General Science Abstracts - Full TextThe HW Wilson Company20000104endatasettext/xml© The HW Wilson CompanyArticleFeature articleBiochemistry (American Chemical Society)us0006-2960BAIGSAGSIen38461999111615070150778The human glycoprotein hormones chorionic gonadotropin (CG), thyrotropin (TSH), lutropin (LH), and follitropin (FSH) are heterodimers, composed of a common a subunit assembled to a hormone- specific b subunit. The subunits combine noncovalently early in the secretory pathway and exist as heterodiniers, but not as multimers. Little information is available regarding the steps associated with the assembly reaction. It is unclear if the initial ab engagment results either in the formation of only mature heterodimer or if the nascent complex is reversible and can undergo an exchange of subunits or combine transiently with an additional subunit. This is relevant for the case of LH and FSH, because both are synthesized in the same cell (i.e., pituitary gonadotrophs) and several of the a subunit sequences required for association with either the LHb or FSHb subunits are different. Such features could favor the generation of short-lived, multi-subunit forms prior to completion of assembly. Previously, we showed that the CGb or FSHb subunit genes can be genetically fused to the a gene to produce biologically active single chains, CGba and Fba, respectively. Studies using monoclonal antibodies sensitive to the conformation of the hCG subunits suggested that in contrast to the highly compact heterodimer, the interactions between the b and a domains in the single chain are in a more relaxed configuration. That the tethered domains do not interact tightly predicts that they could combine with an additional subunit to form triple domain complexes. We tested this point by cotransfecting CHO cells with the genes encoding Fba and the CGb subunit or the CGba and FSHb monomer. The CGb subunit combined noncovalently with Fba to form a Fba /CGb complex. Ternary complex formation was not restricted to a specific set of single chain/monomeric subunit, because a CGba/FSHb complex was also detected implying that triple domain intermediates could be transiently generated along the secretory pathway. Monoclonal antibodies specific for the CG heterodimer recognized the Fba/CGb complex, which suggests that the epitopes unique for dimeric CG were established. In addition, media containing Fba/CGb displayed high-affinity binding to both CG and FSH receptors. The presence of CG activity is presumptive for the existence of a functional Fba/CGb complex, because neither Fba nor the uncombined CGb subunit binds to CG receptor. These data show that the a subunit of the tether, although covalently linked to the FSHb domain, can functionally interact with a different b subunit implying that the contacts in the nascent ab dimer are reversible. The formation of a functional single chain/subunit complex was not restricted to the FSH single chain/CGb subunit since CG single chain interacts with the monomeric FSHb subunit and exhibits FSH activity. The presence of the triple domain configuration does not abolish bioactivity, suggesting that although the gonadotropins are heterodimers, the cognate receptor is capable of recognizing a larger ligand composed of three subunit domains. Copyright 1999, American Chemical Society.Ben-MenahemDavidHydeRiciaPixleyMarySynthesis of multi-subunit domain gonadotropin complexes: a model for a/b heterodimer formation1999IllustrationsDimerizationsGonadotropinss