The human B1 bradykinin receptor exhibits high ligand-independent, constitutive activity - Roles of residues in the fourth intracellular and third transmembrane domains

The B1 bradykinin (BK) receptor (B1R) is a seven-transmembrane domain, G protein-coupled receptor that is induced by injury and important in inflammation and nociception. Here, we show that the human B1R exhibits a high level of ligand-independent, constitutive activity. Constitutive activity was identified by the increase in basal cellular phosphoinositide hydrolysis as a function of the density of the receptors in transiently transfected HEK293 cells. Several B1R peptide antagonists were neutral antagonists or very weakly efficacious inverse agonists, Constitutive B1R activity was further increased by alanine mutation of Asn(121) in the third transmembrane domain of the receptor (B1A(121)). This mutant resembled the agonist preferred receptor state since it also exhibited increased agonist affinity and decreased agonist responsiveness. A dramatic loss of constitutive activity occurred when the fourth intracellular C-terminal domain (IC-IV) of the human B2 BK receptor subtype (B2R), which exhibits minimal constitutive activity, was substituted in either B1R or B1A121 to make B1(B2ICIV) and B1(B2ICIV)A(121), respectively. Activity was partially recovered by subsequent alanine mutation of a cluster of two serines and two threonines in IC-IV of either B1(B2ICIV) or B1(B2ICIV)A121, a cluster that is important for B2R desensitization. The ligand-independent, constitutive activity of B1R therefore depends on epitopes in both transmembrane and intracellular domains. We propose that the activity is primarily due to the lack of critical epitopes in IC-IV that regulate such activity.