Dopamine receptors are widely distributed in the central nervous system and are important therapeutic targets for treatment of various psychiatric and neurological diseases. Here, we report three cryo-electron microscopy structures of the D1 dopamine receptor (D1R)-Gs complex bound to two agonists, fenoldopam and tavapadon, and a positive allosteric modulator LY3154207. The structure reveals unusual binding of two fenoldopam molecules, one to the orthosteric binding pocket (OBP) and the other to the extended binding pocket (EBP). In contrast, one elongated tavapadon molecule binds to D1R, extending from OBP to EBP. Moreover, LY3154207 stabilizes the second intracellular loop of D1R in an alpha helical conformation to efficiently engage the G protein. Through a combination of biochemical, biophysical and cellular assays, we further show that the broad conformation stabilized by two fenoldopam molecules and interaction between TM5 and the agonist are important for biased signaling of D1R.

G protein–coupled receptors (GPCRs) comprise the largest family of membrane receptors and are the most important drug targets. An agonist-bound GPCR engages heterotrimeric G proteins and triggers the exchange of guanosine diphosphate (GDP) with guanosine triphosphate (GTP) to promote G protein activation. A complete understanding of molecular mechanisms of G protein activation has been hindered by a lack of structural information of GPCR–G protein complex in nucleotide-bound states. Here, we report the cryo-EM structures of the D1 dopamine receptor and mini-G s complex in the nucleotide-free and nucleotide-bound states. These structures reveal major conformational changes in Gα such as structural rearrangements of the carboxyl- and amino-terminal α helices that account for the release of GDP and the GTP-dependent dissociation of Gα from Gβγ subunits. As validated by biochemical and cellular signaling studies, our structures shed light into the molecular basis of the entire signaling events of GPCR-mediated G protein activation.

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