Calnexin regulated gonadotropin-releasing hormone receptor plasma membrane expression

A significant proportion of human gonadotropin-releasing hormone receptors (GnRHRs) are normally retained in the endoplasmic reticulum (ER); however, nearly all rat GnRHRs are routed to the plasma membrane. When mutations are introduced into either receptor, considerably more of the proteins are recognized by the quality control system (QCS) as misfolded and retained compared with wild-type (WT) receptor, resulting in decreased signaling in the presence of agonist. Calnexin, a component of the QCS, decreased plasma membrane expression of the GnRHRs, an effect that was mediated by a physical interaction between the receptor and the calnexin. Only the human receptor showed reduced signaling because it had fewer spare receptors compared with the rat GnRHR, allowing calnexin to affect signaling. Calnexin did not affect receptor signaling when K(191) was deleted from the human WT GnRHR. Removal of this amino acid decreases receptor misfolding and increases plasma membrane expression. K(191) is not present in the rat WT GnRHR. A pharmacological chaperone that corrects GnRHR misfolding, increased expression of the human WT GnRHR in the presence of calnexin. Calnexin apparently retains misfolded GnRHRs but routes correctly folded receptors to the plasma membrane. Mutation of a calnexin protein kinase C consensus phosphorylation site promoted increased retention of the human GnRHR, suggesting that calnexin phosphorylation controls the retention mechanism. We conclude that a proportion of the human and the rat WT GnRHR appears to be retained in the ER by calnexin, an effect that decreases GnRHR signaling capacity.