The GGAs pick up their coat

Intracellular transport in the secretory and endocytic pathways involves the regulated assembly of cytoplasmic coat proteins on appropriate donor membranes. ARF proteins, constituting a family of small GTPases, are crucial for the membrane recruitment of coat proteins such as COP I and the adaptor complexes AP-1, AP-3 and AP-4. ARFs cycle between a GDP-bound, cytosolic form and a GTP-bound, active form that is associated with membranes. The ARF GTP–GDP cycle is regulated by GDP–GTP exchange factors (GEFs) that load ARFs with GTP and by GTPase-activating proteins (GAPs) that induce hydrolysis of GTP bound to ARFs, but the mechanistic details of this regulation remain sketchy. GGAs (Golgi-associated, γ-adaptin homologous, ARF-interacting proteins) constitute a recently discovered family of conserved proteins that associate with membranes of the trans-Golgi network (TGN) and are implicated in regulating ARF-dependent trafficking events 1xThe GGAs promote ARF-dependent recruitment of clathrin to the TGN. Puertollano, R. et al. Cell. 2001; 105: 93–102Abstract | Full Text | Full Text PDF | PubMed | Scopus (185)See all References, 2xA family of proteins with γ-adaptin and VHS domains that facilitate trafficking between the trans-Golgi network and the vacuole/lysosome. Hirst, J. et al. J. Cell Biol. 2000; 149: 67–79Crossref | PubMed | Scopus (242)See all References, 3xA selective transport route from Golgi to late endosomes that requires the yeast GGA proteins. Black, M. and Pelham, H. J. Cell Biol. 2000; 151: 587–600Crossref | PubMed | Scopus (110)See all References. In yeast, deletion of both the genes encoding GGAs results in missorting of the vacuolar enzyme carboxypeptidase Y and of the yeast syntaxin Pep12p, and a defective vacuolar morphology phenotype 2xA family of proteins with γ-adaptin and VHS domains that facilitate trafficking between the trans-Golgi network and the vacuole/lysosome. Hirst, J. et al. J. Cell Biol. 2000; 149: 67–79Crossref | PubMed | Scopus (242)See all References, 3xA selective transport route from Golgi to late endosomes that requires the yeast GGA proteins. Black, M. and Pelham, H. J. Cell Biol. 2000; 151: 587–600Crossref | PubMed | Scopus (110)See all References.Mammalian and yeast GGAs have a modular structure, comprising a VHS, GAT, hinge and γ-adaptin ear domain. The GAT domain of the GGAs binds directly to the GTP-bound form of several members of the ARF family and is sufficient to target a GFP reporter protein to the Golgi complex. The VHS domain is implicated in interactions with vesicle cargo. Puertollano et al. have now performed a structure–function analysis of the GAT and hinge domains of GGAs, focusing on human GGA3 1xThe GGAs promote ARF-dependent recruitment of clathrin to the TGN. Puertollano, R. et al. Cell. 2001; 105: 93–102Abstract | Full Text | Full Text PDF | PubMed | Scopus (185)See all References1. They identify crucial residues in a highly conserved stretch of the GAT domain required for the Golgi targeting and ARF-binding activities. Most interestingly, the GAT domain of GGA3 can inhibit the GTPase activity of ARF1 induced by recombinant GAPs in vitro (ASAP1 or ARF GAP1). In line with this observation, overexpression of the GAT domain stabilizes Golgi-bound ARF1–GTP on Golgi membranes in vivo 1xThe GGAs promote ARF-dependent recruitment of clathrin to the TGN. Puertollano, R. et al. Cell. 2001; 105: 93–102Abstract | Full Text | Full Text PDF | PubMed | Scopus (185)See all References1. Puertollano et al. show that GGAs can bind to clathrin in vitro through their hinge domain 1xThe GGAs promote ARF-dependent recruitment of clathrin to the TGN. Puertollano, R. et al. Cell. 2001; 105: 93–102Abstract | Full Text | Full Text PDF | PubMed | Scopus (185)See all References1. Binding of clathrin to liposomes is enhanced by full-length recombinant His6-GGA1 if these binding reactions additionally contain GTP-bound ARF, whereas GDP–ARF has no effect. This suggests that GGAs might promote recruitment of clathrin to membranes in cells. Indeed, expression of a Myc-tagged full-length GGA1 construct increases the staining for clathrin in the trans-Golgi complex, whereas a truncated GGA1 just containing the VHS and GAT domains results in a strong decrease in clathrin staining in the Golgi region. Thus, the GGAs could function to link clathrin to membrane-bound ARF–GTP.GGAs and GAPs might compete for interaction with ARF1–GTP, and binding of GGAs to ARF–GTP might allow for enough time for recruitment of the coat protein clathrin. Expression of a GGA1 VHS–GAT domain construct causes a striking accumulation of the cation-independent mannose 6-phosphate receptor at the TGN, but had no effect on the transferrin receptor 1xThe GGAs promote ARF-dependent recruitment of clathrin to the TGN. Puertollano, R. et al. Cell. 2001; 105: 93–102Abstract | Full Text | Full Text PDF | PubMed | Scopus (185)See all References1. This is another hint as to the transport step(s) that the GGAs mediate. It will be interesting to watch the tale of this intriguing family of proteins unfold further in the future.