Abstract

Binding of the peptide hormone angiotensin II (AngII) to the type 1 (AT_1A) receptor and the subsequent activation of phospholipase C-mediated signaling, involves specific determinants within the AngII peptide sequence. In contrast, the contribution of such determinants to AT_1A receptor internalization, phosphorylation and activation of mitogen-activated protein kinase (MAPK) signaling is not known. In this study, the internalization of an enhanced green fluorescent protein-tagged AT_1A receptor (AT_1A-EGFP), in response to AngII and a series of substituted analogs, was visualized and quantified using confocal microscopy. AngII-stimulation resulted in a rapid, concentration-dependent internalization of the chimeric receptor, which was prevented by pretreatment with the nonpeptide AT₁receptor antagonist EXP3174. Remarkably, AT_1A receptor internalization was unaffected by substitution of AngII side chains, including single and double substitutions of Tyr⁴ and Phe⁸ that abolish phospholipase C signaling through the receptor. AngII-induced receptor phosphorylation was significantly inhibited by several substitutions at Phe⁸ as well as alanine replacement of Asp¹. The activation of MAPK was only significantly inhibited by substitutions at position eight in the peptide and specific substitutions did not equally inhibit inositol phosphate production, receptor phosphorylation and MAPK activation. These results indicate that separate, yet overlapping, contacts made between the AngII peptide and the AT_1A receptor select/induce distinct receptor conformations that preferentially affect particular receptor outcomes. The requirements for AT_1A receptor internalization seem to be less stringent than receptor activation and signaling, suggesting an inherent bias toward receptor deactivation.