G-protein-coupled receptors (GPCRs) form the largest family of receptors encoded by the human genome (around 800 genes). They transduce signals by coupling to a small number of heterotrimeric G proteins (16 genes encoding different α-subunits). Each human cell contains several GPCRs and G proteins. The structural determinants of coupling of Gs to four different GPCRs have been elucidated1,2,3,4, but the molecular details of how the other G-protein classes couple to GPCRs are unknown. Here we present the cryo-electron microscopy structure of the serotonin 5-HT1B receptor (5-HT1BR) bound to the agonist donitriptan and coupled to an engineered Go heterotrimer. In this complex, 5-HT1BR is in an active state; the intracellular domain of the receptor is in a similar conformation to that observed for the β2-adrenoceptor (β2AR)3 or the adenosine A2A receptor (A2AR)1 in complex with Gs. In contrast to the complexes with Gs, the gap between the receptor and the Gβ-subunit in the Go–5-HT1BR complex precludes molecular contacts, and the interface between the Gα-subunit of Go and the receptor is considerably smaller. These differences are likely to be caused by the differences in the interactions with the C terminus of the Go α-subunit. The molecular variations between the interfaces of Go and Gs in complex with GPCRs may contribute substantially to both the specificity of coupling and the kinetics of signalling.
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This work was funded by a grant from the European Research Council (EMPSI 339995), Heptares Therapeutics and core funding from the Medical Research Council (MRC U105197215). We thank J. Espinosa and L. Renault for their help with data collection at NeCEN; S. Scheres and P. da Fonseca for useful discussions and C. Savva and G. Cannone for microscopy technical support.Reviewer information:
Nature thanks the anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, Representative micrograph (magnification 75,000×, defocus −0.6 μm) of the 5-HT1BR–Go complex collected using a Titan Krios with the Falcon III detector and Volta phase plate. b, Representative 2D class averages of the 5-HT1BR–Go complex. c, FSC curve of the final reconstruction showing an overall resolution of 3.8 Å using the gold-standard FSC of 0.143. Both masked and unmasked FSC curves are shown to highlight the lack of masking artefacts. d, Final reconstruction coloured by subunit. Inset shows a magnified view of the weak density for ICL3. The magnified region corresponds to a map sharpened with B = –50 to remove noise from lower density levels. e, Local resolution estimation of the 5-HT1BR map as calculated by Resmap.
a, Transmembrane helices of 5-HT1BR. b, The α5-helix of Go. c, Donitriptan and the neighbouring side chains in the orthosteric binding site. d, FSC of the refined model versus the map (green curve) and FSCwork/FSCtest validation curves (blue and red curves, respectively).
Micrographs were collected during nine sessions on the Titan Krios (either 24 h or 48 h) and each session was processed independently. The number of images and particles from one 48-h session is indicated on the flowchart as a guide. At the bottom of the figure, the final number of particles is shown. Each dataset was corrected separately for drift, beam-induced motion and radiation damage. After CTF estimation, particles were picked using a Gaussian blob and submitted to either one or two rounds of reference-free 2D classification (see Methods). A 3D classification was performed on the selected particles using an ab initio model generated from ten thousand particles. Classification was performed in parallel in three and four classes. The models with best features were refined on their own; if there were two classes of similar high quality, these were then re-refined together (the resolution of the models refers to the resolution after refinement and calculation of gold-standard FSC = 0.143). The set of particles that obtained the best map quality and resolution were saved and merged with the best particles from other datasets. A final model with 730,118 particles was refined and achieved a global resolution of 3.78 Å.
a, Amino acid sequence of the 5-HT1BR construct used for the cryo-EM structure determination. Residues are coloured according to how they have been modelled. Black, good density allows the side chain to be modelled; red, limited density for the side chain, therefore the side chain has been truncated to Cβ; blue, no density observed and therefore the residue was not modelled. Regions highlighted in grey represent the transmembrane α-helices, and amphipathic helix 8 is highlighted in yellow. b, Model of 5-HT1BR showing the Cα positions of amino acid residues with poor density (spheres) and unmodelled regions (dotted lines).
Diamonds above the sequences indicate amino acid residues in Gαs in which the side chains make atomic contacts to residues in β2AR (β2 con) or A2AR (2A con). Ovals indicate amino acid residues in Gαs in which only the main chain atoms make contacts with the receptor. Secondary structural elements are indicated as grey bars with positions numbered according to the CGN numbering system.
Extended Data Fig. 7 Similarity of Gα structures and the difference poses of the α5-helices in Gαo and Gαs coupled to receptors.
a, The structures of the α-subunits in complex with 5-HT1BR, β2AR3 and A2AR1 were superimposed over the whole of their sequence in Pymol. Blue, Gαo coupled to 5-HT1BR; green, Gαs coupled to A2AR; Gαs coupled to β2AR. b, 5-HT1BR (blue), β2AR3 (green) and A2AR1 (red) were superimposed based on H3, H5 and H6. Two different views are shown with the red arrows indicating differences in orientation of Gαs and Gαo.
Residues in grey correspond to the α-helical region that does not make contact with GPCRs and was deleted during the construction of mini-Go. Secondary structural elements are depicted as grey bars with the CGN numbers shown to aid comparisons. Amino acids are highlighted as follows: pink, stabilizing residues required to generate mini-Go; yellow; residues in Gαo that are different from residues conserved in all three Gαi sequences; blue, residues that are non-conserved in Gαi sequences. #, the affinity tag on mini-Go used for purification (MGHHHHHHENLYFQG).
The α5 helices in the cryo-EM structures of A2AR–Gs (carbon, green) and 5-HT1BR–Go (carbon, light blue) were aligned (in Pymol) along their whole sequence and displayed in different poses: cartoon depiction (a); Gs (green spheres), Go (blue sticks) (b); Go (blue spheres), Gs (green sticks) (c).