Receptors located on the cell surface are responsible for recognition of extracellular first messengers. The largest number of receptors belongs to a G protein-coupled receptor superfamily. They showed common structural features characterized by seven transmembrane regions and three connecting extracellular and intracellular loops. Stimulation of a receptor by an agonist activates the coupled G protein. From in vitro mutagenesis analyses of β2-adrenergic, muscarinic acetylcholine and α1-adrenergic receptors, it was shown that the intracellular third loop and carboxyl terminus of the receptor molecule are involved in coupling with a G protein. One intriguing observation was that mutation at a single site of the intracellular third loop could induce the active state of receptors without agonist stimulation. Receptor heterogeneity generated from distinct genes or alternative splicing can be seen at the intracellular third loop and carboxyl terminus that is assumed to play an important role of coupling with G proteins. There are several examples that receptor isoforms arising from alternative splicing have their own counterparts of G proteins.
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