TY - JOUR T1 - The D<sub>1</sub> Dopamine Receptor Is Constitutively Phosphorylated by G Protein-Coupled Receptor Kinase 4 JF - Molecular Pharmacology JO - Mol Pharmacol SP - 759 LP - 769 DO - 10.1124/mol.105.019901 VL - 69 IS - 3 AU - Michele L. Rankin AU - Paul S. Marinec AU - David M. Cabrera AU - Zheng Wang AU - Pedro A. Jose AU - David R. Sibley Y1 - 2006/03/01 UR - http://molpharm.aspetjournals.org/content/69/3/759.abstract N2 - G protein-coupled receptor (GPCR) kinases (GRKs) phosphorylate agonist-activated GPCRs, initiating their homologous desensitization. In this article, we present data showing that GRK4 constitutively phosphorylates the D1 receptor in the absence of agonist activation. This constitutive phosphorylation is mediated exclusively by the α isoform of GRK4; the β, γ, and δ isoforms are ineffective in this regard. Mutational analysis reveals that the constitutive phosphorylation mediated by GRK4α is restricted to the distal region of the carboxyl terminus of the receptor, specifically to residues Thr428 and Ser431. Phosphorylation of the D1 receptor by GRK4α results in a decrease in cAMP accumulation, an increase in receptor internalization, and a decrease in total receptor number—all of which are abolished in a D1 receptor mutant containing T428V and S431A. The increase in internalized D1 receptors induced by GRK4α phosphorylation is due to enhanced receptor internalization rather than retarded trafficking of newly synthesized receptors to the cell surface. The constitutive phosphorylation of the D1 receptor by GRK4α does not alter agonist-induced desensitization of the receptor because dopamine pretreatment produced a similar decrease in cAMP accumulation in control cells versus cells expressing GRK4α. These observations shift the attenuation of D1 receptor signaling from a purely agonist-driven process to one that is additionally modulated by the complement of kinases that are coexpressed in the same cell. Furthermore, our data provide direct evidence that, in contrast to current dogma, GRKs can (at least in some instances) constitutively phosphorylate GPCRs in the absence of agonist activation resulting in constitutive desensitization. ER -