Membrane dynamic processes including vesicle biogenesis depend on Arf guanosine
triphosphatase (GTPase) activation by guanine nucleotide exchange factors (GEFs)
containing a catalytic Sec7 domain and a membrane-targeting module such as a
pleckstrin homology (PH) domain. The catalytic output of cytohesin family Arf
GEFs is controlled by autoinhibitory interactions that impede accessibility of
the exchange site in the Sec7 domain. These restraints can be relieved through
activator Arf-GTP binding to an allosteric site comprising the PH domain and
proximal autoinhibitory elements (Sec7-PH linker and C-terminal helix).
Small-angle X-ray scattering and negative-stain electron microscopy were
used to investigate the structural organization and conformational dynamics of
cytohesin-3 (Grp1) in autoinhibited and active states. The results support a
model in which hinge dynamics in the autoinhibited state expose the activator
site for Arf-GTP binding, while subsequent C-terminal helix unlatching
and repositioning unleash conformational entropy in the Sec7-PH linker to
drive exposure of the exchange site.
