# Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation
## 3 Membrane Trafficking Machinery
### Tethers (MTCs and Coiled-Coil Tethers)
#### 3.1.1 Coiled-Coil Tethers
Golgin-84 directly interacts with another Golgi tether, CASP, [86]and the COG7 subunit of the COG complex [87], and its siRNA-driven depletion in HeLa cells causes glycosylation defects [88]. Surprisingly, GOLGA5 is dispensable for mouse embryonic development and postnatal survival [89], indicating the redundancy in Golgi coiled-coil tethers. It was also demonstrated that Golgi-84 can directly capture intra-Golgi vesicles carrying glycosylation enzymes via its N-terminus [62]. GOLGA1 (Golgin-97): This protein was identified as an autoanti in patients with [PERSON]'s syndrome [90]. Golgi-97 is localized at the TGN where it tethers endosome-derived vesicles [91]. It associates with the centrosome [92], plays a role in the retrograde trafficking from endosomes to TGN [93] and its silencing leads to Golgi fragmentation [94]. This protein has garnered particular interest in the field of virology due to its requirement in poxvirus replication [95]. GOLGA4 (Golgin-245): Like Golgi-97, Golgi-245 too was identified as an autoanti in patients with [PERSON]'s syndrome. It is involved in the retrograde trafficking from endosomes to TGN and contributes to Golgi positioning around the centrosome. In a HeLa cell line, siRNA-mediated depletion of Golgi-245 resulted in TGN dispersal and fragmentation of the Golgi ribbon into mini stacks [96]. Displacement of Golgi-245 from the TGN or siRNA silencing disrupts the anterograde transport of cell surface destined cargo from the TGN [97; 98; 99]. In a more recent study, this protein has also been shown to function in phagophore formation during autophagy [100]. It was also demonstrated that Golgi-245 could directly capture endosomal-derived vesicles via its N-terminus [62]. GCC1 (GCC88): This GRIP-domain containing TGN localized Golgi is involved in endosome to Golgi retrograde transport [101] and maintains the tubular organization of the TGN [102; 103]. GCC88 KD in HeLa cells led to mislocalization of the Qc SNARE syntaxin 6 which prevented the recycling of TGN38 and CI-M6 PR from endosomes to the TGN [101]. Due to impaired CI-M6 PR recycling, lysosomal activity was perturbed and the lysosomal enzyme Cathepsin D's processing was impaired [103]. GCC2 (GCC185): This is another GRIP domain-containing protein targeted to the TGN and has the dual functions of tethering vesicle and cytoskeletal organization by providing a microtubule attachment site [104]. It binds the TGN Qa SNARE STX16 [105]. Rab9 localizes GCC185 at the endosomes while Rab6 and Ar11 are suggested to localize it at the TGN [106; 107; 108], but Rab6 KD does not mislocalize GCC185 from the Golgi [109]. It functions in the CI-M6 PR recycling from endosomes to TGN. KD in HeLa cells fragments the Golgi [106; 110]. Despite a fragmented Golgi in GCC185 KD cells, anterograde trafficking is unperturbed, but a subset of the retrograde cargo, Shiga toxin, for example, is blocked in endosomes [110]. TMF/ARA160: Tata element modulatory factor (TMF/ARA160) is a multifunctional Golgi tether that also functions as a transcriptional factor. Smg1, the yeast homologue, is recruited to the TGN by Ypt6-GTP and its deletion in yeast does not produce any trafficking defect [111; 112]. TMF KD causes a dispersal of the Golgi cisternae [112], blocks endosome to TGN retrograde trafficking and mislocalizes GalNAc-T2 [113]. It directs the proteasomal degradation of Stat3, a cell growth regulator and p65 [114; 115]. Interestingly TMF null male and female mice develop normally, but males are sterile with defects in the testis and epididymis. These mice produce acrosome-deficient-morphologically-defective immotile sperms, and have low testosterone levels [116; 117]. It interacts with the microtubules and plays a critical role in Golgi orientation during spermatogenesis [118]. More recently, it was also found to be responsible for the transport of vesicles containing GLUT4, a receptor that drives glucose uptake in an insulin-dependent manner [119].