Abstract
Human CG (hCG) consists of a commonα -subunit and a hormone-specific β-subunit. Similarly, its receptor is also composed of two domains, an extracellular N-terminal half (exodomain) and a membrane-associated C-terminal half (endodomain). hCG initially binds the exodomain of the receptor after which the resulting hCG/exodomain complex is thought to interact with the endodomain. This secondary interaction is considered responsible for signal generation. Despite the importance, it is unclear which hormone subunit interacts with the exodomain or the endodomain. As a step to determine the mechanisms of the initial and secondary interactions and signal generation, we investigated the interaction of the hormone-specific β-subunit in hCG with the receptor’s exodomain. A photoactivable hCG derivative consisting of the wild-type α-subunit and a photoactivableβ -subunit derivative was prepared and used to label the exodomain. The analysis and immunoprecipitation of photoaffinity labeled exodomain demonstrate that the β-subunit in hCG makes the direct contact with the exodomain.

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