Sec16 and Sed4 interdependently function as interaction and localization partners at ER exit sites

Abstract

COPII proteins assemble at ER exit sites (ERES) to form transport carriers. The initiation of COPII assembly in the yeast Saccharomyces cerevisiae is triggered by the ER membrane protein Sec12. Sec16, which plays a critical role in COPII organization, localizes to ERES independently of Sec12. However, the mechanism underlying Sec16 localization is poorly understood. Here, we show that a Sec12 homolog, Sed4, is concentrated at ERES and mediates ERES localization of Sec16. We found that the interaction between Sec16 and Sed4 ensures their correct localization to ERES. Loss of the interaction with Sec16 leads to redistribution of Sed4 from the ERES specifically to high-curvature ER areas, such as the tubules and edges of the sheets. The luminal domain of Sed4 mediates this distribution, which is required for Sed4, but not for Sec16, to be concentrated at ERES. We further show that the luminal domain and its O-mannosylation are involved in the self-interaction of Sed4. Our findings provide insight into how Sec16 and Sed4 function interdependently at ERES.

Remodeling of the secretory pathway is coordinated withde novomembrane formation in budding yeast gametogenesis

“…7G). This is consistent with the finding of mutual localization between Sec16 and Sed4 40 . These observations suggest that Sed4 together with Sec16 is also involved in ERES regulation by PP1-Gip1.…”

“…Although TANGO1 is absent in budding yeast, development-specific regulation of Sec16 recruitment could exist. We found that the mutant of Sed4, a regulator of the Sar1 GTPase cycle and Sec16 localization to ERES 40,41 , partially phenocopied gip1 Δ, suggesting involvement of these factors in the ERES formation during meiosis.…”

“…Yorimitsu et al have revealed that Sec16 foci formation is mediated through the interaction with Sed4 40 . Sed4 is a non-essential paralog of Sec12, the ER-resident transmembrane guanine-nucleotide-exchange-factor (GEF) for Sar1 GTPase, which has no catalytic activity toward Sar1, but is proposed to stimulate Sar1 activity 41–43 .…”