The full-length TSH receptor is stabilized by TSH ligand

The receptor for thyroid stimulating hormone (TSHR), a GPCR, is the primary antigen in autoimmune hyperthyroidism (Graves' disease) caused by stimulating TSHR antibodies. While we have previously published a full length model of the TSHR, including its leucine rich domain (LRD), linker region (LR) and transmembrane domain (TMD), to date, only a partial LRD (aa 21-261) stabilized with TSHR autoantibodies has been crystallized. Recently, however, cryo-EM structures of the full-length TSHR have been published but they include only an incomplete LR. We have now utilized the cryo-EM models, added disulfide bonds to the LR and performed longer (3000 ns) molecular dynamic (MD) simulations to update our previous model of the entire full-length TSHR, with and without the presence of TSH ligand. As in our earlier work, the new model was embedded in a lipid membrane and was solvated with water and counterions. We found that the 3000 ns Molecular Dynamic simulations showed that the structure of the LRD and TMD were remarkably constant while the LR, known more commonly as the "hinge region", again showed significant flexibility, forming several transient secondary structural elements. Analysis of the new simulations permitted a detailed examination of the effect of TSH binding on the structure of the TSHR. We found a structure-stabilizing effect of TSH, including increased stability of the LR, which was clearly demonstrated by analyzing several intrinsic receptor properties including hydrogen bonding, fluctuation of the LRD orientation, and radius of gyration. In conclusion, we were able to quantify the flexibility of the TSHR and show its increased stability after TSH binding. These data indicated the important role of ligands in directing the signaling structure of a receptor.