Characterization of a Third Human Thyroid Hormone Receptor Coexpressed with Other Thyroid Hormone Receptors in Several Tissues

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Abstract
A cDNA that encodes a third type of human thyroid hormone receptor (hTRα1) has been isolated from a skeletal muscle library. The cDNA encodes a 410 amino acid protein, Mr = 46,820. When expressed and translated in vitro, hTRα1 binds T3 with an association constant (ka) of 1.8 × 109 M−1. Comparison of the DNA sequence of hTRα1 and a previously identified α type thyroid hormone receptor (hTRα2) suggests that they could be transcribed from the same gene, and that alternative RNA splicing results in the synthesis of either hTRα1 or hTRα2. Two mRNA (3.2 kilobases and 6 kilobases) of hTRα1 have been detected in several tissues. At least three types of thyroid hormone receptors (hTRα1, α2, β), which possess similar affinities for hormone ligands, can be expressed in the same tissue.