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Vol. 293, Issue 3, 735-746, June 2000
Departments of Biochemistry (K.K., W.K.K., E.I.G., J.E.S., B.L.R.),
Psychiatry (D.L.W., B.L.R.), and Neurosciences (B.L.R.), Case Western
Reserve University Medical School, Cleveland, Ohio; and Department of
Medicinal Chemistry, Medical College of Virginia, Richmond, Virginia
(R.A.G.)
Discovering the molecular and atomic mechanism(s) by which
G-protein-coupled receptors (GPCRs) are activated by agonists remains an elusive goal. Recently, studies examining two representative GPCRs
(rhodopsin and
1b-adrenergic receptors) have suggested that the disruption of a putative "salt-bridge" between highly conserved residues in transmembrane (TM) helix III, involving aspartate
or glutamate, and helix VII, involving a basic residue, results in
receptor activation. We have tested whether this is a general mechanism
for GPCR activation by constructing a model of the 5-hydroxytryptamine
(5-HT)2A receptor and characterizing several mutations at
the homologous residues (Asp-155 and Asn-363) of the 5-HT2A
serotonin receptor. All of the mutants (D155A, D155N, D155E, D155Q, and
S363A) resulted in receptors with reduced basal activity; in no case
was evidence for constitutive activity revealed. Structure-function
studies with tryptamine analogs and various Asp-155 mutants
demonstrated that Asp-155 interacts with the terminal, and not indole,
amine moiety of 5-HT2A agonists. Interestingly, the D155E
mutation interfered with the membrane targeting of the 5-HT2A receptor, and an inverse relationship was discovered
when comparing receptor activation and targeting for a series of
Asp-155 mutants. This represents the first known instance in which a
charged residue located in a putative TM helix alters the membrane
targeting of a GPCR. Thus, for 5-HT2A receptors, the
TMIII aspartic acid (Asp-155) is involved in anchoring the
terminal amine moiety of indole agonists and in membrane targeting and
not in receptor activation by salt-bridge disruption.
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