Molecular mechanisms of inverse agonism via κ-opioid receptor–G protein complexes
Aaliyah S. Tyson, Saif Ur Rehman Khan, Zenia Motiwala, Gye Won Han, Zixin Zhang, Mohsen Ranjbar, Daniel Styrpejko, Nokomis Ramos‐Gonzalez, Stone Woo, Kelly Villers, Delainey Landaker, Terry Kenakin, Ryan A. Shenvi, Susruta Majumdar, Cornelius Gati
Abstract
Opioid receptors, a subfamily of G protein-coupled receptors (GPCRs), are key therapeutic targets. In the canonical GPCR activation model, agonist binding is required for receptor–G protein complex formation, while antagonists prevent G protein coupling. However, many GPCRs exhibit basal activity, allowing G protein association without an agonist. The pharmacological impact of agonist-free receptor–G protein complexes is poorly understood. Here we present biochemical evidence that certain κ-opioid receptor (KOR) inverse agonists can act via KOR–Gi protein complexes. To investigate this phenomenon, we determined cryo-EM structures of KOR–Gi protein complexes with three inverse agonists: JDTic, norBNI and GB18, corresponding to structures of inverse agonist-bound GPCR–G protein complexes. Remarkably, the orthosteric binding pocket resembles the G protein-free ‘inactive’ receptor conformation, while the receptor remains coupled to the G protein. In summary, our work challenges the canonical model of receptor antagonism and offers crucial insights into GPCR pharmacology. This study uncovers that certain κ-opioid receptor inverse agonists form receptor–G protein complexes, even in inactive states, challenging the classic GPCR activation model.