Protein kinase C (PKC) phosphorylates the C-terminus of the type 1 angiotensin II receptor (AT1), although the exact site(s) of phosphorylation are unidentified. In the present study, we examined the phosphorylation of epitope-tagged wild-type AT1A receptors, transiently expressed in Chinese hamster ovary K1 cells, in response to angiotensin II (AngII) and following selective activation and inhibition of PKC. This phosphorylation was compared with mutant receptors where C-terminal serine residues (Ser331, Ser338 and Ser348) within three putative PKC consensus sites were replaced with alanine, either individually or in combination. Stimulation by AngII or the phorbol ester PMA to activate PKC induced an increase in phosphorylation of the wild-type AT1A receptor, which was prevented by truncation of the receptor C-terminus to remove the last 34 amino acids, including Ser331, Ser338 and Ser348. Whereas single alanine mutation (Ser331Ala, Ser338Ala and Ser348Ala) resulted in decreased receptor phosphorylation, no single mutant completely inhibited either AngII- or PMA-induced phosphorylation. Combined mutation of the three PKC consensus sites caused an ≈ 70% reduction in PMA-mediated phosphorylation. The ≈ 60% reduction in AngII (1 μM)-induced phosphorylation of this triple mutant and the partial inhibition of wild-type receptor phosphorylation by bisindolylmaleimide, a specific PKC inhibitor, suggest a significant contribution of PKC to agonist-stimulated regulation. The ratio of PKC to total receptor phosphorylation was greatest at low doses of AngII (1 nM), consistent with the idea that PKC phosphorylates and regulates receptor function at low levels of stimulation, whereas phosphorylation by other kinases is more prevalent at high levels of agonist stimulation. To determine if a single PKC site is favoured when the contribution of PKC varies, the phosphorylation of wild-type and mutant receptors was examined over a range of AngII concentrations (0, 1, 10 and 100 nM). At all AngII concentrations, single mutation of Ser331, Ser338 or Ser348 was incapable of completely preventing receptor phosphorylation, suggesting no clear preference for PKC consensus-site utilization. Together, these results indicate a redundancy in PKC phosphorylation of the AT1A receptor, whereby all three consensus sites are utilized to some degree following homologous (AngII) and heterologous (PMA) stimulation. The contribution of PKC phosphorylation to receptor regulation is unclear, but multiple PKC phosphorylation of the AT1A receptor may allow independent and/or complementary events to occur at the three separate sites of the C-terminus.

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