Schwarz S, Krude H, Nelboeck E, Berger P, Merz W E, Wick G
Institute of General & Experimental Pathology, Faculty of Medicine, University of Innsbruck, Austria.
J Recept Res. 1991;11(1-4):437-58. doi: 10.3109/10799899109066420.
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.
当hCG与受体结合时，在其先前确定的总共14个表面表位（参考文献10、11、12）中，只有2个表位（即β3和β5）能被相应的125I-MCA检测到。显然，这表明hCG在这种特定状态下的取向是非随机的（参考文献1）。现在我们报告，在与受体结合的去唾液酸化hCG（脱唾液酸hCG）以及与受体结合的去糖基化hCG（去糖基hCG）上，β3和β5表位以及其余表位均无法被125I-MCA识别，尽管这两种变体在未与受体结合时，其表位的数量和拓扑结构与天然hCG并无差异。因此，hCG的碳水化合物（CHO）单元似乎既不是这14个抗原位点的一部分，也不影响受体结合亲和力：这两种变体的亲和力甚至比天然hCG更高。然而，由于已知CHO对于触发hCG刺激的靶细胞的受体后反应是必不可少的，这分别表现为脱唾液酸hCG和去糖基hCG的生物学活性降低50%或完全丧失，因此这里所展示的表位可及性的明显差异可能与受体结合取向的差异有关，这些差异反映了与受体相互作用的信号转导活性和无活性模式。我们认为，CHO虽然本身不参与受体结合，但它的作用是确保hCG在配体识别域中的正确定位，从而使配体与受体之间能够进行适当的蛋白质-蛋白质相互作用，进而实现最佳激活，并将外部信号传递到细胞内部。此外，大多数表面表位在与受体结合的hCG上被掩盖这一事实，为迄今尚未得到证实的假说提供了首个实验支持，即最近克隆的hCG受体（hCG-R）（参考文献23、24）异常长（即341个氨基酸残基）的细胞外N'末端结构域确实参与配体识别。
