# 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
The c. 163 A > G (p.M55V) is a missense mutation at the start codon for the STX5S, leading to complete loss of STX5S protein. This mutation led to spontaneous abortion or death shortly after birth due to liver failure. Clinical presentations of this CDG include skeletal deformities, dysmorphia hypotonia, hepatomegaly, and elevated cholesterol. IEF of serum transferrin and ApoEIII together indicated N- and O- glycosylation defects which were confirmed by lectin binding analysis on patients' fibroblasts. Despite the essential role of SNAREs in Golgi trafficking, no other CDG has been reported so far. Embryonic lethality could result in mutations going undetected; however, due to redundancy and some degree of promiscuity among SNAREs, studies using KD or KO approaches in cell lines and other model organisms have yielded only mild phenotypes. Disruption of the Qb and Qc STX5 partners GOSR1 (GS28) and BET1L (GS15), only produce mild phenotypes. Overexpression of a BET1L lacking transmembrane domain caused Golgi disruption and mislocalization of the glycosylation enzyme MANN2 [188]. YKT6, the R SNARE in the Golgi STX5 SNARE complex, lacks a transmembrane domain and is integrated into the Golgi membrane via a lipid anchor, and its deletion is lethal in yeast [198]. Ykt6 has an essential role in Golgi organization which is disrupted when Ykt6 cannot localize to the Golgi [199]. SNAP29 is a Qbc SNARE that is reported to be membrane localized and has a cytosolic pool as well [200]. The Golgi is one of the membranes that SNAP29 is present on [190]. Among various other SNARE partners [201], it also partners promiscuously with the Golgi SNAREs STX5, STX6 [190]) and YKT6 [191], as well as the COG complex subunit, COG6 [202]. It has roles in synaptic transmission, endocytosis, cell cycle regulation and autophagy [203]. In humans, mutations in SNAP29 give rise to CEDNIK (cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma) syndrome or the neuromuscular disorder PMLD (Pelizaeus-Merzbacher-like disorder) [204,205]. Zebrafish carrying CEDNIK syndrome-causing mutations have multi-systemic developmental defects [206] and the mutant male mice carrying SNAP29 mutations, in addition to phenocopying the human mutant phenotypes, are also sterile [207]. SNAP29 deletion in Drosophila disrupts the Golgi organization [208] and controlled RNAi mediated silencing leads to lethality at high temperatures [209], but in C. elegans, it is the endosomal system that is disrupted and not the Golgi, upon SNAP29 KD [210]. Glycoslation defects in SNAP29 mutants have not yet been investigated. At the TGN, KD of any of the STX16 SNARE complex subunits alters endosome to TGN transport. In humans, a 4.4-kb microdeletion in the STX16 locus leads to autosomal dominant pseudohypoparathyroidism Type Ib [211]. KD of any one of the four STX16 SNARE partners at the trans-Golgi leads to Golgi fragmentation [212] yet STX16 mice are viable [213]. Both Vti1a KO and Vti1b KO mice are viable, with the latter presenting with progressive neurodegeneration [214].