Unmasking of an immunoreactive site on the alpha subunit of human choriogonadotropin bound to the extracellular domain of its receptor.

To define the human choriogonadotropin (hCG) hormone's contact points with its receptor, we examined five monoclonal anti-hCG antibodies for their binding ability to the hCG-intact receptor complex and to the hCG-truncated extracellular N-terminal half receptor complex. hCG-producing CHO cells were transfected with the N-terminal 297 residues of the porcine LH/CG receptor and the secreted complexes were detected by two-site immunoassays based on anti-receptor and anti-hCG antibodies. Four antibodies did not show any differences toward the two types of complexes. In contrast, a particular antibody, directed to the alpha-subunit of hCG, recognized the hCG-truncated receptor complex but not the hCG-intact receptor complex. These results substantiate recent reports indicating that, if most of the whole alpha/beta dimer is bound to the N-terminal half of the receptor, some regions of the alpha-subunit might be contacting the C-terminal half.