# Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation
## 3 Membrane Trafficking Machinery
### Golgi-Associated Retrograde Protein Complex (GARP)  
The GARP complex is another Golgi-localized multisubunit vesicle tethering complex of the CATCHR family [150; 151; 152]. Its subunits, VPS52, VPS53, and VPS54, were first identified in a Saccharomyces cerevisiae genetic screen which was designed to find new vacuolar protein sorting (VPS) genes essential in Carboxypeptidase Y (CPY) sorting [153]. These novel trimeric protein complexes were initially named as VPS fifty-three (VFT) [111]. Later, a fourth subunit, VPS51, was discovered in another genetic screen, a mutation of which has a similar defect in CPY sorting [154; 155]. A co-immunoprecipitation experiment demonstrated that VPS51 belongs to the same complex as VFT [154]. Hence, this complex is composed of four different subunits named VPS51, VPS52, VPS53, and VPS54, and each of these four subunits forms an obligatory 1:1:1:1 complex [156; 157]. All of these GARP subunits require the coiled-coil motif for assembly into the complex [154; 155; 158]. The GARP complex is evolutionary conserved, with homologues discovered in worms, flies, mammals, and plants [150; 159; 160; 161]. GARP is localized at the TGN. In human cells, the small GTPases ARL5 and ARFRP1 localized at the TGN are responsible for the recruitment of GARP [162]. The GARP complex shares VPS51, VPS52, and VPS53 with another tethering complex-endosome associated recycling protein (EARP) whereas VPS50 is a unique subunit of EARP and VPS54 is a unique subunit of GARP [163]. The VPS54 subunit of the GARP complex has its N-terminal domain crucial in assembly and stability of the GARP complex, whereas the C-terminal domain mediates the localization to an early endocytic compartment [164]. GARP functions in the recycling receptors for lysosomal hydrolase precursors, transmembrane proteins, and SNAREs to the Golgi [157; 165; 166]. Depletion of the GARP subunit blocked the retrograde trafficking of the lysosomal hydrolase receptor Mannose-6-phosphate receptor, TGN protein TGN46, and the B subunit of the Shiga toxin. As a consequence of the blockade in retrograde trafficking, there is missorting of the precursor of the acid hydrolase Cathepsin D [166]. GARP is also essential for the maintenance of cholesterol and sphingolipid homeostasis, disruption of which can lead to neurodegenerative diseases known as \"progressive cerebello-cereb atrophy type 2\" and \"Hereditary spastic paraparesis\" [167; 168; 169; 170]. Auxin-induced acute depletion of yeast GARP subunits resulted in the missorting of plasma membrane proteins of two different functional groups, amino-phospholipid flippases and cell wall biosynthesis proteins [171]. Exome sequencing of a 6-year-old Caucasian female identified compound heterozygous mutations (c.1468C \(>\) T and c.2232 delC) in the gene encoding the VPS51 resulting in a neurodevelopmental disorder [172]. The VPS52 subunit of GARP has shown to interact with [PERSON]'s disease kinase LRRK2 (Leucine-rich repeat kinase 2) and contributes to intracellular trafficking by stabilizing the GARP-SNAREs complex formation [173]. Depletion of GARP exacerbates the neurodegeneration in C. elegans [173] (Table 1). In mice, a null mutation in the VPS54 subunit of the GARP complex causes embryonic lethality [174; 175]. The \"wobbler mouse\" phenotype is caused by a spontaneous missense mutation L967Q near the C terminus of VPS54. The \"wobbler mutations\" characteristics, such as cellular transport abnormalities, progressive motor neuron degeneration, and neuroinflammation, are quite similar to human Amyotrophic lateral sclerosis, although there are no known GARP genetic abnormalities associated with human ALS patients. The \"wobbler mutation\" causes the VPS54 protein, as well as the whole GARP complex, to become unstable and impairs retrograde vesicular trafficking. This affects spermiogenesis and also leads to spinal muscular atrophy [176; 177; 178; 179]  
GARP is structurally and functionally similar to other multisubunit tethering complexes such as COG, Ds1, and Exocyst [181].