Structural flexibility of the Gas α-helical domain in the β2-adrenoceptor Gs complex

The active-state complex between an agonist-bound receptor and a guanine nucleotide-free G protein represents the fundamental signaling assembly for the majority of hormone and neurotransmitter signaling. We applied single-particle electron microscopy (EM) analysis to examine the architecture of agonist-occupied beta(2)-adrenoceptor (beta(2)AR) in complex with the heterotrimeric G protein Gs (G alpha s beta gamma). EM 2D averages and 3D reconstructions of the detergent-solubilized complex reveal an overall architecture that is in very good agreement with the crystal structure of the active-state ternary complex. Strikingly however, the alpha-helical domain of G alpha s appears highly flexible in the absence of nucleotide. In contrast, the presence of the pyrophosphate mimic foscarnet (phosphonoformate), and also the presence of GDP, favor the stabilization of the alpha-helical domain on the Ras-like domain of G alpha s. Molecular modeling of the alpha-helical domain in the 3D EM maps suggests that in its stabilized form it assumes a conformation reminiscent to the one observed in the crystal structure of G alpha s-GTP gamma S. These data argue that the alpha-helical domain undergoes a nucleotide-dependent transition from a flexible to a conformationally stabilized state.