Coat protein complex II (COPII) mediates anterograde trafficking from the endoplasmic reticulum (ER). While the core COPII machinery is well-characterized, how cells regulate COPII to accommodate large cargoes, including collagens, remains incompletely understood. Here, we show that the cargo-selecting COPII subunit Sec24D is modified by site-specific O-linked β-N-acetylglucosamine (O-GlcNAc) in its N-terminal intrinsically disordered region upon induction of collagen transport. These glycosylations are required for collagen trafficking in human cells and developing zebrafish. Crosslinking proteomics demonstrated that each O-GlcNAcylation influences the Sec24D interactome in a distinct way, regulating nearly all steps of COPII-mediated transport through protein-protein interactions. In particular, myoferlin interacts with glycosylated Sec24D and unexpectedly facilitates fusion of ER exit sites (ERES) and the ER-Golgi intermediate compartment (ERGIC) to enable collagen transport. Our results establish Sec24D O-GlcNAcylation as a dynamic regulator of COPII protein-protein interactions and collagen trafficking and identify myoferlin as a mediator of this process.
Hirata et al. (Wed,) studied this question.