Activation Of Glycoprotein Hormone Receptors : Role Of Different Receptor Domains In Hormone Binding And Signaling

The glycoprotein hormones, Luteinizing Hormone (LH), human Chorionic Gonadotropin (hCG), Follicle Stimulating Hormone (FSH) and Thyroid Stimulating Hormone (TSH) are heterodimeric proteins with an identical α-subunit associated non-covalently with the hormone specific β-subunit and play important roles in reproduction and overall physiology of the organism [1]. The receptors of these hormones belong to the family of G-protein coupled receptors (GPCR) and have a large extracellular domain (ECD) comprising of 9-10 leucine rich repeats (LRR) followed by a flexible hinge region, a seven helical transmembrane domain (TMD) and a C terminal cytoplasmic tail [2]. Despite significant sequence and structural homologies observed between the ECDs of the receptors and the specific β-subunits of the hormones, the hormone-receptor pairs exhibit exquisite specificity with very low cross-reactivity with other members of the family. The TSH receptor (TSHR) is an especially interesting member of this family as it not only recognizes is cognate ligand, i.e. TSH, but also binds to the non-cognate ligands such as autoantibodies. TSHR autoantibodies come in different flavors; inhibitory antibodies that compete with the hormone for receptor binding and block its action, stimulatory antibodies that activate the receptor in a hormone independent manner and neutral antibodies that bind to the receptor but do not directly influence its functions. The inhibitory autoantibodies cause hypothyroidism and are responsible for Hashimoto’s Thyroiditis, whereas the stimulatory autoantibodies cause Graves’ thyrotoxicosis characterized by hyperthyroid condition [3]. The exact epitopes of these autoantibodies are not well delineated although it has been hypothesized that the blocking type- and the stimulatory type- autoantibodies have predominant epitopes in the TSHR ECD that overlap with hormone binding regions [4]. Insights into the mode of hormone or autoantibody binding to the receptor was primarily derived from the crystal structure of FSHR leucine rich repeat domain (LRRD) bound to single chain analog of FSH, and the crystal structures of TSHR LRRD bound to the stimulatory type human monoclonal antibody M22 [5] and the inhibitory type- monoclonal antibody K1-70 [6]. Both these crystal structures propose LRRDs as the primary ligand binding site which interacts with the hormone through its determinant loop in a hand-clasp fashion [7] while the autoantibodies mimics the hormone binding to a large extent [8] . These structures, while providing detailed understanding of the molecular interactions of the LRRs with the hormone, shed little light on the mechanism by which the signal generated at the LRRD are transduced to the downstream effector regions at the distally situated TMD. Hence, while one understands the ligand binding to a large extent, the activation process is not well understood, one of the central objective of the present study. Ligand-receptor interactions are typically studied by perturbing ligand/receptor structure by mutagenesis or by…