Negative and Positive Regulatory Epitopes in the C-Terminal Domains of the Human B1 and B2 Bradykinin Receptor Subtypes Determine Receptor Coupling Efficacy to G9/11-Mediated Phospholipase Cβ Activity

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Abstract
The human B1 bradykinin (BK) receptor (B1R) is more efficacious than the human B2 BK receptor (B2R) in both ligand-independent and agonist-dependent coupling to Gq/11-mediated phospholipase Cβ activity. In fact, B1R is constitutively active, whereas B2R exhibits little if any constitutive activity. To evaluate the role of the C-terminal domain in receptor Gq/11 coupling, we constructed chimeric and C-terminally truncated receptors. The slopes of the increase in basal and agonist-dependent cellular phosphoinositide hydrolysis as a function of receptor density in transiently transfected human embryonic kidney 293 cells provided parameters of receptor coupling. Exchanging the C-terminal domains between the two receptors revealed that these domains are largely responsible for the difference in coupling. B1R truncation showed that this receptor does not directly depend on the C-terminal domain for efficient coupling, although coupling is dramatically augmented by residues in the membrane-distal portion of the domain downstream from Tyr327. On the other hand, coupling of B2R is absolutely dependent on a membrane-proximal epitope in the C-terminal domain upstream from Lys315. This epitope is adjacent to a basic residue, Arg311, which exerts an inhibitory effect on coupling. Arg311 is not conserved in B1R, and complementary mutations in B2R and B1R showed that this residue, together with previously identified serines and threonines, acts to attenuate the coupling efficacy of B2R. Therefore, the C-terminal domain participates intimately in the efficacy of B1R and B2R Gq/11 coupling by contributing both positive and negative regulatory epitopes.