A conformational contribution of the luteinizing hormone-receptor ectodomain to receptor activation.

Glycoprotein hormone receptors such as the lutropin/chorionic gonadotropin receptor (LHR) are characterized by a large N-terminal ectodomain (ECD), which is responsible for hormone-receptor interactions. For the closely related TSH receptor (TSHR), it has been proposed that the ECD also serves as a tethered inverse agonist. However, the exact role of the LHR-ECD for receptor activation remains elusive. Functional analysis of N-terminally truncated LHR mutants expressed in COS-7 cells revealed that the LHR-ECD does not act as an inverse agonist but facilitates active LHR conformations. This notion is supported by two observations: first, removal of the ECD tended to decrease basal LHR activity and secondly, mutationally induced constitutive receptor activity was diminished for most activating mutations in LHR lacking the ECD. In addition, swapping of the LHR-ECD for the ECD of the closely related TSHR was not sufficient to restore constitutive receptor activity induced by naturally occurring activating heptahelical LHR mutations. Thus, the ECD stabilizes an activation-competent conformation of the heptahelical region. While the full-length LHR fused to the cognate agonist, human chorionic gonadotropin (hCG), showed increased basal activity, fusion proteins between hCG and N-terminally truncated LHR did not yield constitutive receptor activity suggesting an important role of the ECD also for agonist-dependent LHR activity. Our experiments strengthen the concept of a major contribution of the LHR-ECD in the activation mechanism apart from hormone binding and provide evidence for a cooperative model with structural and functional interactions of the ECD and the transmembrane domain.