Fig. 1.

A: Quality control system in the endoplasmic reticulum (ER). Newly synthesized proteins (e.g. GPCRs) are translocated to the lumen of the ER where folding is facilitated by molecular chaperones (oval structures); if folding fails, misfolded proteins are retained in the ER and targeted for degradation by cytosolic proteasomes that cleaved the protein into fragments (short lines) (step1). Pharmacoperones diffuse into the cell (step 2) and selectively bind the misfolded protein to influence folding (step 3) and allow correct routing to the Golgi complex for further processing (e.g. glycosylation) (step 4). Previously synthesized misfolded proteins, retained by the QCS, may still be rescued by pharmacoperones (steps 2 and 3 at left). Mature proteins are then delivered to the cell surface membrane (step 5), where the pharmacoperone is dissociated from the receptor protein (step 6) allowing the receptor to interact with its cognate ligand (step 7). B: Location of the inactivating mutations in the human GnRHR. Grey ovals are mutations that lead to partial hypogonadotropic hypogonadism, whereas black ovals are mutations that provoke complete hypogonadotropic hypogonadism. Also shown is the location of K¹⁹¹ at the second extracellular loop, which destabilizes the formation of the C¹⁴-C²⁰⁰ bridge, as well as the location of D⁹⁸ (in TM2 2) and K¹²¹ (in TM3) that form the surrogate D⁹⁸-K¹²¹ salt bridge upon pharmacoperone action. Transmembrane domains 1, 3, 5, and 7 are numbered. EC: Extracellular domain; TM: Transmembrane domains; IC: Intracellular domains. C: Enlarged diagram showing specific interactions between E⁹⁰ (in TM2), N³¹⁵ (in TM7), K¹²¹ (in TM3) and S¹²⁴ (in TM3), which form a microdomain important for GnRHR stability. D⁹⁸ and K¹²¹ are shown within the dashed line ovals; the dashed line between the ovals represents the surrogate salt bridge that is formed upon pharmacoperone binding.