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Sequential interactions with Sec23 control the direction of vesicle traffic



How the directionality of vesicle traffic is achieved remains an important unanswered question in cell biology. The Sec23p/Sec24p coat complex sorts the fusion machinery (SNAREs) into vesicles as they bud from the endoplasmic reticulum (ER). Vesicle tethering to the Golgi begins when the tethering factor TRAPPI binds to Sec23p. Where the coat is released and how this event relates to membrane fusion is unknown. Here we use a yeast transport assay to demonstrate that an ER-derived vesicle retains its coat until it reaches the Golgi. A Golgi-associated kinase, Hrr25p (CK1δ orthologue), then phosphorylates the Sec23p/Sec24p complex. Coat phosphorylation and dephosphorylation are needed for vesicle fusion and budding, respectively. Additionally, we show that Sec23p interacts in a sequential manner with different binding partners, including TRAPPI and Hrr25p, to ensure the directionality of ER–Golgi traffic and prevent the back-fusion of a COPII vesicle with the ER. These events are conserved in mammalian cells.

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Figure 1: COPII vesicles lose their ability to bind TRAPPI after Uso1p acts.
Figure 2: Hrr25p resides on the Golgi and phosphorylates Sec23p/Sec24p.
Figure 3: Phosphorylation of S742 and T747 blocks ER–Golgi traffic in vitro.
Figure 4: TRAPPI and Sar1p–GTP cannot bind to Sec23p simultaneously.
Figure 5: In the presence of IC261, cargo is exported more rapidly but remains at peripheral pre-Golgi sites.
Figure 6: Sec23p ensures the direction of ER–Golgi traffic.

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We thank R. Schekman, M. Cyert, L. Miller, B. Glick, M. Lowe and E. Mizuno-Yamasaki for strains, constructs, purified protein and antibody; K. Reinisch for discussions; S. Chen, H. Cai and M. Garcia-Marcos for advice; W. Zhou and A. Lougheed for technical assistance. This work was supported by the Howard Hughes Medical Institute. Salary support for S.F.-N., D.B. and S.M. was provided by the Howard Hughes Medical Institute, P.G. was funded by the Burroughs Wellcome Fund and M.G. by the SRP Super Fund Research Program. Work at the University of Montana was supported by National Institutes of Health grant GM-059378 (to J.C.H.) and the National Institutes of Health COBRE Center grant RR-015583.

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C.L., D.B., S.M., D.N. and J.H. performed experiments and analysed data. M.G. did the mass analysis and P.G. did the computational modelling and analysed data. S.F.-N. directed the project, analysed data and wrote the paper. C.L., D.B., J.H. and P.G. co-wrote the paper.

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Correspondence to Susan Ferro-Novick.

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Lord, C., Bhandari, D., Menon, S. et al. Sequential interactions with Sec23 control the direction of vesicle traffic. Nature 473, 181–186 (2011).

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