Fig. 3.

(A) Allosteric Site 1 is located in the extracellular mouth as illustrated in the X-ray structure. The ECL2, TM3, TM5, TM6 and TM7 are highlighted in pink, red, green, orange and cyan, respectively. (B) In the X-ray structure, the M2 receptor exhibits opened extracellular mouth by ∼11 Å as measured between the C_β atoms of Tyr177^ECL2 and Trp422^7.35. The receptor is represented by surface with pink for ECL2, purple for key interacting residues and blue for the remainder of the receptor. The bound probes are rendered as ball-and-sticks. (C) In the active conformer (red), ECL2 (pink) is found to close up the extracellular mouth.