We have mutated a single residue, Thr348, in the C-terminal portion of the third intracellular loop of the α2C10-adrenergic receptor into five different amino acids. In analogy with the effect of similar mutations in the α(1B)- and β2-adrenergic receptors, these substitutions resulted in two major biochemical modifications: 1) increased constitutive activity of the α2-adrenergic receptor leading to agonist-independent inhibition of adenylyl cyclase and 2) increased affinity of the receptor for binding agonist but not antagonists. The increased constitutive activity of the mutated α2-adrenergic receptors could be inhibited by pertussis toxin, clearly indicating that it results from spontaneous ligand-independent receptor coupling to G(i). In contrast, the increased affinity of the mutant receptors for binding agonists was unaffected by pertussis toxin treatment, indicating that this is an inherent property of the receptors not dependent on interaction with G(i). Coexpression of the receptor mutants with the receptor-specific kinase, βARK1, indicated that the constitutively active α2-adrenergic receptors are substrates for β-adrenergic receptor kinase (βARK)-mediated phosphorylation even in the absence of agonist. These findings strengthen the idea that constitutively active adrenergic receptors mimic the 'active' state of a G protein-coupled receptor adopting conformations similar to those induced by agonist when it binds to wild type receptors. In addition, these results extend the notion that in the adrenergic receptor family the C-terminal portion of the third intracellular loop plays a general role in the processes involved in receptor activation.
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|Publication status||Published - Jan 1 1993|
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology