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 α-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 α/β dimer is bound to the N-terminal half of the receptor, some regions of the α-subunit might be contacting the C-terminal half. © 1993 Academic Press, Inc.