The extracellular domain of luteinizing hormone receptor dictates its efficiency of maturation

The processing of luteinizing hormone receptor (LHR) shows marked differences in different species. While the human LHR is predominantly expressed as the mature, 90 kDa species, rat LHR exists mostly in the 70 kDa precursor form. Since the extracellular domain of the LHR is unusually large in comparison with other G protein-coupled receptors, the present studies examined the role of extracellular domain in its processing. FLAG-tagged chimeric LH receptors were constructed by substituting the extracellular domain of the human receptor in rat LHR (hrr) and the extracellular domain of the rat receptor in human LHR (rhh). The intracellular processing, ligand binding and recycling of the chimeric receptors were compared with that of the wild type receptors in 293T cells. The results showed that the human and rat LHR were expressed predominantly as 90 and 70 kDa species, respectively, as expected. The introduction of the rat extracellular domain into the human LHR (rhh) decreased the abundance of the mature form with an increase in the precursor form. Conversely, substitution of the extracellular domain of the rat LHR by the extracellular domain of the human LHR (hrr) led to an increase in the mature form with a corresponding decrease in the precursor form. Changes were also observed in the ligand binding and recycling of the wild type and chimeric receptors. These results suggest that the extracellular domain of the LHR is one of the determinants that confer its ability for proper maturation and cell surface expression.