In G-protein signaling, an activated receptor catalyzes GDP/GTP exchange on the G _α subunit of a heterotrimeric G protein. In an initial step, receptor interaction with G _α acts to allosterically trigger GDP release from a binding site located between the nucleotide binding domain and a helical domain, but the molecular mechanism is unknown. In this study, site-directed spin labeling and double electron–electron resonance spectroscopy are employed to reveal a large-scale separation of the domains that provides a direct pathway for nucleotide escape. Cross-linking studies show that the domain separation is required for receptor enhancement of nucleotide exchange rates. The interdomain opening is coupled to receptor binding via the C-terminal helix of G _α , the extension of which is a high-affinity receptor binding element.