Agonist-induced phosphorylation of the angiotensin II (AT(1A)) receptor requires generation of a conformation that is distinct from the inositol phosphate-signaling state

G protein coupled receptors are thought to isomerize between distinct inact ive and active conformations, an idea supported by receptor mutations that induce constitutive (agonist-independent) activation. The agonist-promoted active state initiates signaling and, presumably, is then phosphorylated an d internalized to terminate the signal. In this study, we examined the phos phorylation and internalization of wild type and constitutively active muta nts (N111A and N111G) of the type 1 (AT(1A)) angiotensin II receptor. Cells expressing these receptors were stimulated with angiotensin II (Ang-II) an d [Sar(1),Ile(4),Ile(8)]AngII, an analog that only activates signaling thro ugh the constitutive receptors, Wild type AT(1A) receptors displayed a basa l level of phosphorylation, which was stimulated by AngII, Unexpectedly, th e constitutively active AT(1A) receptors did not exhibit an increase in bas al phosphorylation nor was phosphorylation enhanced by AngII stimulation. P hosphorylation of the constitutively active receptors was unaffected by pre treatment with the non-peptide AT(1A) receptor inverse agonist, EXP3174, an d was not stimulated by the selective ligand, [Sar(1),Ile(4),Ile(8)]AngII. Paradoxically, [Sar(1),Ile(4),Ile(8)]AngII produced a robust (similar to 85 % of AngII), dose-dependent phosphorylation of the wild type AT(1A) recepto r at sites in the carboxyl terminus similar to those phosphorylated by AngI I, Moreover, internalization of both wild type and constitutive receptors w as induced by AngII, but not [Sar(1),Ile(4),Ile(8)]AngII, providing a diffe rentiation between the phosphorylated and internalized states. These data s uggest that the AT(1A) receptor can attain a conformation for phosphorylati on without going through the conformation required for inositol phosphate s ignaling and provide evidence for a transition of the receptor through mult iple states, each associated with separate stages of receptor activation an d regulation. Separate transition states may be a common paradigm for G pro tein-coupled receptors.