The third extracellular loop of the beta2-adrenergic receptor can modulate receptor/G protein affinity.
Chimeric receptors of the beta2-adrenergic receptor in which the extracellular loops were replaced with the corresponding amino acids of the alpha1a-adrenergic receptor were generated to measure changes in alpha1-antagonist affinity. Although no changes in alpha1-antagonist affinity were measured in the beta2/alpha1a chimeras, a decreased IC50 (10-fold) for agonists as compared with wild type beta2 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.
Zhao
M M
MM
Department of Molecular Cardiology, The Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA.
Gaivin
R J
RJ
Perez
D M
DM
D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D000242:Cyclic AMP / Q000096:biosynthesis; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D008969:Molecular Sequence Data; D018343:Receptors, Adrenergic, beta-2 / Q000737:chemistry* / Q000378:metabolism
D018343:Receptors, Adrenergic, beta-2; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000242:Cyclic AMP; D019204:GTP-Binding Proteins