Identification of a novel Mr = 76-kDa form of beta-adrenergic receptors

The structure of the human beta-adrenergic receptor in purified basal membranes of human placental syncytiotrophoblast was probed using photoaffinity labeling. Basal membranes display a high specific activity of receptors (4-5 pmol/mg protein) and possess both beta 1- and beta 2-adrenergic receptors subtypes. Autoradiography of membranes that were incubated with the beta-adrenergic antagonist [125I]iodoazidobenzylpindolol, photolyzed and then subjected to sodium dodecylsulfate-polyacrylamide gel electrophoresis, identified four radiolabeled peptides, Mr = 65-kDa, 54-kDa, 43-kDa and a novel higher molecular weight 76-kDa form of the receptor. Photoaffinity labeling of each of these four peptides displayed the pharmacological properties expected for true beta-adrenergic receptors. The 76-kDa photoaffinity labeled receptor peptide observed in human placenta basal membranes has not been reported elsewhere. Competition studies with the beta1-selective ligand CGP-20712A demonstrate that the photoaffinity labeled receptor peptides are composed of both beta 1- and beta 2-adrenergic receptor subtypes.