Structural similarities between the plasma membrane binding sites for L-thyroxine and 3,3',5-triiodo-L-thyronine in cultured cells.

Using 125I-labeled L-thyroxine (T4), the binding of [125I]T4 to GH3 rat pituitary tumor cells was studied. At 15 degrees C, the binding of [125I]T4 to cells is saturable and specific. Least squares analysis of binding data showed two classes of binding sites with apparent dissociation constants of 4.3 +/- 0.3 nM and 350 +/- 30nM and binding capacities of (3.8 +/- 0.5) X 10(4) and (9.1 +/- 0.35) X 10(6) sites/cell, respectively. Affinity labeling of cells or purified plasma membranes with N-bromoacetyl-[125I]T4 (BrAc[125I]T4) showed a major specifically labeled protein band with an apparent molecular mass of 55 kilodaltons (kDal). Digestion of the 55-kDal protein from cells and plasma membrane by Staphylococcus aureus V8 protease or elastase gave similar peptide fragments. Thus, the 55-kDal protein labeled from intact cells is the same protein as that from purified plasma membranes. Peptide mapping was further used to compare the 55-kDal protein specifically labeled by either N-bromoacetyl-3,[125I]3',5-triiodo-L-thyronine (BrAc[125I]T3) or BrAc[125I]T4 in intact cells and highly purified plasma membranes. Very similar patterns were obtained. These results indicate that plasma membrane T3 and T4 binding sites have similar hormone binding domains. In addition the plasma membrane T3 and T4 binding sites of Swiss 3T3-4 mouse fibroblasts and A431 human epithelioid carcinoma cells are structurally similar to the T3 and T4 binding sites of GH3 cells.