(a) X-ray structure of a heterotrimeric G protein. The G_α subunit is pink with its Switch I, II, and III segments green, blue, and red, respectively, and its helical domain light orange. Its bound GDP is shown in space-filling form with C green, N blue, O red, and P yellow. The G_β subunit's N-terminal segment is light blue and each blade of its β propeller has a different color. The G_γ subunit is gold. The view is perpendicular to the axis of the G_β subunit's β propeller. [Based on an X-ray structure by Alfred Gilman and Stephan Sprang, University of Texas Southwestern Medical Center. PDBid 1GP2.] (b) X-Ray structure of a heterotrimeric G protein lacking bound nucleotide, in complex with β₂-adrenergic receptor (β₂AR) binding BI167107 (an epinephrine analog). The β₂AR is colored in rainbow order from its N-terminus (blue) to its C-terminus (red). The BI167107, which binds in a cavity between the receptor's seven transmembrane helices that is open to the extracellular side of the membrane, is drawn in space-filling form with C magenta, N blue, and O red. The view is related to that in Part a by a 90° rotation about its horizontal axis. The G-protein is colored as in Part a except that its switch regions are pink. The approximate position of the plasma membrane is indicated by the horizontal lines. Note that the β₂AR binds, almost exclusively, to the G_α subunit's Ras-like domain. [Based on an X-ray structure by Brian Kobilka, Stanford University. PDBid 3SN6.]