Dynamic O-GlcNAcylation of Sec23-interacting protein regulates COPII function.

About one-third of the eukaryotic proteome transits the secretory pathway to reach its correct cellular or extracellular destination. At the earliest stage, transport from the endoplasmic reticulum (ER) to the ER-Golgi intermediate compartment (ERGIC) or Golgi apparatus is mediated by coat protein complex II (COPII). COPII coats consist of inner and outer layers formed by Sec23-Sec24 heterodimers and Sec13-Sec31 heterotetramers, respectively, which initially assemble at ER exit sites (ERES) to form transport carriers. Sec23-interacting protein (Sec23IP) links the inner and outer coats through its interactions with both Sec23A and Sec31A, positioning it as a key potential regulator of COPII function. However, the mechanisms controlling Sec23IP activity remain poorly understood. Here, we investigate how physiological stimuli regulate COPII function through the dynamic modification of Sec23IP by O-linked β-N-acetylglucosamine (O-GlcNAc), a reversible, intracellular form of glycosylation. We first validated Sec23IP as a bona fide O-GlcNAcylated protein. Rescue experiments in Sec23IP knockout cells with a nearly unglycosylatable mutant protein demonstrated the essential role of O-GlcNAcylation in the intrinsically disordered domain in protein transport and in recruiting Sec31A to ERES. Moreover, O-GlcNAcylation of Sec23IP increased during protein transport, coinciding with a reduction in its interaction with Sec31A. These results indicate that distinct site-specific O-GlcNAcylation of Sec23IP spatiotemporally modulates its association with Sec31A to fine-tune ERES recruitment and COPII assembly/disassembly. Our work provides new insight into Sec23IP regulation and suggests that O-GlcNAc on other COPII proteins may govern carrier formation, uncoating, and transport.