# Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation
## 3 Membrane Trafficking Machinery
### Golgi-Associated Retrograde Protein Complex (GARP)
#### 3.1.3 SNAREs  
SNAREs (SNAP Receptors) complete the final step of docking and fusion of the carrier membrane at its target (Figure 3). There are a total of 36 known SNAREs in humans that are present throughout the secretory pathway and confer a degree of compartment identity. Upon vesicle docking at its target membrane, usually, a total of three or four SNAREs (bearing 4 SNARE motifs) on opposing membranes align, intertwine and form a heterologomeric, parallel \(\alpha\) helical complex. The SNARE motif-a distinctive feature of all SNAREs-is a coiled structure composed of about 60-70 amino acid residues. At the center of the SNARE motif has a glutamate or arginine residue, leading to the categorization of SNAREs as Q and R SNAREs [183]. Q SNAREs are further subcategorized into Qa, Qb and Qc based on protein profiling [184]. Typically, each Qa-, Qb-, Qc- and R-SNARE providesone SNARE motif to the SNARE complex. As the name suggests, Qbc SNAREs possess a Qb and Qc SNARE motif. These SNAREs contribute two SNARE motifs to the four-helix SNARE complex. SNARE motifs align from the N to C terminus and form an extremely stable parallel \(\alpha\) helical complex [185]. Aligned SNAREs on opposing membranes are termed _trans_-SNARE complexes. The energy released upon the formation of the _trans_-SNARE complex drives membrane fusion. After the initiation of membrane fusion, the SNARE complex then resides on the same membrane and is termed a _cis_-SNARE complex. This transition of the SNARE complex from _trans_ to _cis_ is thought to play an important role in the creation and enlargement of the fusion pore [186]. SNARE complexes have to be disassembled to participate in the next round of fusion. This is an enzymatic and energy-intensive process that uses the energy of ATP hydrolysis. The enzyme NEM sensitive factor (NSF), named after it, was found to be inactivated by N-ethyl maleimide (NEM), is an AAA + ATPase. NSF is recruited to SNARE complexes by the Soluble NSF Attachment Protein (SNAP). SNAP has three isoforms - \(\alpha\), \(\beta\)- and \(\gamma\)- SNAP which are ubiquitously expressed in all tissues except \(\beta\)-SNAP which is brain-specific. SNAP is believed to bind to the trans-SNARE complex. From there on, the NSF binds SNAP. The free energy released upon SNARE dissociation is thought to contribute to driving membrane fusion. There are three major SNARE complexes operating in the Golgi (Figure 3). The STX5/GOSR2/Bet1/Sec22b complex mostly operates at the ERGIC compartment in mammalian cells [187]. The STX5/GOSR1/Bet1L/Ykt6 complex operates in retrograde trafficking at the Golgi [188] and the STX16/STX6/Vti1a/Vamp4 complex operates between the endosomal compartment and the _trans_ side of the Golgi [189]. An additional Golgi-localized SNARE complex could be formed by STX5, SNAP29 and YKT6 [190; 191]. In the STX5/GOSR2/ Bet1/Sec22b complex, GOSR2 mutations have been identified in the human population. GOSR2 (GS27/membrin) allelic variants in the human population have been associated with cardiovascular diseases and myocardial infarction [192; 193; 194]. A loss of function mutation (c.430G > T, p.Gly144 Trp) in GOSR2 was reported to cause progressive myoclonus epilepsy with early ataxia [195], and heterozygous c.430G > T and  
Figure 3: Schematic representation of the SNARE complexes participating at the Golgi. The STX5/GOSR1/BET1L/YKT6 (green) Golgi SNARE complex operates throughout the Golgi. STX5/SNAP29/YKT6 is another SNARE complex. STX5/GOSR2/BET1/Sec22b operates between the ER/ERGIC and Golgi. The STX16/Vti1a/STX6/VAMP3 or VAMP4 (purple) operates at the TGN. c.82C > T is associated with muscular dystrophy [196]. The effects of GOSR2 mutations on the muscular system point to its essential role in development (Table 1). Recently a CDG causing mutation, c.163 A > G, was identified in the Qa SNARE STX5 [197]. STX5 has two isoforms, long (STX5L) and short (STXSS), encoded by different start codons. The long isoform has an ER-retrieval sequence, while STX5S is Golgi localized.