Energetic analysis of the rhodopsin–G-protein complex links the α5 helix to GDP release

Activated G protein–coupled receptors promote GDP release by their cognate G proteins, through an allosteric mechanism that remains to be fully determined. Now DEER analyses are integrated with previously available structural data and computational work, thus generating a unified model for receptor-mediated G-protein activation. We present a model of interaction of G i protein with the activated receptor (R*) rhodopsin, which pinpoints energetic contributions to activation and reconciles the β 2 adrenergic receptor–G s crystal structure with new and previously published experimental data. In silico analysis demonstrated energetic changes when the Gα C-terminal helix (α5) interacts with the R* cytoplasmic pocket, thus leading to displacement of the helical domain and GDP release. The model features a less dramatic domain opening compared with the crystal structure. The α5 helix undergoes a 63° rotation, accompanied by a 5.7-Å translation, that reorganizes interfaces between α5 and α1 helices and between α5 and β6-α5. Changes in the β6-α5 loop displace αG. All of these movements lead to opening of the GDP-binding pocket. The model creates a roadmap for experimental studies of receptor-mediated G-protein activation.