The originally described cDNA of the turkey β1-adrenergic receptor encodes a receptor with a carboxyl-terminal, 59-amino acid extension that was not found in several mammalian β1- adrenergic receptors. This extension blocks agonist-promoted endocytosis and downregulation of the receptor. This carboxyl-terminal domain is encoded by an exon distinct from that which encodes the body of the receptor, and the originally described cDNA results from removal of an 849-nucleotide intron. Unspliced mRNA encodes a shorter open reading frame whose translated carboxyl terminus is identical with that of the mammalian β1-adrenergic receptors. There is no evidence for other introns in the coding region. Splicing of the intron to produce the nonendocytosing receptor is highest in fetal blood cells, is appreciable in adult brain and heart, and is detectable in other tissues. Thus, different tissues use alternative splicing to express β-adrenergic receptors that either do or do not endocytose and down-regulate in response to agonist.
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