What is the main function of ARF-GTP in vesicle formation?
Budding of AP1, AP3 and GGA clathrin-coated vesicles at trans-Golgi network and endosomes requires activated ARF1 small GTPase cycling between GDP and GTP states. ARF1-GDP cytosolic bound to GDI-like factor; activation by large ARF GEFs BIG1-BIG2 at TGN and GBF1 at Golgi exchange GDP for GTP upon membrane recruitment mediated by HDS domains sensing curvature and lipid composition. ARF-GTP undergoes structural rearrangement exposing N-terminal myristoylated amphipathic helix inserting into outer leaflet and effector binding interface. Effectors include heterotetrameric AP1 via gamma subunit trunk, AP3 via delta subunit, AP4, and monomeric GGAs via GAT domain, plus activation of PI4KIIIbeta generating PI4P enhancing adaptor affinity and phospholipase D producing phosphatidic acid promoting negative curvature. Coordinated recruitment concentrates lysosomal cargo such as mannose-6-phosphate receptors bearing acidic cluster dileucine motifs. ARF does not hydrolyze ATP; ATP independent. Scission involves dynamin-family or Arf GAP-induced curvature together with BAR proteins, not SNARE scaffolding. Hydrolysis by ARF GAP1 later triggers partial uncoating for fusion competence and recycling.
Ref: Lodish et al., MCB Chapter 14: ARF1-GTP recruits AP1/GGA to initiate clathrin coat at TGN.