Post-translational Processing in the Golgi Plays a Critical Role in the Trafficking of the Luteinizing Hormone/Human Chorionic Gonadotropin Receptor to the Cell Surface

Point mutations in the luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor have been shown to cause constitutive activation which results in precocious puberty in affected males. We introduced one of these mutations, Asp-556 → Gly, into the rat LH/hCG receptor and demonstrated that the mutant receptor constitutively activated adenylate cyclase in transfected 293 T cells. The cell surface expression of the mutant receptor was lower than that of the wild type receptor. Pulse-chase studies showed that the 73-kDa precursor of both the mutant and wild type receptors was synthesized at comparable efficiencies. However, post-translational processing of the mutant receptor to the mature 92-kDa form, which has N-linked complex type oligosaccharide chains, was impaired. Sensitivity of the mutant receptor to peptide-N-glycanase F and endoglycosidase H, and insensitivity to sialidase indicated that the 73-kDa species represents the high mannose form that has not yet been trafficked through the medial and trans Golgi. Additionally, although the wild type receptor was palmitoylated, the mutant receptor was not. Although the high mannose 73-kDa species is capable of binding LH/hCG, our results show that post-translational processing in the Golgi is required for the mature 92-kDa receptor to reach the cell surface.