Cryo-EM structure of the serotonin 5-HT receptor coupled to heterotrimeric G.

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 G to four different GPCRs have been elucidated, 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-HT receptor (5-HTR) bound to the agonist donitriptan and coupled to an engineered G heterotrimer. In this complex, 5-HTR is in an active state; the intracellular domain of the receptor is in a similar conformation to that observed for the β-adrenoceptor (βAR) or the adenosine A receptor (AR) in complex with G. In contrast to the complexes with G, the gap between the receptor and the Gβ-subunit in the G-5-HTR complex precludes molecular contacts, and the interface between the Gα-subunit of G 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 G α-subunit. The molecular variations between the interfaces of G and G in complex with GPCRs may contribute substantially to both the specificity of coupling and the kinetics of signalling.