ABSTRACT
G protein-coupled receptors (GPCRs) represent the single largest class of membrane proteins in the human genome and they are the largest group of targets for the pharmaceutical industry. GPCRs share a common structural signature of seven hydrophobic segments predicted to be membrane-spanning domains with an extracellular amino terminus and an intracellular carboxyl terminus (Fig. 5.1). While the vast majority of GPCRs have been shown to activate one or more cytoplasmic heterotrimeric GTPbinding proteins (G proteins), there is now considerable evidence that some GPCRs can activate signaling pathways that don’t involve heterotrimeric G proteins.1,2 For this reason, the terms seven transmembrane (TM) receptor or heptahelical receptor have also been used in place of GPCR.
