TI - N - terminal tyrosine modulation of the endocytic adaptor function of the beta-arrestins . AB - The highly homologous beta-arrestin1 and -2 adaptor proteins play important roles in the function of G protein -coupled receptors . Either beta-arrestin variant can function as a molecular chaperone for clathrin-mediated receptor internalization . This role depends primarily upon two distinct , contiguous C-terminal beta-arrestin motifs recognizing clathrin and the beta-adaptin subunit of AP2 . However , a molecular basis is lacking to explain the different endocytic efficacies of the two beta-arrestin isoforms and the observation that beta-arrestin N-terminal substitution mutants can act as dominant negative inhibitors of receptor endocytosis . Despite the near identity of the beta-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 beta-arrestin1 and -2 differentially regulate mu-adaptin binding . Our results indicate that the beta-arrestin1 Tyr-54 lessens the interaction with mu-adaptin and moreover is a Src phosphorylation site . A gain of endocytic function is obtained with the beta-arrestin1 Y54F substitution , which improves both the beta-arrestin1 interaction with mu-adaptin and the ability to enhance beta2-adrenergic receptor internalization . These data indicate that beta-arrestin2 utilizes mu-adaptin as an endocytic partner , and that the inability of beta-arrestin1 to sustain a similar degree of interaction with mu-adaptin may result from coordination of Tyr-54 by neighboring residues or its modification by Src kinase . Additionally , these naturally occurring variations in beta-arrestins may also differentially regulate the composition of the signaling complexes organized on the receptor .