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Vol. 61, Issue 5, 1041-1052, May 2002
Center for Basic Research in Digestive Diseases, Departments of
Internal Medicine and Biochemistry/Molecular Biology, Mayo Clinic and
Foundation, Rochester, Minnesota (X.-Q.D., L.J.M., D.I.P.); and
Department of Chemistry and Center for Structural Biology, Vanderbilt
University, Nashville, Tennessee (K.E.F., T.P.L.)
Insight into the molecular basis of cholecystokinin (CCK) binding to
its receptor has come from receptor mutagenesis and photoaffinity labeling studies, with both contributing to the current hypothesis that
the acidic Tyr-sulfate-27 residue within the peptide is situated adjacent to basic Arg197 in the second loop of the
receptor. Here, we refine our understanding of this region of
interaction by examining a structure-activity series of these positions
within both ligand and receptor and by performing three-dimensional
molecular modeling of key pairs of modified ligand and receptor
constructs. The important roles of Arg197 and
Tyr-sulfate-27 were supported by the marked negative impact on binding
and biological response with their natural partner molecule when the
receptor residue was replaced by acidic Asp or Glu and when the peptide
residue was replaced by basic Arg, Lys, p-amino-Phe,
p-guanidino-Phe, or p-methylamino-Phe.
Complementary ligand-receptor charge-exchange experiments were unable
to regain the lost function. This was supported by the molecular
modeling, which demonstrated that the charge-reversed double mutants
could not form a good interaction without extensive rearrangement of receptor conformation. The models further predicted that R197D and
R197E mutations would lead to conformational changes in the extracellular domain, and this was experimentally supported by data
showing that these mutations decreased peptide agonist and antagonist
binding and increased nonpeptidyl antagonist binding. These receptor
constructs also had increased susceptibility to trypsin degradation
relative to the wild-type receptor. In contrast, the relatively
conservative R197K mutation had modest negative impact on peptide
agonist binding, again consistent with the modeling demonstration of
loss of a series of stabilizing inter- and intramolecular bonds. The
strong correlation between predicted and experimental results support
the reported refinement in the three-dimensional structure of the
CCK-occupied receptor.
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