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Published 25 September 2006. doi:10.1083/jcb.200605106
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 174, Number 7, 973-983
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Article

 Exomer: a coat complex for transport of select membrane proteins from the trans-Golgi network to the plasma membrane in yeast

Chao-Wen Wang1,2, Susan Hamamoto1,2, Lelio Orci3, and Randy Schekman1,2

1 Department of Molecular Cell Biology and 2 Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, CA 94720
3 Department of Cell Physiology and Metabolism, University Medical Center, 1121-Geneva 4, Switzerland

Correspondence to Randy Schekman: schekman{at}berkeley.edu

Ayeast plasma membrane protein, Chs3p, transits to the mother–bud neck from a reservoir comprising the trans-Golgi network (TGN) and endosomal system. Two TGN/endosomal peripheral proteins, Chs5p and Chs6p, and three Chs6p paralogues form a complex that is required for the TGN to cell surface transport of Chs3p. The role of these peripheral proteins has not been clear, and we now provide evidence that they create a coat complex required for the capture of membrane proteins en route to the cell surface. Sec7p, a Golgi protein required for general membrane traffic and functioning as a nucleotide exchange factor for the guanosine triphosphate (GTP)–binding protein Arf1p, is required to recruit Chs5p to the TGN surface in vivo. Recombinant forms of Chs5p, Chs6p, and the Chs6p paralogues expressed in baculovirus form a complex of approximately 1 MD that binds synthetic liposomes in a reaction requiring acidic phospholipids, Arf1p, and the nonhydrolyzable GTP{gamma}S. The complex remains bound to liposomes centrifuged on a sucrose density gradient. Thin section electron microscopy reveals a spiky coat structure on liposomes incubated with the full complex, Arf1p, and GTP{gamma}S. We termed the novel coat exomer for its role in exocytosis from the TGN to the cell surface. Unlike other coats (e.g., coat protein complex I, II, and clathrin/adaptor protein complex), the exomer does not form buds or vesicles on liposomes.

Abbreviations used in this paper: ARF, ADP ribosylation factor; BFA, brefeldin A; COP, coat protein complex; FPLC, fast protein liquid chromatography; GEF, guanine nucleotide exchange factor; mArf1p, myristoylated Arf1p; NTA, nitrilotriacetic acid; PA, phosphatidic acid; PE, phosphatidylethanoamine.


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