Spatial relationships of the human chorionic gonadotropin (hCG) subunits in the assembly of the hCG-receptor complex in the luteinized rat ovary

Relationship of orientation with affinity and activity of receptor-bound glycosylation variants of human chorionic gonadotropin (hCG), as visualized by monoclonal antibodies (MCA)

When hCG was receptor-bound, only 2 epitopes (i.e. beta 3 and beta 5) out of its previously mapped total of 14 surface epitopes (10, 11, 12) could be detected by the respective 125I-MCA. Clearly, this indicates a non-random orientation of hCG in this particular state (1). Now we report that on receptor-bound desialylated (asialo-hCG) as well as on receptor-bound deglycosylated hCG (degly-hCG), the beta 3 and beta 5 epitopes were inaccessible for 125I-MCA as were the remaining epitopes, although both variants, when not receptor-bound, were indistinguishable from native hCG with respect to number and topography of epitopes. Thus, the carbohydrate (CHO) units of hCG neither seem to be part of these 14 antigenic sites nor to contribute to the affinity of receptor binding: both variants had even higher affinities than native hCG. However, since the CHO are known to be obligatory for triggering the postreceptor responses of hCG-stimulated target cells, as seen by the 50% reduced or totally abolished biological potency of asialo-hCG and degly-hCG, respectively, the here demonstrated clear-cut differences in epitope accessibility can be related to differences in receptor-bound orientations which reflect signal transduction-competent and incompetent modes of interaction with the receptor. We believe that the CHO, while not contributing to receptor binding per se, function to assure correct positioning of hCG in the ligand recognition domain to allow for proper protein-protein interactions between ligand and receptor and thus optimal activation and hence transduction of the external signal to the cell's interior. In addition, the fact that most of the surface epitopes were masked on receptor-bound hCG, represents the first experimental support for the sofar unproven hypothesis that this unusually long (i.e. 341 amino acid residues) extracellular N' terminal domain of the recently cloned hCG receptor (hCG-R) (23,24), is indeed involved in ligand recognition.

Evidence for monomeric and oligomeric hormone-binding domains in affinity-purified gonadotropin receptor from rat ovary

Rat ovarian lutropin/choriogonadotropin receptor was purified from a Triton X-100-solubilized membrane preparation by affinity chromatography with Affi-Gel 10 coupled to purified human choriogonadotropin. The affinity-purified receptor preparations contained a single class of high-affinity binding sites for 125I-labeled human choriogonadotropin, with an equilibrium dissociation constant (Kd) of 2.5 x 10(-9) M, which is comparable to the Kd values for membrane-bound and solubilized receptors. The purified receptor appeared as two dominant bands with molecular weights of 135,000 and 92,000 after sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE) under nonreducing conditions. These two bands were also detected in subsequent direct ligand blotting analysis when the purified receptor was electrophoretically transferred to a nitrocellulose membrane after SDS/PAGE under nonreducing conditions. When the individual affinity-purified receptor bands were electroeluted from the gel and analyzed again by SDS/PAGE under nonreducing conditions, both the Mr 92,000 and the 135,000 proteins retained their original molecular form even when 8 M urea was included in the gel. However, when the electrophoretically purified Mr 92,000 and 135,000 bands were subjected to SDS/PAGE under reducing conditions, the Mr 135,000 species was almost completely converted to a Mr 92,000 band, but the Mr 92,000 species did not undergo any alteration in molecular weight. The results suggest that the lutropin/choriogonadotropin receptor from rat ovary exists in two molecular forms, and the higher molecular weight form appears to be composed of disulfide-linked Mr 92,000 subunit, which comprises the hormone-binding domain.

Probing the receptor-interaction of glycoprotein hormones with monoclonal antibodies

Having recently analyzed with monoclonal antibodies (MCA) the immunologic surface of human chorionic gonadotropin (hCG) as consisting of 9 distinct epitopes exposed on the molecule in a characteristic topographical manner (Schwarz, S., Berger, P., and Wick, G., Endocrinology 118, 189-197, 1986) we now attempted to confirm this result on a more general basis, e.g. by incorporating MCA that were just recently obtained and previously not included. What we were, however, interested most was the question whether the tentative model of the epitope map of hCG could represent a "target" with which hCG-related hormones such as LH, FSH, and TSH (the family of glycoprotein hormones, GPH) would (partially) match. Indeed, repeating various immunizations with GPH of human as well as animal origin revealed a remarkable reproducibility in terms of several anticipated epitope specificities of MCA. This indicates that MCA can be regarded as reliable probes for mapping epitopes and, as we have presumed, of receptor interaction domains of GPH as well. Extending the originally used 2-site MCA binding exclusion approach by an interspecies crossreactivity (Xr) analysis we now are able to refine our epitope model of hCG such that 2 additional epitopes were found which were not previously resolvable. Most surprisingly, two of 5 epitopes on the alpha subunit were now also detected on various non-human GPH, which is in striking contrast to a seemingly well established dogma. Yet all five alpha epitopes of hCG are present on hLH, hFSH, and hTSH as well and arranged in the same spatial relationship to each other as on hCG. Even the 2 conformational epitopes and their close topographical relationship to the alpha-epitopes appear to be remarkably conserved on all human GPH. Among the beta-epitopes we have found one that is not shared by hLH and that - surprisingly - is not the C-terminal peptide (CTP) by which hLH differs from hCG. On the basis of this refined epitope map a way was paved along which it should be feasible to elucidate the sterical relationship of the epitopes to the receptor interaction domain(s) of hCG. To this end the MCA were tested in principally two ways: first, as to which of the 11 MCA with different epitope specificities would be able (or not) to inhibit by preincubation the binding of radiolabeled hCG (or hLH, respectively) to rat testis LH/hCG receptors? Secondly (and inversely) which of the 11 epitopes of hCG would still be accessible to binding by radiolabeled MCA when the (unlabeled) hormone is bound to the receptor?(ABSTRACT TRUNCATED AT 400 WORDS)

Further studies on the covalent crosslinking of thyrotropin to its receptor: evidence that both the alpha and beta subunits of thyrotropin are crosslinked to the receptor

Highly purified alpha- and beta-subunits of thyrotropin were individually radioiodinated and, subsequently, recombined with their unlabeled complementary subunits. This procedure resulted in the formation of [125I]thyrotropin(TSH) hybrid molecules which were labeled on only one hormone subunit. Characterization of the binding properties of these two hybrid molecules demonstrated that both yielded nonlinear Scatchard plots with Kd and Bmax values similar to those obtained with radioiodinated native TSH and that both were capable of interaction with the high- and low-affinity binding components of the TSH receptor. The recombined [125I]TSH molecules were then crosslinked to the TSH receptor using disuccinimidyl suberate. Following electrophoresis and autoradiography, two labeled TSH-receptor complexes with Mr of 68,000 and 80,000 were observed. These two complexes exhibited hormone specificity and electrophoretic mobility identical to those previously observed using native [125I]TSH. Crosslinking with increasing concentrations of disuccinimidyl suberate suggested that the formation of the 68,000 and 80,000 complexes was sequential with the 68,000 appearing before the 80,000. Furthermore, the two bands were labeled regardless of which TSH subunit of the hybrid TSH was radioiodinated. These data strongly suggest that the 68,000 and 80,000 TSH-receptor complexes are the result of crosslinking to the TSH alpha-beta dimer and not to one subunit in the case of the 68,000 complex and to the TSH alpha-beta dimer in the case of the 80,000 complex, as had been hypothesized previously.