Protein export from the endoplasmic reticulum (ER), the first step in protein transport through the secretory pathway, is mediated by coatomer protein II (COPII)-coated vesicles at ER exit sites. COPII coat assembly on the ER is well understood and the conserved large hydrophilic protein Sec16 clearly has a role to play in COPII coat dynamics. Sec16 localizes to ER exit sites, its loss of function impairs their functional organization in all species where it has been studied, and it interacts with COPII coat subunits. However, its exact function in COPII dynamics is debated, as Sec16 is proposed to act as a scaffold to recruit COPII components and as a device to regulate the Sar1 activity in uncoating, in such a way that the coat is released only when the vesicle is fully formed and loaded with cargo. Furthermore, Sec16 has been shown to respond to nutrient signalling, thus coupling environmental stimuli to secretory capacity.

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