Integrating three views of Arf1 activation dynamics.

The proteins Arno and Gea2 of the Sec7 family can promote GDP-GTP exchange on Arf1, a small GTP-binding protein, which coordinates coated vesicle formation for protein transport within the cell. Crystal structures of the essential Sec7 domain (Sec7d) of Gea2 in the free and Arf1-bound forms suggest that conformational dynamics of the Sec7d as well as those of the G-protein play a role in nucleotide exchange. Starting from a set of complementary crystal structures, we compared the collective movements of unbound Gea2 and Arno Sec7 domains, Arf1-GDP, and the Arf1-Gea2(Sec7d) nucleotide-free complex using normal modes analyses. In all unbound Sec7d analyses, significant low-energy movements were found to lead to closure of the hydrophobic groove towards the form seen in the Arf1-Gea2(Sec7d) complex, suggesting that groove closure is a general feature of the Sec7 family. Low-energy movements in Arf1-GDP implicate critical switch 1 and 2 residues which are coupled to modifications in the myristoylated N-terminal-helix binding site at the other end of the "interswitch" beta hairpin. It is suggested that Sec7d groove closure upon docking of the two molecules may permit extraction of switch 1 from Arf1-GDP and prepare the complex for movement of the interswitch, which is central to the membrane-linked exchange activity. Large-scale collective movements in the Arf1-Sec7d complex appear to participate in the insertion of the Sec7d Glu finger into the GDP binding site to promote actual nucleotide release.