Serotonin receptors are important targets for established therapeutics and drug development as they are expressed throughout the human body and play key roles in cell signaling. There are 12 serotonergic G protein-coupled receptor members encoded in the human genome, of which the 5-hydroxytryptamine (5-HT)_5A receptor (5-HT_5AR) is the least understood and lacks selective tool compounds. Here, we report four high-resolution (2.73-2.80 Å) structures of human 5-HT_5ARs, including an inactive state structure bound to an antagonist AS2674723 by crystallization and active state structures bound to a partial agonist lisuride and two full agonists, 5-carboxamidotryptamine (5-CT) and methylergometrine, by cryo-EM. Leveraging the new structures, we developed a highly selective and potent antagonist for 5-HT_5AR. Collectively, these findings both enhance our understanding of this enigmatic receptor and provide a roadmap for structure-based drug discovery for 5-HT_5AR.

The MRGPRX family of receptors (MRGPRX1-4) is a family of mas-related G-protein-coupled receptors that have evolved relatively recently¹. Of these, MRGPRX2 and MRGPRX4 are key physiological and pathological mediators of itch and related mast cell-mediated hypersensitivity reactions^2-5. MRGPRX2 couples to both G_i and G_q in mast cells⁶. Here we describe agonist-stabilized structures of MRGPRX2 coupled to G_i1 and G_q in ternary complexes with the endogenous peptide cortistatin-14 and with a synthetic agonist probe, respectively, and the development of potent antagonist probes for MRGPRX2. We also describe a specific MRGPRX4 agonist and the structure of this agonist in a complex with MRGPRX4 and G_q. Together, these findings should accelerate the structure-guided discovery of therapeutic agents for pain, itch and mast cell-mediated hypersensitivity.