A Golgi-bypass secretion mechanism for proTGFα involving TMED9 and GRASP65
We recently demonstrated that the endoplasmic reticulum export of proTGF is tightly gated, and that proteostasis factors, including the intramembrane protease RHBDL4, can promote its release by an unknown mechanism. While investigating a potential link between RHBDL4-regulated p24/TMED family proteins and proTGF trafficking, we uncovered an RHBDL4-independent pathway in which the cargo receptor TMED9 induces unconventional protein secretion (UcPS) of ectopically expressed proTGF. This pathway selectively requires GRASP65, but not the closely related GRASP55, and depends on components of the autophagic machinery and the ESCRT-associated protein ALIX, consistent with a Golgi-bypass secretion route. Although largely based on overexpression systems, our findings identify a previously unrecognized UcPS mechanism and provide insight into alternative protein trafficking pathways that may be exploited under conditions of proteostasis stress and in pathological contexts such as cancer and inflammation.