Synthesis of multi-subunit domain gonadotropin complexes: a model for alpha/beta heterodimer formation.
The human glycoprotein hormones chorionic gonadotropin (CG), thyrotropin (TSH), lutropin (LH), and follitropin (FSH) are heterodimers, composed of a common alpha subunit assembled to a hormone-specific beta subunit. The subunits combine noncovalently early in the secretory pathway and exist as heterodimers, but not as multimers. Little information is available regarding the steps associated with the assembly reaction. It is unclear if the initial alpha beta engagement 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 alpha subunit sequences required for association with either the LH beta or FSH beta 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 CG beta or FSH beta subunit genes can be genetically fused to the alpha gene to produce biologically active single chains, CG beta alpha and F beta alpha, 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 beta and alpha 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 F beta alpha and the CG beta subunit or the CG beta alpha and FSH beta monomer. The CG beta subunit combined noncovalently with F beta alpha to form a F beta alpha/CG beta complex. Ternary complex formation was not restricted to a specific set of single chain/monomeric subunit, because a CG beta alpha/FSH beta 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 F beta alpha/CG beta complex, which suggests that the epitopes unique for dimeric CG were established. In addition, media containing F beta alpha/CG beta displayed high-affinity binding to both CG and FSH receptors. The presence of CG activity is presumptive for the existence of a functional F beta alpha/CG beta complex, because neither F beta alpha nor the uncombined CG beta subunit binds to CG receptor. These data show that the alpha subunit of the tether, although covalently linked to the FSH beta domain, can functionally interact with a different beta subunit implying that the contacts in the nascent alpha beta dimer are reversible. The formation of a functional single chain/subunit complex was not restricted to the FSH single chain/CG beta subunit since CG single chain interacts with the monomeric FSH beta 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.