Signaling through ␤-arrestins is a recently appreciated mechanism used by seven-transmembrane receptors. Because G protein-coupled receptor kinase (GRK) phosphorylation of such receptors is generally a prerequisite for ␤-arrestin binding, we studied the roles of different GRKs in promoting ␤-arrestin-mediated extracellular signal-regulated kinase (ERK) activation by a typical seven-transmembrane receptor, the Gs-coupled V2 vasopressin receptor. Gsand ␤-arrestin-mediated pathways to ERK activation could be distinguished with H89, an inhibitor of protein kinase A, and ␤-arrestin 2 small interfering RNA, respectively. The roles of GRK2, -3, -5, and -6 were assessed by suppressing their expression with specific small interfering RNA sequences. By using this approach, we demonstrated that GRK2 and -3 are responsible for most of the agonist-dependent receptor phosphorylation, desensitization, and recruitment of ␤-arrestins. In contrast, GRK5 and -6 mediated much less receptor phosphorylation and ␤-arrestin recruitment, but yet appeared exclusively to support ␤-arrestin 2-mediated ERK activation. GRK2 suppression actually increased ␤-arrestin-stimulated ERK activation. These results suggest that ␤-arrestin recruited in response to receptor phosphorylation by different GRKs has distinct functional potentials. extracellular signal-regulated kinase ͉ phosphorylation ͉ desensitization ͉ small interfering RNA

Binding of Sonic Hedgehog (Shh) to Patched (Ptc) relieves the latter's tonic inhibition of Smoothened (Smo), a receptor that spans the cell membrane seven times. This initiates signaling which, by unknown mechanisms, regulates vertebrate developmental processes. We find that two molecules interact with mammalian Smo in an activation-dependent manner: G protein-coupled receptor kinase 2 (GRK2) leads to phosphorylation of Smo, and beta-arrestin 2 fused to green fluorescent protein interacts with Smo. These two processes promote endocytosis of Smo in clathrin-coated pits. Ptc inhibits association of beta-arrestin 2 with Smo, and this inhibition is relieved in cells treated with Shh. A Smo agonist stimulated and a Smo antagonist (cyclopamine) inhibited both phosphorylation of Smo by GRK2 and interaction of beta-arrestin 2 with Smo. beta-Arrestin 2 and GRK2 are thus potential mediators of signaling by activated Smo.