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Vol. 63, Issue 5, 961-972, May 2003
Departments of Biochemistry (J.A.G., A.B., B.L.R.), Psychiatry
(B.L.R.), and Neurosciences (B.L.R.), Case Western Reserve University
School of Medicine, Cleveland, Ohio; and Department of Pharmacology,
Vanderbilt University, Nashville, Tennessee (V.V.G.)
5-HT2A serotonin receptors are unusual among G-protein
coupled receptors in that they can be internalized and desensitized, in
some cell types, in an arrestin-independent manner. The molecular basis
of the arrestin-insensitivity of 5-HT2A receptors is
unknown but is probably caused, in part, by the apparent lack of
agonist-induced 5-HT2A receptor phosphorylation. Because
the arrestin-insensitivity of 5-HT2A receptors is cell-type
selective, we used a "constitutively active" arrestin mutant that
can interact with agonist-activated but nonphosphorylated receptors. We
show here that this "constitutively active" arrestin mutant
(Arr2-R169E) can force 5-HT2A receptors to be regulated by
arrestins. Cotransfection of 5-HT2A receptors with
Arr2-R169E induced agonist-independent 5-HT2A receptor
internalization, and a constitutive translocation of the Arr2-R169E
mutant to the plasma membrane, whereas wild-type Arrestin-2 had no
effect. Additionally, Arr2-R169E, unlike wild-type arrestin-2, induced
a significant decrease in efficacy of agonist-induced phosphoinositide
hydrolysis with an unexpected increase in agonist potency. Radioligand
binding assays demonstrated that the fraction of receptors in the
high-affinity agonist binding-state increased with expression of
Arr2-R169E, indicating that Arr2-R169E stabilizes the agonist-high
affinity state of the 5-HT2A receptor (R*). Intriguingly,
the agonist-independent interaction of Arr2-R169E with
5-HT2A receptors was inhibited by inverse agonist treatment
and is thus probably caused by the high level of 5-HT2A
receptor constitutive activity. This is the first demonstration that a
constitutively active arrestin mutant can both induce
agonist-independent internalization and stabilize the agonist-high
affinity state of an arrestin-insensitive G protein coupled receptor.
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