Figure 7.

Contributions to G‐protein signaling of extended TM5/TM6. A) (Up) Surface and cartoon representations of Gα_s are shown, with the surface of Gα_s depicted as transparent. (Down)The binding surface of TM5 with Gα_s of D1R is shown, with TM5 beyond I^5.69 noted as extended helix. Residues that interact with Gα are highlighted in green. B) (Up)Surface and cartoon representations of Gα_i are shown, with the surface of Gα_i depicted as transparent. (Down) The binding surface of TM6 with Gα_i/o of 5‐HT_1A. TM6 beyond R^6.29 was considered as extended helix. C) The additional contact areas of G_s‐ or G_i/o‐coupled receptors between the extended TM5/TM6 with Gα_s/Gα_i/o, respectively. D) Schematic representation of the TM5‐ (up) or TM6‐ (down) breaking mutants. E) Dose‐dependent curves for histamine induced cAMP accumulation or G‐protein dissociation in HEK293 cells expressing the truncated receptors (n  = 3). (Up) The TM5 of H₂R was respectively truncated by four amino acids and eight amino acids. The cAMP assay validated that the length of TM5 was closely related to the activation of the G_s signal pathway. (Down) The TM6 of H₃R was respectively truncated by eight amino acids and nine amino acids. The NanoBit assay validated that the length of TM6 did not correlate with the dissociation of the G_i‐protein. F) Schematic representation of the geometry determinants for G‐protein selectivity of GPCRs.