Similarities and differences in interactions of thyroid stimulating and blocking autoantibodies with the TSH receptor

Abstract

Binding of a new thyroid-stimulating human monoclonal autoantibody (MAb) K1–18 to the TSH receptor (TSHR) leucine-rich domain (LRD) was predicted using charge–charge interaction mapping based on unique complementarities between the TSHR in interactions with the thyroid-stimulating human MAb M22 or the thyroid-blocking human MAb K1–70. The interactions of K1–18 with the TSHR LRD were compared with the interactions in the crystal structures of the M22–TSHR LRD and K1–70–TSHR LRD complexes. Furthermore, the predicted position of K1–18 on the TSHR was validated by the effects of TSHR mutations on the stimulating activity of K1–18. A similar approach was adopted for predicting binding of a mouse thyroid-blocking MAb RSR-B2 to the TSHR. K1–18 is predicted to bind to the TSHR LRD in a similar way as TSH and M22. The binding analysis suggests that K1–18 light chain (LC) mimics binding of the TSH-α chain and the heavy chain (HC) mimics binding of the TSH-β chain. By contrast, M22 HC mimics the interactions of TSH-α while M22 LC mimics TSH-β in interactions with the TSHR. The observed interactions in the M22–TSHR LRD and K1–70–TSHR LRD complexes (crystal structures) with TSH–TSHR LRD (comparative model) and K1–18–TSHR LRD (predictive binding) suggest that K1–18 and M22 interactions with the receptor may reflect interaction of thyroid-stimulating autoantibodies in general. Furthermore, K1–70 and RSR-B2 interactions with the TSHR LRD may reflect binding of TSHR-blocking autoantibodies in general. Interactions involving the C-terminal part of the TSHR LRD may be important for receptor activation by autoantibodies.