Abstract

We examined the functional significance of two residues present in the second (Asp100) and seventh (Asn391) transmembrane domains of the rat cholecystokininB (CCKB) receptor that are highly conserved among the members of the G protein-coupled receptor family. Substitution of Asn for Asp100 by using site-directed mutagenesis did not change the affinity and selectivity for agonists but slightly increased the affinity of three CCKB-selective antagonists of different chemical structures. Cells expressing the mutant receptor exhibited a 50% reduction in CCKB-induced phosphoinositide turnover compared with cells expressing the wild-type receptor, suggesting a critical role for this residue in the coupling of the CCKB receptor to G protein. This latter was shown to be insensitive to pertussis toxin treatment and could therefore belong to the Gq family. Replacement of Asn391 by Asp located in the seventh transmembrane domain did not change agonist binding or phosphoinositide turnover. This suggests that in contrast to the gonadotropin-releasing hormone receptor, there is no direct interaction in the CCKB receptor between Asp100 and Asn391. However, a rhodopsin-based molecular modeling of the CCKB receptor showed a spatial proximity between Asp100 and the carboxyl terminal part of the third intracellular loop, known to interact with G protein. This could explain the reduction in phosphoinositide turnover observed with the Asn100 mutant.