title: Thyrotropin, follitropin, and chorionic gonadotropin expressed as a single multifunctional unit reveal remarkable permissiveness in receptor-ligand interactions.
abstract: The glycoprotein hormones [chorionic gonadotropin (CG), FSH, LH, and TSH] are composed of a common alpha-subunit and a hormone-specific beta-subunit. Subunit assembly is vital to the in vivo function of these hormones. However, recent in vitro studies using double domain (beta-alpha) and triple domain (beta-beta-alpha) single chains have shown that gonadotropin receptor recognition can accommodate conformationally modified ligands. To investigate the extent of flexibility of ligand-receptor interactions, we constructed a single chain tetramer containing three different beta-subunits (TSHbeta, FSHbeta, and CGbeta) and a single alpha-subunit. This analog was inefficiently secreted from transfected Chinese hamster ovary cells, but surprisingly, the protein exhibited all activities comparable to the corresponding heterodimers. Because the alpha-subunit presumably cannot form the entire array of heterodimeric contacts with all beta-subunits simultaneously in the tetra-domain analog, the data show that the complete quaternary subunit-subunit interactions are essential for the efficient intracellular trafficking of the glycoprotein hormones, but not for receptor recognition. From an evolutionary perspective, the organization of such a multifunctional analog is consistent with the hypothesis that glycoprotein hormone genes were originally linked in tandem and subsequently evolved as independent genes. Our results also indicate that both gonadal and thyroid stimulatory functions can be combined in a unique analog.
authors.affiliation: Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
authors.forename: Vicenta
authors.initials: V
authors.lastname: Garcia-Campayo
authors.forename: T Rajendra
authors.initials: TR
authors.lastname: Kumar
authors.forename: Irving
authors.initials: I
authors.lastname: Boime
mesh_terms: D000818:Animals; D016466:CHO Cells; D006063:Chorionic Gonadotropin / Q000235:genetics / Q000378:metabolism*; D006224:Cricetinae; D005640:Follicle Stimulating Hormone / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D009154:Mutation / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000235:genetics; D011956:Receptors, Cell Surface / Q000378:metabolism*; D015398:Signal Transduction; D013972:Thyrotropin / Q000235:genetics / Q000378:metabolism*
chemical_list: D006063:Chorionic Gonadotropin; D008024:Ligands; D011956:Receptors, Cell Surface; D005640:Follicle Stimulating Hormone; D013972:Thyrotropin