A Golgi-bypass secretion mechanism for proTGFα involving TMED9 and GRASP65

We recently demonstrated that the endoplasmic reticulum export of proTGFα 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 proTGFα trafficking, we uncovered an RHBDL4-independent pathway in which the cargo receptor TMED9 induces unconventional protein secretion (UcPS) of ectopically expressed proTGFα. 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. TMED9- and GRASP65-induced proTGFα secretion is insensitive to Brefeldin A treatment and involves trafficking through pre-Golgi compartments. 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. At present, our data establish the existence of a TMED9/GRASP65-dependent Golgi-bypass trafficking route under conditions of ectopic cargo expression, whereas its endogenous physiological relevance remains to be established.