G Protein-coupled Receptors

Nearly 2000 G protein-coupled receptors (GPCRs)1 have been reported since bovine opsin was cloned in 1983 (1) and the β-adrenergic receptor in 1986 (2). They are classified into over 100 subfamilies according to the sequence homology, ligand structure, and receptor function. A substantial degree of amino acid homology is found among members of a particular subfamily, but comparisons between subfamilies show significantly less or no similarity.