Interruption of the ionic lock in the bradykinin B2 receptor results in constitutive internalization and turns several antagonists into strong agonists.

The DRY motif with the highly conserved R3.50 is a hallmark of family A G protein-coupled receptors (GPCRs). The crystal structure of rhodopsin revealed a salt bridge between R135(3.50) and another conserved residue, E247(6.30), in helix 6. This ionic lock was shown to maintain rhodopsin in its inactive state. Thus far, little information is available on how interruption of this ionic bond affects signaling properties of nonrhodopsin GPCRs, because the focus has been on mutations of R3.50, although this residue is indispensable for G protein activation. To investigate the importance of an ionic lock for overall receptor activity in a nonrhodopsin GPCR, we mutated R128(3.50) and E238(6.30) in the bradykinin (BK) B(2) receptor (B(2)R) and stably expressed the constructs in HEK293 cells. As expected, mutation of R3.50 resulted in lack of G protein activation. In addition, this mutation led to considerable constitutive receptor internalization. Mutation of E6.30 (mutants E6.30A and E6.30R) also caused strong constitutive internalization. Most intriguingly, however, although the two E6.30 mutants displayed no increased basal phosphatidylinositol hydrolysis, they gave a response to three different B(2)R antagonists that was almost comparable to that obtained with BK. In contrast, swapping of R3.50 and E6.30, thus allowing the formation of an inverse ionic bond, resulted in rescue of the wild type phenotype. These findings demonstrate for the first time, to our knowledge, that interruption of the ionic lock in a family A GPCR can have distinctly different effects on receptor internalization and G protein stimulation, shedding new light on its role in the activation process.

Alanine screening of the intracellular loops of the human bradykinin B receptor--effects on receptor maintenance, G protein activation and internalization.

The bradykinin B(2) receptor is coupled to G protein G(q/11) and becomes sequestered into intracellular compartments after activation. To more closely define the receptor sequences involved in these processes and their functions, we systematically mutated all three intracellular loops (ICLs), either as point mutations or in groups of three to five amino acids to Ala, obtaining a total of 14 mutants. All constructs were stably expressed in HEK 293 cells and, with the exception of triple mutant DRY --> AAA, retained the ability to specifically bind [(3)H]bradykinin. The binding affinities at 4 or 37 degrees C of several mutants differed considerably from those determined for the wild-type receptor, indicating an allosteric connection between the conformation of the binding site and that of the ICLs. Mutations in ICL-1 strongly reduced surface expression without affecting G protein signaling or [(3)H]bradykinin internalization. Two cluster mutants in the middle of ICL-2 containing basic residues displayed considerably reduced potencies, whereas two mutations in ICL-3 resulted in receptor conformations that were considered to be semi-active, based on the observation that they responded with phosphoinositide hydrolysis to compounds normally considered to be antagonists. This, and the fact that a cluster mutant at the C-terminal end of ICL-3 was signaling incompetent, hint at the involvement of ICL-2 and ICL-3 in G(q/11) activation, albeit with different functions. None of the mutants displayed reduced ligand-induced receptor internalization, indicating that the loops are not essential for this process. No conclusion could be drawn, however, with regard to the role of the DRY sequence, as the corresponding triplet mutation lacked binding capability.