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 Asp(125) with Ala and Arg(126) 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 AT(1) receptor through beta-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.