The adenosine class of G protein-coupled receptors mediates the important role of extracellular adenosine in many physiological processes and is antagonized by caffeine. We have determined the crystal structure of the human A 2A adenosine receptor in complex with a high affinity subtypeselective antagonist, ZM241385, to 2.6 Å resolution. Four disulfide bridges in the extracellular domain combined with a subtle repacking of the transmembrane helices relative to the adrenergic and rhodopsin receptor structures defines a pocket distinct from that of other structurally determined GPCRs. The arrangement allows for the binding of the antagonist in an extended conformation perpendicular to the membrane plane. The binding site highlights an integral role for the extracellular loops, together with the helical core in ligand recognition by this class of GPCRs, and suggests a role for ZM241385 in restricting the movement of a tryptophan residue important in the activation mechanism of the class A receptors.

The role of cholesterol in eukaryotic membrane protein function has been attributed primarily to an influence on membrane fluidity and curvature. We present the 2.8 A resolution crystal structure of a thermally stabilized human beta(2)-adrenergic receptor bound to cholesterol and the partial inverse agonist timolol. The receptors pack as monomers in an antiparallel association with two distinct cholesterol molecules bound per receptor, but not in the packing interface, thereby indicating a structurally relevant cholesterol-binding site between helices I, II, III, and IV. Thermal stability analysis using isothermal denaturation confirms that a cholesterol analog significantly enhances the stability of the receptor. A consensus motif is defined that predicts cholesterol binding for 44% of human class A receptors, suggesting that specific sterol binding is important to the structure and stability of other G protein-coupled receptors, and that this site may provide a target for therapeutic discovery.

The recent progress in crystallography of G-protein coupled receptors opens an unprecedented venue for structure-based GPCR drug discovery. To test efficiency of the structure-based approach, we performed molecular docking and virtual ligand screening (VLS) of more than 4 million commercially available "drug-like" and ''lead-like'' compounds against the A 2A AR 2.6 Å resolution crystal structure. Out of 56 high ranking compounds tested in A 2A AR binding assays, 23 showed affinities under 10 µM, eleven of those had sub-µM affinities, and two compounds had affinities under 60 nM. The identified hits represent at least 9 different chemical scaffolds and are characterized by very high ligand efficiency (0.3-0.5 kcal/mol per heavy atom). Significant A 2A AR antagonist activities were confirmed for 10 out of 13 ligands tested in functional assays. High success rate, novelty and diversity of the chemical scaffolds and strong ligand efficiency of the A 2A AR antagonists identified in this study suggest practical applicability of receptor-based VLS in GPCR drug discovery.

Factor H (FH) is the key regulator of the alternative pathway of complement. The carboxyl-terminal domains 19-20 of FH interact with the major opsonin C3b, glycosaminoglycans, and endothelial cells. Mutations within this area are associated with atypical haemolytic uremic syndrome (aHUS), a disease characterized by damage to endothelial cells, erythrocytes, and kidney glomeruli. The structure of recombinant FH19-20, solved at 1.8 Å by X-ray crystallography, reveals that the short consensus repeat domain 20 contains, unusually, a short a-helix, and a patch of basic residues at its base. Most aHUS-associated mutations either destabilize the structure or cluster in a unique region on the surface of FH20. This region is close to, but distinct from, the primary heparin-binding patch of basic residues. By mutating five residues in this region, we show that it is involved, not in heparin, but in C3b binding. Therefore, the majority of the aHUS-associated mutations on the surface of FH19-20 interfere with the interaction between FH and C3b. This obviously leads to impaired control of complement attack on plasma-exposed cell surfaces in aHUS.