The vesicle coat assembly process is initiated by:
Coat assembly regulated spatiotemporally to occur only at correct donor membranes and when cargo present ensuring energy not wasted. Initiating event activation of small GTPases Sar1 and ARF families from GDP cytosolic to GTP membrane state. Sar1 activated by Sec12 GEF integral ER protein exchanges GDP for GTP exposing amphipathic helix embedding ER leaflet inducing curvature recruiting Sec23-Sec24 heterodimer. Similarly ARF1 activated by GBF1 at ERGIC cis-Golgi and BIG1/2 at TGN upon membrane recruitment exposes myristoylated helix plus effector binding switch regions recruiting COPI coatomer
Ref: Lee & Miller, Annu Rev Cell Dev Biol: Sar1 and ARF GTPases initiate coat assembly.