TMED3 Complex Mediates ER Stress‐Associated Secretion of CFTR, Pendrin, and SARS‐CoV‐2 Spike

Under ER stress conditions, the ER form of transmembrane proteins can reach the plasma membrane via a Golgi‐independent unconventional protein secretion (UPS) pathway. However, the targeting mechanisms of membrane proteins for UPS are unknown. Here, this study reports that TMED proteins play a critical role in the ER stress‐associated UPS of transmembrane proteins. The gene silencing results reveal that TMED2, TMED3, TMED9 and TMED10 are involved in the UPS of transmembrane proteins, such as CFTR, pendrin and SARS‐CoV‐2 Spike. Subsequent mechanistic analyses indicate that TMED3 recognizes the ER core‐glycosylated protein cargos and that the heteromeric TMED2/3/9/10 complex mediates their UPS. Co‐expression of all four TMEDs improves, while each single expression reduces, the UPS and ion transport function of trafficking‐deficient ΔF508‐CFTR and p.H723R‐pendrin, which cause cystic fibrosis and Pendred syndrome, respectively. In contrast, TMED2/3/9/10 silencing reduces SARS‐CoV‐2 viral release. These results provide evidence for a common role of TMED3 and related TMEDs in the ER stress‐associated, Golgi‐independent secretion of transmembrane proteins.