Expression of mRNA of β₁- and β₂-adrenergic receptors in Xenopus oocytes results from structurally distinct receptor mRNAs

Poly(A)⁺-selected RNA prepared from cells or tissues that express a homogeneous population of either β₁- or β₂-adrenergic receptors was isolated and then microinjected into Xenopus laevis oocytes. Following microinjection, the expression of β-adrenergic receptors was assessed by equilibrium radioligand binding analysis using the antagonist ligand [³H]dihydroalprenolol. The pharmacology of theh newly- expressed β-adrenergic receptors in oocyte membranes was the same as that of the original tissue used as a source of RNA. Hybridization of nick-translated cDNA of hamster β₂-adrenergic receptor to poly(A)⁺-selected RNA from tissues containing β₂-adrenergic receptors was to a mRNA species of 2.2 kilobases. In contrast, hybridization of the cDNA probe to poly(A)⁺-selected RNA from tissues containing β₁-adrenergic receptors was to a mRNA species of 2.0 kilobases. A single-stranded fragment of hamster β₂-adrenergic receptor cDNA corresponding to nucleotides 730-886 was isolated and uniformly radiolabeled. This region of the gene is predicted to encode for the entire second exofacial loop (L_4-5), the entire fifth transmembrane-spanning region, and the first 5 amino acid residues of the third cytoplasmic loop (L_5-6) of the β₂-adrenergic receptor. Hybridization at 48 and 56 °C of poly(A)⁺-selected RNA prepared from sources that express either β₁-or β₂-adrenergic receptors to the antisense orientation strand of this region of the β₂-adrenergic receptor cDNA was followed by S1 endonuclease digestion of nonhybridized sequences. At 48 °C, S1-resistant hybrids from both sources of RNA protected the probe from S1 endonuclease digestion. At 56 °C, however, only the RNA prepared from the source of β₂-adrenergic receptors protected the probe from S1 endonuclease digestion. These results demonstrate that the mRNAs encoding for the structurally homologous β₁- and β₂-adrenergic receptors are distinct in the pharmacological specificity of their translation products and in their size and structure.