N-terminal Tyrosine Modulation of the Endocytic Adaptor Function of the β-Arrestins

The highly homologous β-arrestin1 and -2 adaptor proteins play important roles in the function of G protein-coupled receptors. Either β-arrestin variant can function as a molecular chaperone for clathrin-mediated receptor internalization. This role depends primarily upon two distinct, contiguous C-terminal β-arrestin motifs recognizing clathrin and the β-adaptin subunit of AP2. However, a molecular basis is lacking to explain the different endocytic efficacies of the two β-arrestin isoforms and the observation that β-arrestin N-terminal substitution mutants can act as dominant negative inhibitors of receptor endocytosis. Despite the near identity of the β-arrestins throughout their N termini, sequence variability is present at a small number of residues and includes tyrosine to phenylalanine substitutions. Here we show that corresponding N-terminal (Y/F)VTL sequences in β-arrestin1 and -2 differentially regulate μ-adaptin binding. Our results indicate that the β-arrestin1 Tyr-54 lessens the interaction with μ-adaptin and moreover is a Src phosphorylation site. A gain of endocytic function is obtained with the β-arrestin1 Y54F substitution, which improves both the β-arrestin1 interaction with μ-adaptin and the ability to enhance β2-adrenergic receptor internalization. These data indicate that β-arrestin2 utilizes μ-adaptin as an endocytic partner, and that the inability of β-arrestin1 to sustain a similar degree of interaction with μ-adaptin may result from coordination of Tyr-54 by neighboring residues or its modification by Src kinase. Additionally, these naturally occurring variations in β-arrestins may also differentially regulate the composition of the signaling complexes organized on the receptor.