Sugar on the Coat: Site-Specific O-GlcNAc and Myoferlin Adapt COPII for Collagen Export.

Recent work by Hirata and colleagues identifies site-specific O-GlcNAc modification of the cargo-selecting COPII subunit Sec24D as a dynamic regulatory switch that promotes collagen export from the endoplasmic reticulum (ER), providing new insight into how COPII accommodates cargoes far larger than canonical transport vesicles. Two glycosites in the poorly characterized N-terminal intrinsically disordered region (IDR) of Sec24D, T9 and S13, display strikingly different dynamics: S13 is constitutively glycosylated, whereas T9 is rapidly modified following a synchronized burst of collagen transport and returns to baseline within minutes. Both sites are required for collagen I trafficking and rescue of Sec24D-deficient bulldog zebrafish, while T9 is also required for efficient transport of the smaller cargo E-cadherin, suggesting that a single intrinsically disordered region can encode cargo-selective information. Crosslinking proteomics further reveals distinct glycosylation-dependent changes in the Sec24D interactome and identifies myoferlin (MYOF) as a partner proposed to promote fusion of ER exit sites (ERES) with ER-Golgi intermediate compartment (ERGIC) membranes. In this commentary, we discuss how these findings recast the COPII inner coat as a signal-integrating platform, intersect with TANGO1-dependent ERGIC recruitment and competing models of large collagen carriers, and raise key mechanistic questions concerning signal transmission across the ER membrane, glycan stoichiometry, and how the recruitment or tethering function proposed for MYOF is coupled to membrane fusion.