A CONSTITUTIVELY ACTIVE MUTANT BETA(2)-ADRENERGIC RECEPTOR IS CONSTITUTIVELY DESENSITIZED AND PHOSPHORYLATED

The beta2-adrenergic receptor (beta2AR) can be constitutively activate d by mutations in the third intracellular loop. Whereas the wild-type receptor exists predominantly in an inactive conformation (R) in the a bsence of agonist, the mutant receptor appears to spontaneously adopt an active conformation (R). We now demonstrate that not only is the m utant beta2AR constitutively active, it is also constitutively desensi tized and down-regulated. To assess whether the mutant receptor can co nstitutively engage a known element of the cellular desensitization ma chinery, the receptor was purified and reconstituted into phospholipid vesicles. These preparations retained the essential properties of the constitutively active mutant receptor: agonist-independent activity [ to stimulate guanine nucleotide-binding protein (G(s))-GTPase] and ago nist-specific increase in binding affinity. Moreover, the purified mut ant receptor, in the absence of agonist, was phosphorylated by recombi nant betaAR-specific kinase (betaARK) in a fashion comparable to the a gonist-occupied wild-type receptor. Thus, the conformation of the muta ted receptor is equivalent to the active conformation (R), which stim ulates G(s) protein and is identical to the betaARK substrate.