The surface of alpha-subunit loop 1 distant from the subunit interface is exposed in the hCG lutropin receptor complex

Interactions of the placental glycoprotein hormone human choriogonadotropin (hCG) with lutropin receptors (LHR) are required for maintenance of early pregnancy. Knowledge of how hCG interacts with LHR is useful for understand ing the mechanism of receptor function, an issue of considerable debate. A large surface of hCG remains exposed after the hormone binds the LHR and ca n be readily detected with monoclonal antibodies. Here we show that the sur face of hCG alpha-subunit loop 1 furthest from the beta-subunit interface c an also be recognized by a monoclonal antibody when hCG is bound to the LHR . This extends the area of hCG known to be exposed in the hormone receptor complex, an observation that further restricts models of hCG-LHR interactio n. (C) 2000 Elsevier Science B.V. All rights reserved.