## 5 Gorab, a Scaffolding Protein for COPI, Is Involved in Geroderma Osteodysplastica  
The RAB6-interacting protein GORAB, also known as NTKL-binding protein 1, was also dubbed \"Scyl1-binding protein 1\" after its interaction with Scyl1/NTKL protein [84]. Scyl1, a component of the Scyl1-like family of catalytically inactive protein kinases, was involved in COPI-mediated retrograde trafficking because of its association with isoform 2 of \(\gamma\)COP subunit, through a C-terminal RKXX-COO- dibasic motif [85; 86]. _GORAB_ loss-of-function mutations have been linked to Geroderma osteodysplastica GO, OMIM 231070; [87], an autosomal recessive inherited disorder characterized by lax and wrinkly skin, which gives patients a prematurely aged appearance [87; 88]. Moreover, individuals suffering from GO exhibit a severe osteoporosis with reduced bone mass, susceptibility to fractures and malar and mandibular hypoplasia [87; 88]. To date, the molecular mechanisms that link loss of GORAB with the onset of GO disease remain to be clarified. Evidence indicates that GORAB is a key regulator of the E3 ligase Mdm2, promoting both its transcription, by binding _Mdm2_ promoter [89], and Mdm2 protein self-ubiquitylation [90]. A recent work implicated the _Drosophila_ homologue of GORAB in centriole structure and duplication [91]. Although the above studies proved a subcellular localization of GORAB in the cytoplasm and in the nucleus, the GO disease has been mostly associated with Golgi-associated functions [87; 92]. Indeed, in dermal fibroblasts and osteoblasts, which represent the mostly affected cell types in GO, GORAB protein was detected almost exclusively at the Golgi apparatus [87; 92]. Originally classified as a member of the Golgin family, GORAB localizes to the _trans_-Golgi through the association with the active, GTP-bound, form of the small RAB6 and ARF5 GTPases through the \"internal Golgi-targeting RAB6 and ARF5 binding domain\" (IGRAB) [87; 92]. However, a recent study has suggested that GORAB functions as a scaffolding protein for COPI assembly at the _trans_-Golgi, rather than behaving as a member of the Golgin family [66]. [PERSON] and coauthors [66] demonstrated that GORAB undergoes oligomerization and localizes to discrete puncta throughout the _trans_-Golgi where Scyll1 and GORAB directly interact via their respective \(N\)-terminal domains. Moreover, a mutational analysis indicated that the ability of GORAB to self-associate in oligomers is involved in the assembly of the coatomer. Taken together, these data indicate that the GORAB/Scyll1 complex is necessary and sufficient to recruit the COPI complex to the Golgi membranes [66]. GORAB and Scyll1 were also shown to directly bind to the active GTP-bound form Arf1, indicating that they might function as Arf1 effector proteins. Of note, the predominant Arf1 binding coat proteins at the _trans_-Golgi network (TGN) are AP1 and the GGAs [93]. [PERSON] and coauthors propose that GORAB and Scyll1 oligomers provide discrete domains at the TGN, where high levels of Scyll1 and GTP loaded Arf1 favor the selective COPI coat assembly at the expense of AP1 ([66]; Figure 3). The roles of GORAB protein in the COPI-mediated retrograde trafficking also involve maintenance of the proper set of glycosyltransferases in the Golgi _trans_-cisterna. Indeed, in HeLa cells depleted of GORAB, the \(\beta\)-galactosidase \(\alpha\)-2,6-sialyltransferase 1(ST6 GAL1) undergoes a shift towards the later compartments of Golgi [66]. Moreover, \(N\)-glycome analysis by mass spectrometry from fibroblasts of GO patients and from skin samples of GORAB-knockout mice revealed a significant decrease of sialylated complex glycans [66]. Recent work indicated the requirement for GORAB protein for \(O\)-glycosylation. An analysis of a GORab null full knock out in a mouse model revealed a substantial reduction of the dermatan sulfate glycosaminoglycan (GAG) attached to proteoglycans decorin and biglycan through \(O\)-glycosylation [94].