Human choriogonadotropin (hCG) consists of an α subunit and a β subunit. The existing evidence from various studies using truncation, substitution, synthetic hormone peptides, and hCG crystals suggests that the C-terminal region of the α subunit contacts the luteinizing hormone/choriogonoadotropin (LH/CG) receptor and is involved in receptor activation. Despite a deluge of the speculation and the important role of the αC-terminal region, direct evidence for its interaction with the receptor has been elusive. Because of the significant biological activity, it is imperative to prove the interaction of the αC-terminal region. For this purpose, decamer peptides corresponding to the α subunit sequence from His83 to Ser92 (α83-92) were derivatized with the N-hydroxysuccinimide ester of 4- azidobenzoylglycine (ABG) and radioiodinated. The resulting ABG-125I- α83-92 was capable of binding and activating the LH/CG receptor. Furthermore, UV-sensitive ABG-125I-α83-92 exclusively photoaffinity- labeled an ~86-kDa molecule. This labeled molecule was shown to be the LH/CG receptor by various methods including immunoprecipitation by anti-LH/CG receptor antiserum. In addition, evidence is presented that the amino group of αLys91 of α83-92 is in such close proximity to a carboxyl group of the receptor that this pair is cross-linked to form an amide, a zero length cross-link. This low affinity contact of α83-92 and the receptor is sufficient for receptor activation and is crucial for the full understanding of the mechanistics of the receptor activation steps.
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