Orientation of Follicle-stimulating Hormone (FSH) Subunits Complexed with the FSH Receptor

Follicle-stimulating hormone (FSH) comprises an α subunit and a β subunit, whereas the FSH receptor consists of two halves with distinct functions: the N-terminal extracellular exodomain and C-terminal membrane-associated endodomain. FSH initially binds to exodomain, and the resulting FSH/exodomain complex modulates the endodomain and generates signal. However, it has been difficult to determine which subunit of FSH contacts the exodomain or endodomain and in what orientation FSH interacts with them. To address these crucial issues, the receptor was Ala-scanned and the hormone subunits were probed with photoaffinity labeling with receptor peptides corresponding to the N-terminal region of the exodomain and exoloop 3 of the endodomain. Our results show that both regions of the receptors are important for hormone binding and signal generation. In addition, the FSH β subunit is specifically labeled with the N-terminal peptide, whereas the α subunit is labeled with the exoloop 3 peptide. These contrasting results show that the FSH β subunit is close to the N-terminal region and that the α subunit is projected toward exoloop 3 in the endodomain. The results raise the fundamental question whether the α subunit, common among the glycoprotein hormones, plays a major role in generating the hormone signal common to all glycoprotein hormones.