STRUCTURAL SIMILARITIES IN THE PLASMA MEMBRANE 3,3?,5-TRIIODO-L-THYRONINE RECEPTORS FROM HUMAN, RAT AND MOUSE CULTURED CELLS. ANALYSIS BY AFFINITY LABELING

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Abstract
Affinity labeling of Swiss 3T3-4 mouse fibroblasts with 0.3 nM N-bromoacetyl-3,[125I]3',5-triiodo-L-thyronine (BrAc[125I]T3) at 4 C for 0.5 h showed three labeled protein bands with apparent molecular masses of 55, 53 and 33 kilodaltons (kDal) in a ratio of 85:8:7. Only the labeling of 55-kDal protein was selectively reduced to approximately 50% by 15 microM unlabeled T3. Affinity labeling of the purified plasma membranes under identical conditions yielded similar results. One-dimensional peptide mapping by Staphylococcus aureus V8 or elastase digestion of the 55-kDal proteins from cells or plasma membranes gave identical peptide fragments. Thus the 55-kDal protein is a membrane-associated protein. Furthermore, affinity labeling of human epithelioid carcinoma A431 cells and purified plasma membranes gave similar results as those of Swiss 3T3 cells. Peptide mapping of elastase or S. auerus digestion of 55-kDal proteins from A431, Swiss 3T3 and GH3 rat pituitary tumor cells gave identical patterns. These results indicate that plasma membrane T3 receptors from three species have structural similarity in the hormone binding domains. Thus, the plasma membrane T3 receptor probably are highly conserved.

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