The role of helix 8 and of the cytosolic C-termini in the internalization and signal transduction of B(1) and B(2) bradykinin receptors.

Determinants for desensitization and sequestration of G protein-coupled receptors often contain serine or threonine residues located in their C-termini. The sequence context, however, in which these residues have to appear, and the receptor specificity of these motifs are largely unknown. Mutagenesis studies with the B(2) bradykinin receptor (B(2)wt), stably expressed in HEK 293 cells, identified a sequence distal to N338 (NSMGTLRTSI, including I347 but not the basally phosphorylated S348) and in particular the TSI sequence therein, as a major determinant for rapid agonist-inducible internalization and the prevention of receptor hypersensitivity. Chimeras of the noninternalizing B(1) bradykinin receptor (B(1)wt) containing these B(2)wt sequences sequestered poorly, however, suggesting that additional motifs more proximal to N338 are required. In fact, further substitution of the B(1)wt C-terminus with corresponding B(2)wt regions either at C330(7.71) following putative helix 8 (B(1)CB(2)) or at the preceding Y312(7.53) in the NPXXY sequence (B(1)YB(2)) resulted in chimeras displaying rapid internalization. Intriguingly, however, exchange performed at K322(7.63) within putative helix 8 generated a slowly internalizing chimera (B(1)KB(2)). Detailed mutagenesis analysis generating additional chimeras identified the change of V323 in B(1)wt to serine (as in B(2)wt) as being responsible for this effect. The slowly internalizing chimera as well as a B(1)wt point-mutant V323S displayed significantly reduced inositol phosphate accumulation as compared to B(1)wt or the other chimeras. The slow internalization of B(1)KB(2) was also accompanied by a lack of agonist-induced phosphorylation, that in contrast was observed for B(1)YB(2) and B(1)CB(2), suggesting that putative helix 8 is either directly or indirectly (e.g. via G protein activation) involved in the interaction between the receptor and receptor kinases.