The bombesin receptor subtypes have distinct G protein specificities

Abstract

We used an in situ reconstitution assay to examine the receptor coupling to purified G protein α subunits by the bombesin receptor family, including gastrin-releasing peptide receptor (GRP-R), neuromedin B receptor (NMB-R), and bombesin receptor subtype 3 (BRS-3). Cells expressing GRP-R or NMB-R catalyzed the activation of squid retinal Gα(q) and mouse Gα(q) but not bovine retinal Gα(t) or bovine brain Gα(i/o). The GRP-R- and NMB-R- catalyzed activations of Gα(q) were dependent upon and enhanced by different βγ dimers in the same rank order as follows: bovine brain βγ > β1γ2 >> β1γ1. Despite these qualitative similarities, GRP-R and NMB-R had distinct kinetic properties in receptor-G protein coupling. GRP-R had higher affinities for bovine brain βγ, β1γ1, and β1γ2 and squid retinal Gα(q). In addition, GRP-R showed higher catalytic activity on squid Gα(q). Like GRP-R and NMB-R, BRS-3 did not catalyze GTPγS binding to Gα(i/o) or Gα(t). However, BRS-3 showed little, if any, coupling with squid Gα(q) but clearly activated mouse Gα(q). GRP-R and NMB-R catalyzed GTPγS binding to both squid and mouse Gα(q), with GRP-R activating squid Gα(q) more effectively, and NMB-R also showed slight preference for squid Gα(q). These studies reveal that the structurally similar bombesin receptor subtypes, in particular BRS-3, possess distinct coupling preferences among members of the Gα(q) family.