverse the Golgi apparatus. The enzymatic reactions needed for processing of glycans from oligomannose to hybrid and complex are compartmentalized into Golgi cisternae. Mannose trimming enzymes are in the cis and medial Golgi, GlcNAc addition and associated branching in the medial, while capping with galactose and sialic acid in the trans. The differential distribution of these enzymes is maintained through vesicular sorting, with COPI-coated vesicles moving them in the retrograde direction.

Figure 3

Intra-Golgi retrograde vesicle tethering and targeting. Vesicles carrying glycosylation enzymes are targeted to the correct cisternae by different combinations of interactions between trafficking proteins, including Rabs, golgins, and the COG complex. This allows for a complex targeting system leading to the maintenance of enzyme localization within the Golgi, which is pertinent to glycan processing. Golgins grab vesicles at a long distance from the cisternal membranes. This tethering and the movement of the vesicle to the cisternal membrane at the base of the golgin are assisted by Rabs and COG. Possible mechanisms for these are depicted and explained in the text.

Figure 4

Interactions of the COG complex. The COG complex forms a bi-lobed structure with subunits Cog1-4 making up lobe A and lobe B consisting of Cog5-8. Each subunit (except Cog1) has been demonstrated to interact with numerous proteins involved in Golgi trafficking and tethering such as golgins, SNAREs and Rabs. ^*Indicates an interaction with the COG complex as a whole.

Figure 5

Trafficking defects at the Golgi apparatus. Normal Golgi trafficking, as predicted by the cisternal maturation model, in which COPI-vesicles transport enzymes in the retrograde direction to sort them to their respective cisternal destinations. Different forms of trafficking defects can manifest themselves in a number of ways. COG-CDGs and golgin mutations are likely to lead to the mislocalization of a specific subset of cargo containing vesicles depending on the COG subunit or golgin that is affected (left side). In this case some enzymes, for example sialyltransferases, could be lost from cisternae. More global glycosylation defects, which would likely be embryonically lethal but can be observed in tissue culture, may be the result of an unstable COG complex as a whole, for example due to deletion of a full subunit (right side). This would lead to the loss of several enzymes, but could also broaden the distribution of some enzymes, such as mannosidase I.