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Summary The third intracellular loop (ICL3) of the β1-adrenergic receptor (β1AR) plays a critical role in regulating G protein coupling, yet the structural basis has remained unclear due to truncations of ICL3 in all available structures of the β1AR in complex with Gs or a G protein mimetic nanobody. To address this, we used cell-free cotranslational insertion of full-length human β1AR into nanodiscs and determined its cryo-electron microscopy (cryo-EM) structure in complex with Gs. In this structure, ICL3 extends transmembrane helix 5, resulting in enhanced interactions with Gαs and in a slight rotation of the engaged G protein. This repositioning enables new polar interactions between Gαs, ICL2 and helix 8, while ICL1 and helix 8 form additional contacts with Gβ. These structural insights, supported by mutational analysis, demonstrate that ICL3 enhances G protein activation and downstream cAMP signaling by promoting more extensive interactions between the receptor and the heterotrimeric G protein.
Merino et al. (Mon,) studied this question.