The beta 1 and beta 2 adrenergic receptors are members of the super-family of G protein-coupled receptors (GPCRs). This receptor super-family comprises some of the most important pharmacological targets for the treatment of cardiovascular (e.g. heart failure, hypertension) and pulmonary diseases (e.g. asthma). A major cause for the observed attenuation of adrenergic response in heart failure and asthma is receptor downregulation, which results from chronic agonist exposure. Recently, ubiquitination of cell-surface receptors has been implicated as an important mechanism for post-endocytic sorting to lysosomes. Ubiquitination is a post-translational modification of proteins orchestrated by a well-defined process involving a cascade of three enzymatic activities. Of these the final step, catalyzed by the enzyme, E3 ubiquitin ligase, determines the specificity of substrate ubiquitination. We have demonstrated that two representative GPCRs, the beta2 adrenergic receptor and the V2 vasopressin receptor and their adaptor protein beta-arrestin become ubiquitinated upon agonist stimulation. Ubiquitination of beta-arrestin is crucial for receptor internalization, whereas ubiquitination of the receptor is essential for the proper sorting and downregulation of the internalized and activated receptors. We hypothesize that ubiquitination and deubiquitination coordinate the trafficking and signaling of GPCRs and involve a specific set of endocytic adapter proteins including beta-arrestins. The specific aims are: 1) to define the molecular mechanisms of receptor ubiquitination leading to receptor downregulation, and 2) to determine the roles of deubiquitinating enzyme(s) in receptor trafficking. Unraveling the molecular mechanisms governing the regulation of GPCRs, especially beta ARs, by ubiquitination could have a great impact on the development of novel therapeutic strategies for cardiovascular and pulmonary diseases.