The p24-family member, TMED9, coordinates clearance of misfolded GPI-anchored proteins from the ER to the Golgi via the Rapid ER Stress-Induced Export pathway

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, misfolded GPI-APs dissociate from the endoplasmic reticulum (ER)-resident chaperone calnexin and move to the Golgi by the Rapid ER stress-induced Export (RESET) pathway, and are subsequently trafficked to lysosomes via the plasma membrane. We used biochemical and imaging approaches in cultured cells to demonstrate that misfolded GPI-APs dissociate from calnexin and associate with TMED9 during RESET. Inhibition of TMED9’s function through siRNA-induced depletion or chemical inhibitor, BRD4780, blocked ER-export of misfolded GPI-APs. By 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 a block to 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.