# Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation
## 2 Glycoslation Enzyme Compartmentalization  
Apart from the glycosylation enzymes, the Golgi glycosylation machinery also includes ion channels and sugar transporters that maintain intraluminal pH and bring in cofactors and charged monosaccharides required for glycosylation. The key quality control for glycosylation is the proper cisternal compartmentalization of the glycosylation machinery [9; 18; 19; 20]. Glycan processing/addition occurs in the sequence in which the enzymes encounter their substrates. As the _cis_-Golgi matures, eventually becoming the TGN and carrying cargo forward, the correct compartmentalization of the glycosylation machinery must be maintained within the framework of cisternal maturation. At the TGN, the cargo is sorted into appropriate anterograde carriers targeted to various destinations. All Golgi glycosylation enzymes are transmembrane (TM) proteins and the majority of them are type II TM proteins. They possess a catalytic C-terminal domain in the lumen, a luminal stem region, a transmembrane domain (TMD) and a short N-terminal cytosolic tail. Evidence suggests that the cytoplasmic tail and TMD of Golgi enzymes, cytosolic adaptor and coat proteins, and membrane lipid composition play a role in the sorting and correct cisternal localization of the glycosylation machinery. The current models for sorting and retention at the Golgi have been described by [PERSON] et al. in [21; 22] and more recently by [PERSON] et al. in [23]. Here we summarize the retention and recycling mechanisms that have been described. \"Kin recognition\" was the earliest proposed model for enzyme retention in the Golgi [24]. Kin oligomerization was first identified in the medial Golgi enzymes N-Acetylglucosaminyltransferase-1 (GlcNAcCT1/MGAT1) and Mannosidase-2 (ManII/MAN2A1) [25]. These enzymes interact with each other via their TMDs and stem regions. Because enzyme oligomers would be too big to enter transport vesicles exiting the Golgi, they are retained in the Golgi. The enzyme \(\alpha\)-2,6-sialyltransferase-1 (ST6 Gal1) is believed to possess trans Golgi localization signals in its cytoplasmic tail, TMD and stem region because mutations in these regions lead to the secretion of the enzyme [26; 27]. ST6 Gal1 not only forms homodimers but also heteroligomeries with \(\beta\)-1,4-Galactosyltransferase-1 (B4 GALT1) [28]. All the currently known oligomers of the known Golgi enzymes in various glycosylation pathways have been described by [PERSON] et al. in [29]. Transmembrane domain-dependent sorting relies on compatibility between the TMD and membrane thickness [30]. Membrane thickness is believed to gradually increase from the ER to the PM due to different lipid compositions. Phospholipids are enriched in ER and Golgi membranes, while the enrichment of sphingolipids and sterols gradually increases in the TGN and post-Golgi compartments with the highest abundance in the PM. An extensive and detailed comparison of the TM proteins found that ER and Golgi residents have shorter TMDs compared to post-Golgi residents and bulkier residues were distributed closer to the exoplasmic side of the bilayer TMDs [31]. Simply stated, shorter TMDs reside in thinner membranes. Thus, Golgi enzymes are kept from entering the post-Golgi compartments by the mere incompatibility of their short TMDs in thicker membranes. Golgi enzymes can cycle between the Golgi cisternae [32; 33] as well as between the Golgi and ER [34], and the majority of recycling occurs in COPI-coated vesicles. Yeast \(\alpha\)-1,6-mannosyltransferase (Mnn9p) is one such example [35; 36]. This _cis_-Golgi enzyme is retrieved in COPI vesicles at the TGN [35]. A few other glycosylation enzyme complexes such as the Man-Pol-I (Mann9p and Van1p) and Man-Pol-II (Mnn9p, Hoc1p, Amp1, Mnn10p, and Mnn11p), in yeast, cycle through the ER in COPI vesicles. Selective sorting into retrograde COPI-coated carriers either occurs via direct interaction between the cytosolic tails or via an adaptor.