## 4 GOLPH3 Interacts with COPI to Recruit Glycostransferases to Golgi Compartments
After releasing of the COPI vesicle from the Golgi membrane, GORAB and Scyl1 detach from the complex to start a new cycle. (C2-O-sLex), a ligand involved in selectin-mediated leukocyte trafficking and tumorigenesis [72]. GOLPH3 protein interacts with the CT of C2 GnT1 via a LLRRR sequence and controls Golgi localization of C2 GnT1 in KG1a and K562 cells [71]. Moreover, depletion of GOLPH3 affects synthesis of C2-O-sLex associated with P-selectin glycoprotein ligand-1 with effects on cell tethering, rolling and adhesion [71]. The involvement of GOLPH3 in \(O\)-glycosylation was further demonstrated by its interaction with the CT of POMGnT1, the \(O\)-mannosyl-\(\beta\)-1,2-\(N\)- acetylglucosamineaminyltransferase 1 required for \(O\)-mannosylation of \(\alpha\)-dystroglycan [73]. \(\alpha\)-dystroglycan, a key component of the \(\alpha\)-dystroglycan glycoprotein complex mediates the association with laminin and other components of the extracellular matrix [74]. Perturbation of the glycosylation status of \(\alpha\)-dystroglycan has been linked to various forms of congenital muscular dystrophies [75]. Loss of GOLPH3/ POMGnT1 interaction impairs Golgi localization of POMGnT1 and results in a reduction in IIH6 immunoreactivity indicating a defective glycosylation status of \(\alpha\)-dystroglycan. [PERSON] and coauthors [76] showed that GOLPH3 controls the ability of Golgi to retain exostosins, a class of glycosyltransferases implicated in \(O\)-glycosylation of heparan sulfate proteoglycans. Exostosins catalyze the addition of alternating \(\beta\)1-4-linked glucuronic acid (GlcA) and \(\alpha\)1-4-linked \(N\)-acetylglucosamine (GlcNAc) disaccharides in the biosynthesis of glycosaminoglycan chains [77]. In humans, mutations affecting the exosostins EXT1 and EXT2 cause multiple osteochondrosoma (MO), an autosomal dominant skeletal disease characterized by multiple cartilaginous tumors [78; 79; 80]. _Drosophila_ GOLPH3 physically interacts with the exosostin EXT1 and EXT2. Moreover, knockdown or overexpression of GOLPH3 affects the distribution of EX1 and EX2 proteins within the Golgi cisternae, resulting in defective HSPGs and Hedgehog signaling [76]. Consistent with the results in _Drosophila_, Golgi localization of EXT1 and EXT2 is sensitive to GOLPH3 protein levels in cells cultivated from bone cells such as osteosarcoma cells (U2 OS, MG63), chondrosarcoma cells (SW1353), and rhabdomyosarcoma (RD) cells [76]. Several studies have supported the role of the GOLPH3 protein in \(N\)-glycosylation ([65; 81]; Figure 3). [PERSON] and coworkers [81] showed that GOLPH3 knockdown (KD) affects integrin-mediated cell migration which correlates with defective sialylation of \(N\)-glycans, especially \(\alpha\)2,6-sialylation on \(\beta\)1-integrin. They further demonstrated that GOLPH3 associates with \(\alpha\)2,6-sialyltransferase-I (ST6 GAL1) in HeLa cells and that expression of \(\alpha\)2,6-sialyltransferase-I (ST6 GAL1) rescues integrin-dependent cell migration and cellular signaling defects in GOLPH3 KD cells. [PERSON] and coauthors [65] reported a role of human GOLPH3 in spatial regulation of ST6 GAL1. They showed that GOLPH3 protein directly interacts with Core \(2N\)-acetylglucosamineaminyltransferase (C2 GnT) and ST6 GAL1, and directs incorporation of these enzymes into COPI coated vesicles. Conversely, galactosyltransferase, an enzyme that does not bind to GOLPH3, is not incorporated into COPI vesicles, and does not depend on GOLPH3 for its Golgi localization. Remarkably, previous studies reported that increased \(\alpha\)2-6 sialylation of \(\beta\)1-integrins affect the adhesive and migratory capacity of tumor cells, and may contribute to colon tumor progression [82; 83]. Collectively, these data suggest a strong correlation between GOLPH3-dependent glycosylation defects and tumor progression.