When the wild type β1-adrenergic receptor (WT-β1AR) was expressed in Sf9 cells, the β1AR-stimulated adenylyl cyclase activities were desensitized by prior treatment with isoproterenol. The extent of β1AR desensitization was not modified, and the onset was not promoted by the overexpression of G protein-coupled receptor kinase 2 (GRK2), GRK5 or GRK6. However, overexpression of the dominant negative mutant of GRK2 appeared to inhibit desensitization of the β1AR. The change of the potential protein kinase A phosphorylation site located at the intracellular third loop did not affect β1AR desensitization. Desensitization of the truncated mutant, in which nearly all of the serine and threonine residues from the carboxyl terminus were eliminated, was the same as that of the WT-β1AR. A deletion mutant that lacked serine and threonine residues of the intracellualr third loop was also desensitized by isoproterenol stimulation. Furthermore, the deletion of serine and threonine residues from both the intracellular third loop and carboxyl terminus did not affect desensitization of the β1AR. These results suggested that phosphorylation by endogenous GRKs in Sf9 cells contributed to desensitization of the β1AR and that the regions other than third intracellular loop and carboxyl terminus may be responsible for β1AR desensitization.
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