TFG facilitates outer coat disassembly on COPII transport carriers to promote tethering and fusion with ER–Golgi intermediate compartments

Significance

The endoplasmic reticulum (ER) serves as a platform for the packaging of most secretory proteins into conserved coat protein complex II (COPII)-coated transport carriers destined for ER–Golgi intermediate compartments (ERGIC) in animal cells. In this work, we demonstrate that Trk-fused gene (TFG), a protein implicated in multiple neurodegenerative diseases and oncogenesis, functions in this pathway by interacting directly with the COPII protein Sec23. Specifically, we show that TFG outcompetes interactions between the inner and outer layers of the COPII coat, indicating that TFG promotes the uncoating process after transport carriers undergo scission from the ER. Moreover, we demonstrate that TFG simultaneously captures and concentrates COPII transport carriers at the ER/ERGIC interface to enable the rapid movement of secretory cargoes to the ERGIC.

Abstract

The conserved coat protein complex II (COPII) mediates the initial steps of secretory protein trafficking by assembling onto subdomains of the endoplasmic reticulum (ER) in two layers to generate cargo-laden transport carriers that ultimately fuse with an adjacent ER–Golgi intermediate compartment (ERGIC). Here, we demonstrate that Trk-fused gene (TFG) binds directly to the inner layer of the COPII coat. Specifically, the TFG C terminus interacts with Sec23 through a shared interface with the outer COPII coat and the cargo receptor Tango1/cTAGE5. Our findings indicate that TFG binding to Sec23 outcompetes these other associations in a concentration-dependent manner and ultimately promotes outer coat dissociation. Additionally, we demonstrate that TFG tethers vesicles harboring the inner COPII coat, which contributes to their clustering between the ER and ERGIC in cells. Together, our studies define a mechanism by which COPII transport carriers are retained locally at the ER/ERGIC interface after outer coat disassembly, which is a prerequisite for fusion with ERGIC membranes.