Rab5 association with the angiotensin II type 1A receptor promotes Rab5 GTP binding and vesicular fusion.

Previous studies have demonstrated that the internalization of the angiotensin II type 1A receptor (AT(1A)R) may be mediated by both beta-arrestin-sensitive and -insensitive mechanisms. Therefore, we have used the AT(1A)R carboxyl-terminal tail to screen a rat brain yeast two-hybrid expression library for novel AT(1A)R-interacting proteins that might contribute to the regulation of AT(1A)R internalization. We have identified Rab5a as an AT(1A)R-binding protein that selectively associates with the AT(1A)R and not with the beta2-adrenergic receptor. A Rab5a-S34N mutant defective in GTP binding does not prevent the internalization of the AT(1A)R but does prevent the trafficking of the AT(1A)R into larger hollow cored vesicular structures. Agonist activation of the AT(1A)R promotes both the formation of Rab5a.AT(1A)R protein complexes and Rab5a GTP binding. Rab5a interactions with the AT(1A)R are mediated in part by the last 10 amino acid residues of the AT(1A)R 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 AT(1A)R activation modulates Rab5a activity leading to the homotypic fusion of endocytic vesicles. These observations suggest that vesicular cargo proteins, such as the AT(1A)R, may control their targeting between intracellular compartments by directly regulating the activity of components of the intracellular trafficking machinery such as Rab5a.