Control-quality checkpoints regulate the delivery of neo-synthesized polytopic plasma membrane proteins, such as receptors, ion channels or transporters, from the Endoplasmic Reticulum (ER) to the plasma membrane. Cargo plasma membrane proteins are loaded in COPII transport vesicles at ER exit sites (ERES). Cargo capture requires the recognition of export signals, such as di-acidic DXE/D motifs, by the COPII vesicle adaptor protein Sec24. Retention mechanisms in the ER, on the other hand, control cargo access to COPII vesicles. Arginine-based (RXR) retention motifs were identified, which supposedly mediate cargo interaction with putative ER resident gatekeepers, such as PRAF2.
The present study uncovered the involvement of PRAF2 in the RXR-dependent retention of the Cystic Fibrosis Transmembrane Conductance Regulator CFTR in the ER. CFTR is mutated and almost totally absent at the cell surface in cystic fibrosis patients. PRAF2 binds to and retains both wild type CFTR and the cystic fibrosis-causing CFTR-F508del mutant upstream of ERES. Because of its lower abundance, compared to wild type CFTR, CFTR-F508del recruitment into COPII vesicles is suppressed by the ER-resident PRAF2.
Figure legend: Transport of mutated CFTR (CFTR-F508 del) to ER exit sites from which COPII vesicles bud. In untreated patients, the malformed CFTR-F508 del (orange) is scarce and completely retained by PRAF2. In the presence of VX-445, CFTR-F508 del is better folded (grey), more abundant and manages to quantitatively overcome the retention by PRAF2. It can then be included in the COPII vesicles thanks to its interaction with Sec 24 and addressed to the cell surface.
Reducing the concentration of endogenous PRAF2 or mutation of the PRAF2-interaction motif both enhanced the maturation of CFTR-F508del. Moreover, second generation pharmacological chaperones used to treat CF patients targeted PRAF-2 dependent CFTR-F508del retention. Whereas VX-809 and VX-661 enhanced the flow of CFTR-F508del to override PRAF2 retention, VX-445 behaved as a specific negative modulator of CFTR-PRAF2 interaction.
These findings open new therapeutic perspectives for diseases caused by the impaired cell surface trafficking of mutant PMPs, which contain RXR-based retention motifs that might be recognized by PRAF2 (V2 vasopressin receptor, mutated in the nephrogenic diabetes insipidus, calcium sensing CaSR receptors, mutated in familial hypocalciuric hypercalcemia, gonadotrophin GnRH receptor, mutated in hypogonadotropic hypogonadism…).
Reference
Saha K et al. Pharmacological chaperone-rescued cystic fibrosis CFTR-F508del mutant overcomes PRAF2-gated access to endoplasmic reticulum exit sites. Cellular and Molecular Life Sciences. (2022) 79:530, DOI : 10.1007/s00018-022-04554-1
