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Vol. 53, Issue 5, 878-885, May 1998
Département de Pharmacochimie Moléculaire et
Structurale, Institut National de la Santé et de la Recherche
Médicale U266-Centre National de la Recherche Scientifique,
Unité de Recherche Associée D1500, Unité de Formation et de
Recherche des Sciences Pharmaceutiques et Biologiques, 75270 Paris
Cedex 06, France
The functional significance of the extracellular amino-terminal region
and of three highly conserved aromatic residues present in the fifth
(TM-V) and sixth (TM-VI) transmembrane domains of the rat
cholecystokinin (CCK)B receptor, transfected in Cos-7 cells, was investigated by site-directed mutagenesis. The
amino-terminal region of the CCKB receptor seemed to be
weakly involved in CCK binding in that the affinities of
CCK8 and selective agonists and antagonists were not
modified by truncation of this region. Substitution of Phe347 in TM-VI
with alanine produced a mutant receptor that displays the same affinity
and selectivity as the wild-type receptor for agonists, but a slightly
increased affinity for the selective CCKB antagonist
L-365,260. However, the addition of saturating CCK8
concentrations to cells expressing this mutant did not result in the
production of inositol phosphates, demonstrating the critical role of
Phe347 in CCKB receptor to G protein coupling. Substitution
of Phe227 with alanine was without effect on the affinities of
CCKB ligands and on phosphoinositide turnover but modified
the affinity of the CCKA antagonist L-364,718. Residue Trp351 located within the CCKB receptor TM-VI is involved
in the binding of CCK8 and CCK4 and of the
CCK4-based antagonist PD-134,308, as illustrated by the
decreased affinities of these ligands in W351A mutant. The lower
affinity for CCK8 observed with this mutated CCKB receptor accounts for the higher EC50
value for phosphotidylinositol hydrolysis. This study suggests that at
least part of the binding site for the agonist is located inside the
transmembrane domain of the CCKB receptor, partially
overlapping that of antagonists, and gives new insights into the
regions involved in the transduction process.
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