Cryo-EM structure of the human adenosine A1 receptor–G12-protein complex bound to its endogenous agonist
- Drug Discovery Biology and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville 3052, Victoria, Australia.
- Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany.
- Norvatis Institutes for Biomedical Research, Norvatis Pharma AG, 4002 Basel, Switzerland.
G protein-coupled (GPCRs) are responsible for the majority of cellular responses to hormones, neurotransmitters, and a variety of other small molecules. In recent years there has been an exponential growth in the amount of determined inactive GPCR structures, however, there still remains a dearth of active-state, G-protein-bound, GPCRs. To date, all solved active-GPCRs have been coupled to the stimulatory Gs protein. The class A adenosine A1 receptor (A1R) is a GPCR that preferentially couples to the inhibitory Gi/o family of heterotrimeric G-proteins, has been implicated in numerous diseases, yet remains poorly targeted. We have recently solved the 3.6 Å structure of the human A1R in complex with adenosine and heterotrimeric Gi2-protein determined by Volta phase plate cryo-electron microscopy. Compared to inactive A1R, there is contraction at the extracellular surface in the wide orthosteric binding site that is mediated via movement of transmembrane domains 1 and 2. At the intracellular surface, the G-protein engages the A1R primarily via amino acids in the C-terminus of the Gαi α5 helix, concomitant with a 10.5 Å outward movement of A1R transmembrane domain 6. Comparison with the agonist-bound β2adrenergic receptor-Gs-protein complex reveals distinct orientations for each G-protein subtype upon engagement with its receptor. This active A1R structure provides novel molecular insights into receptor/G-protein selectivity.