Pro-thyrotropin-releasing hormone
adenylate cyclase-activating G protein-coupled receptor signaling pathway
Hormone- and antibody-mediated activation of the thyrotropin receptor.
Thyroid-stimulating hormone (TSH), through activation of its G-protein-coupled thyrotropin receptor (TSHR), controls the synthesis of thyroid hormone-an essential metabolic hormone<sup>1-3</sup>. Aberrant signalling of TSHR by autoantibodies causes Graves' disease (hyperthyroidism) and hypothyroidism, both of which affect millions of patients worldwide<sup>4</sup>. Here we report the active structures of TSHR with TSH and the activating autoantibody M22<sup>5</sup>, both bound to the allosteric agonist ML-109<sup>6</sup>, as well as an inactivated TSHR structure with the inhibitory antibody K1-70<sup>7</sup>. Both TSH and M22 push the extracellular domain (ECD) of TSHR into an upright active conformation. By contrast, K1-70 blocks TSH binding and cannot push the ECD into the upright conformation. Comparisons of the active and inactivated structures of TSHR with those of the luteinizing hormone/choriogonadotropin receptor (LHCGR) reveal a universal activation mechanism of glycoprotein hormone receptors, in which a conserved ten-residue fragment (P10) from the hinge C-terminal loop mediates ECD interactions with the TSHR transmembrane domain<sup>8</sup>. One notable feature is that there are more than 15 cholesterols surrounding TSHR, supporting its preferential location in lipid rafts<sup>9</sup>. These structures also highlight a similar ECD-push mechanism for TSH and autoantibody M22 to activate TSHR, therefore providing the molecular basis for Graves' disease.