Cryo-EM structure of the human adenosine A1 receptor–G12-protein complex bound to its endogenous agonist

Draper-Joyce CJ1, Khoshouei M2,3, Thal DM1, Liang YL1, Furness SGB1, May LT1, Wootten D1, Sexton PM1, Glukhova A1 and Christopoulos A1

  1. Drug Discovery Biology and Department of Pharmacology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville 3052, Victoria, Australia.
  2. Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany.
  3. 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.