Abstract
Previous studies have demonstrated that the internalization of the angiotensin II type 1A receptor (AT1AR) may be mediated by both β-arrestin-sensitive and -insensitive mechanisms. Therefore, we have used the AT1AR carboxyl-terminal tail to screen a rat brain yeast two-hybrid expression library for novel AT1AR-interacting proteins that might contribute to the regulation of AT1AR internalization. We have identified Rab5a as an AT1AR-binding protein that selectively associates with the AT1AR and not with the β2-adrenergic receptor. A Rab5a-S34N mutant defective in GTP binding does not prevent the internalization of the AT1AR but does prevent the trafficking of the AT1AR into larger hollow cored vesicular structures. Agonist activation of the AT1AR promotes both the formation of Rab5a·AT1AR protein complexes and Rab5a GTP binding. Rab5a interactions with the AT1AR are mediated in part by the last 10 amino acid residues of the AT1AR carboxyl-terminal tail, and although a mutant receptor lacking these residues internalizes normally, it does not redistribute into larger hollow vesicles. Our data suggest that AT1AR activation modulates Rab5a activity leading to the homotypic fusion of endocytic vesicles. These observations suggest that vesicular cargo proteins, such as the AT1AR, may control their targeting between intracellular compartments by directly regulating the activity of components of the intracellular trafficking machinery such as Rab5a.