Our data suggest that Sec16 helps to organize patches of COPII-coat proteins into clusters that represent tER sites. The Golgi disruption that occurs in the sec16 mutant provides evidence that Golgi structure in budding yeasts depends on tER organization.

COPII vesicles bud from an ER domain known as the transitional ER (tER). Assembly of the COPII coat is initiated by the transmembrane guanine nucleotide exchange factor Sec12. In the budding yeast Pichia pastoris, Sec12 is concentrated at tER sites. Previously, we found that the tER localization of P. pastoris Sec12 requires a saturable binding partner. We now show that this binding partner is Sec16, a peripheral membrane protein that functions in ER export and tER organization. One line of evidence is that overexpression of Sec12 delocalizes Sec12 to the general ER, but simultaneous overexpression of Sec16 retains overexpressed Sec12 at tER sites. Additionally, when P. pastoris Sec12 is expressed in S. cerevisiae, the exogenous Sec12 localizes to the general ER, but when P. pastoris Sec16 is expressed in the same cells, the exogenous Sec12 is recruited to tER sites. In both of these experimental systems, the ability of Sec16 to recruit Sec12 to tER sites is abolished by deleting a C-terminal fragment of Sec16. Biochemical experiments confirm that this C-terminal fragment of Sec16 binds to the cytosolic domain of Sec12. Similarly, we demonstrate that human Sec12 is concentrated at tER sites, likely due to association with a C-terminal fragment of Sec16A. These results suggest that a Sec12–Sec16 interaction has a conserved role in ER export.

No abstract

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.