Constitutive activation of tethered-peptide/corticotropin-releasing factorreceptor chimeras

Constitutive activity, or ligand-independent activity, of mutant G protein- coupled receptors (GPCRs) has been described extensively and implicated in the pathology of many diseases. Using the corticotropin-releasing factor (C RF) receptor and the thrombin receptor as a model, we present a ligand-depe ndent constitutive activation of a GPCR. A chimera in which the N-terminal domain of the CRF receptor is replaced by the amino-terminal 14 residues of CRF displays significant levels of constitutive activation. The activity, as measured by intracellular levels of cAMP, is blocked in a dose-dependent manner by the nonpeptide antagonist antalarmin. These results support a pr opinquity effect in CRF receptor activation, in which the amino-terminal po rtion of the CRF peptide is presented to the body of the receptor in the pr oper proximity for activation. This form of ligand-dependent constitutive a ctivation may be of general applicability for the creation of constitutivel y activated GPCRs that are regulated by peptide ligands such as CRF. These chimeras may prove useful in analyzing mechanisms of receptor regulation an d in the structural analysis of ligand-activated receptors.