The bombesin receptor subtypes have distinct G protein specificities

We used an in situ reconstitution assay to examine the receptor coupling to purified G protein alpha subunits by the bombesin receptor family, includi ng 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 G alpha(q) and mouse G alpha(q) but not bovine retinal G alpha(t) or bovine brain G alpha(i/o). The GRP-R- and NMB-R-catalyzed activations of Ga, Were dependent upon and enhanced by different py dimers in the same rank order as follows: bovine brain beta ga mma > beta(1)gamma(2) >> beta(1)gamma(1). Despite these qualitative similar ities, GRP-R and NMB-R had distinct kinetic properties in receptor-G protei n coupling. GRP-R had higher affinities for bovine brain beta gamma, beta(1 )gamma(1), and beta(1)gamma(2) and squid retinal G alpha(q). In addition, G RP-R showed higher catalytic activity on squid G alpha(q). Like GRP-R and N MB-R, BRS-3 did not catalyze GTP gamma S binding to G alpha(i/o) or G alpha . However, BRS-3 showed little, if any, coupling with squid G alpha(q) but clearly activated mouse G alpha(q). GRP-R and NMB-R catalyzed GTP gamma S b inding to squid and mouse G alpha(q), with GRP-R activating squid G alpha(q ) more effectively, and NMB-R also showed slight preference for squid G alp ha(q). These studies reveal that the structurally similar bombesin receptor subtypes, in particular BRS-3, possess distinct coupling preferences among members of the G alpha(q) family.