TMED9 coordinates the clearance of misfolded GPI-anchored proteins out of the ER and into the Golgi

The p24-family member, TMED9, has recently emerged as a player in secretory pathway protein quality control (PQC) that influences the trafficking and degradation of misfolded proteins. Here, we show that TMED9 plays a central role in the PQC of GPI-anchored proteins (GPI-APs). Typically, upon release from the endoplasmic reticulum (ER)-resident chaperone calnexin, misfolded GPI-APs traffic to the Golgi by an ER-export pathway called Rapid ER stress-induced Export (RESET). From the Golgi, they access the plasma membrane where they are rapidly internalized for lysosomal degradation. We used biochemical and imaging approaches in cultured cells to demonstrate that at steady-state, the majority of misfolded GPI-APs reside in the ER in association with calnexin and TMED9. During RESET, they dissociate from calnexin and increase their association with TMED9. Inhibition of TMED9's function through siRNA-induced depletion or chemical inhibitor, BRD4780, blocked ER-export of misfolded GPI-APs. In contrast, TMED9-inhibition did not prevent ER-export of wild-type GPI-APs, indicating a specific role for TMED9 in GPI-AP PQC. Intriguingly, we discovered that acute treatment with BRD4780 induced a shift in TMED9 localization away from the ER to the downstream Golgi cisternae and blocked the RESET pathway. Upon removal of BRD4780 following acute treatment, TMED9 regained access to the ER where TMED9 was able to associate with the RESET substrate and restore the RESET pathway. These results suggest that TMED9 plays a requisite role in RESET by capturing misfolded GPI-APs that are released by calnexin within the ER and conveying them to the Golgi.