Structural basis for activation of ARF GTPase: Mechanisms of guanine nucleotide exchange and GTP-myristoyl switching

Ras-related GTPases are positively regulated by guanine nucleotide exchange factors (GEFs) that promote the exchange of GDP for GTP. The crystal structure of the Sec7 domain GEF bound to nucleotide-free ARF1 GTPase has been determined at 2.8 Angstrom resolution and the structure of ARF1 in the GTP-analog form determined at 1.6 Angstrom resolution. The Sec7 domain binds to the switch regions of ARF1 and inserts residues directly into the GTPase active site. The interaction leaves the purine-binding site intact but perturbs the Mg2+ and phosphate groups to promote the dissociation of guanine nucleotides. The structure of ARF1 in the GTP-analog form closely resembles Pas, revealing a substantial rearrangement from the GDP conformation. The transition controls the exposure of the myristoylated N terminus, explaining how ARF GTPases couple the GDP-GTP conformational switch to membrane binding.