The G protein–coupled receptors (GPCRs) constitute a large and ancient superfamily of integral cell membrane proteins that play a central role in signal transduction and are activated by an equally diverse array of ligands. GPCRs share a seven hydrophobic α-helical domain structure and transduce signals through coupling to guanine nucleotide–binding regulatory proteins (G proteins). The seven hydrophobic domains are likely to span the membrane and are linked by three extracellular loops that alternate with three intracellular loops. The extracellular NH2 terminus is usually glycosylated and the cytoplasmic COOH terminus is generally phosphorylated. The presence of a large diversity of GPCR genes may be a characteristic of eukaryotic genomes since >1,000 GPCRs have been identified in the Caenorhabditis elegans genome, representing >5% of its total number of genes (Bargmann 1998).
The completion of the sequencing of the...
