Regulation of receptor fate by ubiquitination of activated beta(2)-adrenergic receptor and beta-arrestin

Although trafficking and degradation of several membrane proteins are regul ated by ubiquitination catalyzed by E3 ubiquitin ligases, there has been li ttle evidence connecting ubiquitination with regulation of mammalian G prot ein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (G PCR) function. Agonist stimulation of endogenous or transfected beta (2)-ad renergic receptors (beta (2)ARs) led to rapid ubiquitination of both the re ceptors and the receptor regulatory protein, beta -arrestin. Moreover, prot easome inhibitors reduced receptor internalization and degradation, thus im plicating a role for the ubiquitination machinery in the trafficking of the beta (2)AR. Receptor ubiquitination required beta -arrestin, which bound t o the E3 ubiquitin ligase Mdm2. Abrogation of beta -arrestin ubiquitination , either by expression in Mdm2-null cells or by dominant-negative forms of Mdm2 lacking E3 ligase activity, inhibited receptor internalization with ma rginal effects on receptor degradation. However, a beta (2)AR mutant tackin g lysine residues, which was not ubiquitinated, was internalized normally b ut was degraded ineffectively. These findings delineate an adapter role of beta -arrestin in mediating the ubiquitination of the beta (2)AR and indica te that ubiquitination of the receptor and of beta -arrestin have distinct and obligatory roles in the trafficking and degradation of this prototypic GPCR.