Characterization of the subunit structure of the thyrotropin receptor in the FRTL-5 rat thyroid cell line.

Radioiodinated TSH was covalently crosslinked to monolayers of FRTL-5 rat thyroid cells using the homobifunctional cross-linking agent disuccinimidyl suberate. Analysis of the cross-linked samples by sodium dodecyl sulfatepolyacrylamide gel electrophoresis under reducing conditions demonstrated the presence of three specifically labeled complexes with apparent mol wt of 68,000, 85,000, and 145,000, in addition to the TSH α-β dimer and its α and β-subunits. When bound [125I]iodo-TSH was cross-linked with increasing concentrations of disuccinimidyl suberate, the formation of the 68,000 and 85,000 mol wt complexes was sequential, with the 68,000 complex appearing first. These two complexes were also observed after labeling with radioiodinated TSH hybrid molecules (α*-β or α-β*), in which the label is in only one subunit, or immunoprecipitation with antibodies against either the α or β-subunit of TSH. Similar complexes (65,000, 82,000, and 145,000 mol wt) were also formed after cross-linking with the alkaline...