Evidence for Follicle- stimulating Hormone Receptor as a Functional Trimer*

Background: A carbohydrate of follicle-stimulating hormone (FSH) has been proposed to sterically block other FSH molecules from binding to the putative receptor (FSHR) trimer. Results: FSH increases its receptor binding by 3-fold when the steric hindrance is removed. Conclusion: FSHR forms a functional trimer. Significance: This knowledge may improve designs of therapeutic drugs targeting FSHR. Follicle-stimulating hormone receptor (FSHR), a G-protein coupled receptor, is an important drug target in the development of novel therapeutics for reproductive indications. The FSHR extracellular domains were observed in the crystal structure as a trimer, which enabled us to propose a novel model for the receptor activation mechanism. The model predicts that FSHR binds Asn(52)-deglycosylated FSH at a 3-fold higher capacity than fully glycosylated FSH. It also predicts that, upon dissociation of the FSHR trimer into monomers, the binding of glycosylated FSH, but not deglycosylated FSH, would increase 3-fold, and that the dissociated monomers would in turn enhance FSHR binding and signaling activities by 3-fold. This study presents evidence confirming these predictions and provides crystallographic and mutagenesis data supporting the proposed model. The model also provides a mechanistic explanation to the agonist and antagonist activities of thyroid-stimulating hormone receptor autoantibodies. We conclude that FSHR exists as a functional trimer.