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 1 , 2 , 3 . 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 2 , 3 .