Affinity labeling of the plasma membrane 3,3',5-triiodo-L-thyronine receptor in GH3 cells

The binding of 3,3',5-triiodo-L-thyronine (T3) to GH3 rat pituitary tumor cells was studied at 15 degrees C and was shown to be saturable, reversible, and stereospecific. Least-squares analysis of the binding data showed two classes of binding sites with dissociation constants of 1.8 +/- 0.2 nM and 260 +/- 30 nM and binding capacities of (5.2 +/- 0.2) X 10(4) and (1.6 +/- 0.2) X 10(6) sites per cell, respectively. Affinity labeling of intact cells was carried out by incubation of cells with 0.3 nM N-bromoacetyl-[125I]T3 at 15 degrees C for 1 hr. Analysis of the cellular extracts by sodium dodecyl sulfate gel electrophoresis showed three labeled protein bands with apparent molecular masses of 55, 47, and 33 kilodaltons (kDal) in a ratio of 86:7:7. The labeling of only the 55-kDal protein band was selectively reduced to 50% by 20 microM unlabeled T3. Highly purified plasma membranes of GH3 cells were prepared and shown to be free of nuclei. Affinity labeling of the purified plasma membranes gave the same labeling pattern as with intact cells. Peptide mapping by Staphylococcus aureus V8 digestion of the 55-kDal protein from cells or plasma membranes gave the identical peptide fragments. Thus the 55-kDal protein labeled from intact cells is the same protein as that from purified plasma membranes. These results together with our earlier findings [Horiuchi, R., Cheng, S.-y., Willingham, M. & Pastan, I. (1982) J. Biol. Chem. 257, 3139-3144] suggest that the 55-kDal protein may be involved in mediating the uptake of T3 in GH3 cells.