Crystal structure of the adenosine A2A receptor bound to an antagonist reveals a potential allosteric pocket

The adenosine A(2A) receptor (A(2A)R) has long been implicated in cardiovascular disorders. As more selective A(2A)R ligands are being identified, its roles in other disorders, such as Parkinson's disease, are starting to emerge, and A(2A)R antagonists are important drug candidates for nondopaminergic anti-Parkinson treatment. Here we report the crystal structure of A2A receptor bound to compound 1 (Cmpd-1), a novel A(2A)R/N-methyl D-aspartate receptor subtype 2B (NR2B) dual antagonist and potential anti-Parkinson candidate compound, at 3.5 angstrom resolution. The A(2A) receptor with a cytochrome b562-RIL (BRIL) fusion (A(2A)R-BRIL) in the intracellular loop 3 (ICL3) was crystallized in detergent micelles using vapor-phase diffusion. Whereas A2AR-BRIL bound to the antagonist ZM241385 has previously been crystallized in lipidic cubic phase (LCP), structural differences in the Cmpd-1-bound A2AR-BRIL prevented formation of the lattice observed with the ZM241385-bound receptor. The crystals grew with a type II crystal lattice in contrast to the typical type I packing seen from membrane protein structures crystallized in LCP. Cmpd-1 binds in a position that overlaps with the native ligand adenosine, but its methoxyphenyl group extends to an exosite not previously observed in other A2AR structures. Structural analysis revealed that Cmpd-1 binding results in the unique conformations of two tyrosine residues, Tyr9(1.35) and Tyr271(7.36), which are critical for the formation of the exosite. The structure reveals insights into antagonist binding that are not observed in other A(2A)R structures, highlighting flexibility in the binding pocket that may facilitate the development of A(2A)R-selective compounds for the treatment of Parkinson's disease.