Figure 13-19a


Figure 13-19b

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(a) X-ray structure of a heterotrimeric G protein. The Gα 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 Gβ subunit's N-terminal segment is light blue and each blade of its β propeller has a different color. The Gγ subunit is gold. The view is perpendicular to the axis of the Gβ 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 Gα subunit's Ras-like domain. [Based on an X-ray structure by Brian Kobilka, Stanford University. PDBid 3SN6.]