The Frizzled-1/β2-adrenergic receptor chimera: pharmacological properties of a unique G protein-linked receptor
The frizzled gene family of Wnt receptors encodes proteins that have a seven-transmembrane-spanning motif characteristic of G-protein-coupled receptors. Using a chimeric receptor composed of the exofacial and the transmembrane, ligand-binding domain of the β2-adrenergic receptor (β2AR) fused with the corresponding cytoplasmic domains of the rat Frizzled-1 receptor (Rfz1), we created a unique chimera between distant members of the superfamily of G protein-coupled receptors. Herein, we describe the pharmacological properties of the chimera, which represents a receptor in which the exofacial and cytoplasmic domains are minimized in length. This unique chimera retains much of the pharmacological character of the native β2AR, whereas the coupling can be ascribed to Rfz1 domains which operate via Gαq and not Gαs. The Rfz1 chimera demonstrates a robust agonist (isoproterenol)-induced sequestration. Since the Rfz1 cytoplasmic domains possess canonical sites for several protein kinases, we were able to investigate the effects of kinase inhibitors on Rfz1 chimera sequestration. Only the protein kinase A inhibitor KT5720, but not inhibitors of protein kinase C, calcium/calmodulin-sensitive kinase-2, casein kinase-2, and Src, inhibited agonist-induced sequestration. Expression of a dominant-negative form of β-arrestin blocked sequestration of the β2AR, but only reduced modestly that of the Rfz1 chimera. These data demonstrate that the Frizzled-1 chimera displays cardinal features of a G protein-coupled receptor, including agonist-induced sequestration, but appears to do so largely even in the presence of dominant-negative β-arrestins.
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