G protein-coupled receptor inactivation by an allosteric inverse-agonist antibody

G protein-coupled receptors (GPCRs) are the largest class of cell-surface receptors, and these membrane proteins exist in equilibrium between inactive and active states.(1-13) Conformational changes induced by extracellular ligands binding to GPCRs result in a cellular response through the activation of G-proteins. The A(2A) adenosine receptor (A(2A)AR) is responsible for regulating blood flow to the cardiac muscle and is important in the regulation of glutamate and dopamine release in the brain.(14) In this study, we have successfully raised a mouse monoclonal antibody against human A(2A)AR that prevents agonist but not antagonist binding to the extracellular ligand-binding pocket. The structure of the A(2A)AR-antibody Fab fragment (Fab2838) complex reveals that the fragment, unexpectedly, recognises the intracellular surface of A(2A)AR and that its complementarity determining region, CDR-H3, penetrates into the receptor. CDR-H3 is located in a similar position to the G-protein C-terminal fragment in the active opsin structure(1) and to the CDR-3 of the nanobody in the active β(2) adrenergic receptor structure(2) but locks the A(2A)AR in an inactive conformation. These results shed light on a novel strategy to modulate GPCR activity.