High-resolution structure of an adenosine receptor
Figure: High-resolution structure of
the A2A Adenosine Receptor, where sodium is shown in violet.
Ray Stevens group (The Scripps Research Institute) and collaborators
Ray Stevens' group determined a high-resolution structure of
the A2A adenosine receptor, which revealed the structural basis
for allosteric modulation of GPCRs by sodium ions. During the past 40 years,
allosteric modulation by Na+ has been observed for many GPCRs and
was linked to motifs in helix II, including the highly conserved
Asp2.50. Mutation of this residue has been the subject of many
studies on a multitude of receptors, which have shown that the Na+
effect was largely abrogated when the residue was mutated to alanine or
asparagine. Despite this indirect evidence, the nature of Na+
interactions with GPCRs remained hypothetical, and the sodium ion remained
undetected in crystal structures of GPCRs solved at medium resolution. The
researchers reengineered the human A2A adenosine receptor
(A2AAR) by replacing its third intracellular loop with
apocytochrome b562RIL and solved the structure to 1.8-Å resolution.
This unprecedented high-resolution GPCR structure allowed them to identify 57
ordered water molecules inside the receptor comprising three major clusters.
The central cluster harbors a putative sodium ion bound to the highly
conserved aspartate residue Asp2.50. Additionally, two
cholesterols stabilize the conformation of helix VI, and one of 23 ordered
lipids intercalates inside the ligand-binding pocket. These high-resolution
details shed light on the potential role of structured water molecules,
sodium ions, and lipids/cholesterol in GPCR stabilization and function.
Furthermore, this antagonist-bound structure shows that the Na+
ion is part of a water channel that spans the whole receptor; comparison with
the active state structure shows dramatic rearrangement in the Na+
Liu, W, Chun, E, Thompson, AA, Chubukov, P, Xu, F, Katritch, V, Han, GW,
Roth, CB, Heitman, LH, AP, IJ, Cherezov, V, Stevens, RC. Structural basis for
allosteric regulation of GPCRs by sodium ions, Science 337, 232-236 (2012).