Cryo-EM structure of the adenosine A_2A receptor coupled to an engineered heterotrimeric G protein

The adenosine A_2A receptor (A_2AR) is a prototypical G protein-coupled receptor (GPCR) that couples to the heterotrimeric G protein G_S. Here, we determine the structure by electron cryo-microscopy (cryo-EM) of A_2AR at pH 7.5 bound to the small molecule agonist NECA and coupled to an engineered heterotrimeric G protein, which contains mini-G_S, the βγ subunits and nanobody Nb35. Most regions of the complex have a resolution of ~3.8 Å or better. Comparison with the 3.4 Å resolution crystal structure shows that the receptor and mini-G_S are virtually identical and that the density of the side chains and ligand are of comparable quality. However, the cryo-EM density map also indicates regions that are flexible in comparison to the crystal structures, which unexpectedly includes regions in the ligand binding pocket. In addition, an interaction between intracellular loop 1 of the receptor and the β subunit of the G protein was observed.