Identification of thyroid-stimulating antibody-specific interaction sites in the N-terminal region of the thyrotropin receptor.

Using mutants of the N-terminal region (residues 30-76) of the rat TSH receptor (TSHR), which substitute corresponding segments of rat gonadotropin receptors or hydrophilic (serine) and hydrophobic (alanine) amino acids as appropriate, we show that residues 30-33, 34-37, 42-45, 52-56, and 58-61, in addition to threonine-40, are determinants for the interaction of thyroid-stimulating autoantibodies (stimulating TSHRAbs) with the TSHR. The most important, residues 34-37, 42-45, and 52-56, whose mutants lose stimulating TSHRAb activity with at least 11 of 12 (> 90%) of the Graves' immunoglobulins G tested, are, like threonine-40, in regions of the TSHR that are nonhomologous with gonadotropin receptors. These data establish at least in part, therefore, the basis for the thyroid-specific effects of stimulating TSHRAbs. In no case do the same mutants lose their reactivity with TSH or blocking-type TSHR autoantibodies (blocking TSHRAbs) from hypothyroid patients with idiopathic myxedema. Since the latter have b...