Protein disulfide isomerase chaperone ERP-57 decreases plasma membrane expression of the human GnRH receptor.

Retention of misfolded proteins by the endoplasmic reticulum (ER) is a quality control mechanism involving the participation of endogenous chaperones such as calnexin (CANX). CANX interacts with and restricts plasma membrane expression (PME) of the gonadotropin releasing hormone receptor (GnRHR), a G protein-coupled receptor. CANX also interacts with ERP-57 a thiol oxidoreductase chaperone present in the ER. CANX along with ERP-57 promotes the formation of disulfide bond bridges in nascent proteins. The human GnRH receptor (hGnRHR) is stabilized by two disulfide bond bridges (C(14)-C(200) and C(114)-C(196)), that, when broken, lead to a decrease in receptor expression at the plasma membrane. To determine if the presence of chaperones CANX and ERP-57 exerts an influence over membrane routing and second messenger activation, we assessed the effect of various mutants including those with broken disulfide bridges (Cys --> Ala) along with the hGnRHR. The effect of chaperones on mutants was insignificant, whereas the over expression of ERP-57 led to an hGnRHR retention. This effect was further enhanced by cotransfection with cDNA for CANX showing receptor retention by ERP-57 augmented by CANX, suggesting utilization of these chaperones for quality control of the GnRHR. 2009 John Wiley & Sons, Ltd.