In eukaryotic cells, protein transport between membrane compartments is dependent on membrane carriers, mainly vesicles. The molecular mechanisms of vesicular transport have been studied extensively. A series of protein machineries have been identified to mediate each step of vesicular transport, including vesicle formation, translocation, and docking and fusion with target membrane. Specifically, the machineries for vesicle formation have been defined as coat complexes, which are usually regulated by small GTPases Sar1/ADP ribosylation factor (ARF) (Donaldson and Jackson 2011). Like all other small GTPases, the functions of Sar1/ARFs depend on associated nucleotides. When a Sar1/ARF is GTP-bound, it is in active state to recruit effectors such as coat proteins for downstream events, including vesicular transport. However, its intrinsic GTPase activity enhanced by a negative...