Unconventional homologous internalization of the angiotensin II type-1 receptor induced by G-protein-independent signals

Internalization of a G-protein-coupled receptor (GPCR) is essential to the desensitization, endocytosis, and signal transduction of the receptor. It has been the general view that conventional homologous internalization of a GPCR requires activation of the G-protein(s) coupled to the receptor. However, whether and how GPCR-mediated G-protein-independent signals trigger receptor internalization remains unknown, although G-protein-independent internalization has been reported. Here we show that an angiotensin II (Ang II) type-1 (AT ₁) receptor mutant incapable of activating any G-protein still undergoes normal internalization. Substitution of Asp¹²⁵ with Ala and Arg¹²⁶ with Leu at the highly conserved DRY motif of the AT ₁ receptor disabled the ability of the receptor to activate G-proteins, as shown by various Ang II binding studies, GDP-GTP exchange, and inositol phosphate production assays. Surprisingly, the mutant internalized normally in the presence of Ang II and transactivated the epidermal growth factor receptor (EGFR). Similar to the wild-type receptor, overexpression of a dominant-negative K220R mutant GRK2 diminished the internalization of D125A-R126L but not the transactivation of EGFR. These data indicate that G-protein-independent specific signals may also trigger homologous internalizations of the AT₁ receptor through β-arrestin- dependent and -independent pathways, suggesting a possible mechanism for G-protein-independent activation of G-protein-coupled receptor kinases (GRKs). This may represent a general mechanism for triggering GPCR internalization.