Abstract
Chimeric receptors of the β2-adrenergic receptor in which the extracellular loops were replaced with the corresponding amino acids of the α1a-adrenergic receptor were generated to measure changes in α1-antagonist affinity. Although no changes in α1-antagonist affinity were measured in the β2/α1a chimeras, a decreased IC50 (10-fold) for agonists as compared with wild type β2 control was found because of the replacement of the third extracellular loop (EX3). These agonist high affinity changes were because of a greater proportion of high affinity sites (2-fold) that were convertible to low affinity sites with guanosine 5′-3-O-(thio)triphosphate. Adenylate cyclase activity evoked by the EX3 chimera showed commensurate increases in the basal signal transduction as well as the isoproterenol-stimulated potency, suggesting constitutive activity. However, unlike other constitutively active adrenergic receptor mutants in which the mutation causes G protein-independent changes, the mechanism of the EX3 chimera seems to be attributable to a greater ease with which the active ternary complex is formed because of a higher affinity/coupling of the G protein. Although the changes because of EX3 are indirect and most likely affect helical packing, they support an emerging hypothesis that G protein-coupled receptors have evolved their structure-function relationships to constrain the receptor in an inactive state.
Footnotes
- Received August 19, 1997.
- Accepted November 21, 1997.
-
Send reprint requests to: Dr. Dianne M. Perez, Department of Molecular Cardiology, FF3-01, The Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, Ohio 44195. E-mail:perezd{at}cesmtp.ccf.org
-
↵1 Current affiliation: Division of Hematology, Vanderbilt University, Nashville, TN 37232.
-
This work was supported in part by National Institutes of Health Grant RO1-HL52544 (D.M.P.) and an unrestricted research grant from Glaxo Wellcome. This work was done under the tenure of an Established Investigator Award from the American Heart Association (D.M.P.).
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|