A Full Biological Response to Autoantibodies in Graves' Disease Requires a Disulfide-bonded Loop in the Thyrotropin Receptor N Terminus Homologous to a Laminin Epidermal Growth Factor-like Domain

We observed amino acid homology between the cysteine-rich N terminus of the thyrotropin receptor (TSHR) ectodomain and epidermal growth factor-like repeats in the laminin γ1 chain. Thyroid-stimulating autoantibodies (TSAb), the cause of Graves' disease, interact with this region of the TSHR in a manner critically dependent on antigen conformation. We studied the role of the cluster of four cysteine (Cys) residues in this region of the TSHR on the functional response to TSAb in Graves' patients' sera. As a benchmark we also studied TSH binding and action. Removal in various permutations of the four cysteines at TSHR positions 24, 29, 31, and 41 (signal peptide residues are 1–21) revealed Cys41 to be the key residue for receptor expression. Forced pairing of Cys41 with any one of the three upstream Cys residues was necessary for trafficking to the cell surface of a TSHR with high affinity TSH binding similar to the wild-type receptor. However, for a full biological response to TSAb, forced pairing of Cys41 with Cys29 or with Cys31, but not with Cys24, retained functional activity comparable with the wild-type TSHR. These data suggest that an N-terminal disulfide-bonded loop between Cys41 and Cys29or its close neighbor Cys31 comprises, in part, the highly conformational epitope for TSAb at the critical N terminus of the TSHR. Amino acid homology, as well as cysteine pairing similar to the laminin γ1 chain epidermal growth factor-like repeat 11, suggests conformational similarity between the two molecules and raises the possibility of molecular mimicry in the pathogenesis of Graves' disease.