The ?2-adrenergic receptor (?2AR) is central in normal lung physiology and disease, but key questions about its regulation remain unanswered. Since ?2AR signaling causes airway smooth muscle relaxation, agonists of the receptor are a mainstay therapy for asthma and COPD. Receptor protein down-regulation is a critical mechanism for controlling ?2AR, and works through ubiquitination and subsequent degradation of ?2AR upon prolonged agonist stimulation. This homeostatic mechanism prevents potentially detrimental persistent activation but comes with a price: the efficacy of ?-agonist therapy is often reduced by the very same process. A better understanding of ?2AR degradation would thus benefit both basic receptor biology and translational ?-agonist therapy. Using a unique genome- wide RNA interference (RNAi) screen, we discovered ?-arrestin3, which is encoded by a novel human gene ARRDC3 (arrestin domain containing 3), as a critical new component of the ?2AR degradation machinery. Alpha-arrestin-3 co-localizes and interacts with ?2AR in an agonist-dependent manner. Through a specific interaction, ?-arrestin3 recruits the ubiquitin E3 ligase NEDD4-1 to activated ?2AR to mediate the receptor ubiquitination and subsequent degradation. Building upon these molecular studies, we now propose to examine the function of ?-arrestin3-mediated ?2AR regulation pathway at the physiologic and human biology levels. We hypothesize that ?-arrestin3-mediated ?2AR degradation plays an important physiological role in the lung and that perturbation of the degradation pathway impacts all aspects of functional outcomes of ?2AR signaling. The proposal is to test this hypothesis with three Aims. In Aim 1 we will determine the role of ?-arrestin3 in regulating the physiological function of ?2AR in airway smooth cells. Aim 2 will test the role of ?-arrestin3 in regulating the bronchoprotection provided by -agonist administration in mice in vivo. Aim 3 will investigate the association of genetic variations in the ?-arrestin3-mediated ?2AR degradation pathway with bronchodilator response (BDR) in asthma patients. This highly integrative project combines molecular and cellular biology, animal model, and pharmacogenetics and will establish a critical role for ?-arrestin3 in regulating ?2AR functions in the lung. Moreover, the results will lead to a better understanding of the genetic basis of individual ?-agonist responses, and may ultimately contribute to the development of improved and personalized ?-agonist therapy for asthma and other chronic lung diseases.