Cryo-EM reveals two distinct serotonin-bound conformations of full-length 5-HT(3A) receptor

Serotonin receptor (5-HT(3A)R)(1), a cationic pentameric ligand-gated ion channel (pLGIC), is the clinical target for management of nausea and vomiting associated with radiation and chemotherapies(2). Upon binding, serotonin induces a global conformational change encompassing the ligand-binding extracellular domain (ECD), the transmembrane domain (TMD), and the intracellular domain (ICD), the molecular details of which are unclear. Here, we present two serotonin-bound structures of the full-length 5-HT(3A)R in distinct conformations at 3.32 Å and 3.89 Å resolutions that reveal the mechanism underlying channel activation. When compared to Apo-5-HT(3A)R, serotonin-bound states underwent a large twisting motion in the ECD and TMD leading to the opening of a 165 Å long permeation pathway. Notably, this motion results in creation of lateral portals for ion permeation at the interface of the TMD and ICD. Combined with molecular dynamics simulations, these structures provide novel insights into conformational coupling across domains and functional modulation.