MOLECULAR-BASIS FOR SUBTYPE-SPECIFIC DESENSITIZATION OF INHIBITORY MOLECULAR ADENOSINE RECEPTORS - ANALYSIS OF A CHIMERIC A(1)-A(3) ADENOSINE RECEPTOR

The differing effects of short-term agonist exposure on the two inhibi tory adenosine receptor (AR) subtypes have been examined using Chinese hamster ovary cells stably expressing the hemagglutinin epitope-tagge d human A(1)AR and rat A(3)AR, Under conditions in which exposure of t ransfected cells to 5 mu M (-)-(R)-N-6-(phenylisopropyl)adenosine resu lted in the functional desensitization and phosphorylation of the A(3) AR, neither property was exhibited by the A(1)AR, However, a stably ex pressed chimeric A(1)-A(3)AR, termed A(1)CT3AR, in which the C-termina l domain of the A(1)AR distal to its predicted palmitoylation site was replaced by the corresponding region of the A(3)AR, was able to under go functional desensitization and agonist-stimulated phosphorylation i n a manner similar to that exhibited by the A(3)AR, More over, purifie d G-protein-coupled receptor kinases 2, 3, and 5 were each capable of enhancing the agonist-dependent phosphorylation of the A(3)AR and A(1) CT3AR in vitro, Taken together, these data demonstrate that the C term inal domain of the A(3)AR distal to its predicted palmitoylation site is responsible for this receptor's ability to undergo a rapid agonist- dependent desensitization and are consistent with a model in which pho sphorylation of the A(3)AR within this domain by one or more G-protein -coupled receptor kinases initiates the desensitization process.