Self‐Organization of Transitional ER Sites

We are studying the biogenesis and dynamics of transitional ER (tER) sites using the budding yeast Pichia pastoris . Previously, we showed that P. pastoris tER sites are long‐lived structures that form de novo , fuse upon collision, and grow or shrink to attain a steady‐state size. This behavior can be explained by a self‐organization model. Specifically, we postulate that capture of new tER components is balanced by shrinkage driven by the budding of COPII coated transport vesicles. To test this model, we used a genetic screen to identify Sec16 as a key player in tER organization. Sec16 is a large peripheral ER membrane protein that interacts with multiple COPII components. Our results suggest that Sec16 acts as a negative regulator of ER export. According to this view, Sec16 functions primarily to control tER dynamics rather than to nucleate tER site formation. Sec16 defines a new level of regulation for the ER export system.