beta-arrestin-mediated ADP-ribosylation factor 6 activation and beta(2)-adrenergic receptor endocytosis

beta -Arrestins are multifunctional adaptor proteins known to regulate inte rnalization of agonist-stimulated G protein-coupled receptors by linking th em to endocytic proteins such as clathrin and AP-2. Here we describe a prev iously unappreciated mechanism by which beta -arrestin orchestrates the pro cess of receptor endocytosis through the activation of ADP-ribosylation fac tor 6 (ARF6), a small GTP-binding protein. Involvement of ARF6 in the endoc ytic process is demonstrated by the ability of GTP-binding defective and GT P hydrolysis-deficient mutants to inhibit internalization of the beta (2)-a drenergic receptor. The importance of regulation of ARF6 function is shown by the ability of the ARF GTPase-activating protein GIT1 to inhibit and of the ARF nucleotide exchange factor, ARNO, to enhance receptor endocytosis. Endogenous beta -arrestin is found in complex with ARNO. Upon agonist stimu lation of the receptor, beta -arrestin also interacts with the GDP-liganded form of ARF6, thereby facilitating ARNO-promoted GTP loading and activatio n of the G protein. Thus, the agonist-driven formation of a complex includi ng beta -arrestin, ARNO, and ARF6 provides a molecular mechanism that expla ins how the agonist-stimulated receptor recruits a small G protein necessar y for the endocytic process and controls its activation.