Minisymposium 21: Protein and Vesicle Trafficking

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Abs # M2101: Plant COPII vesicles bud with an AtUso1-containing scaffold that attaches to the cis-side of the Golgi matrix to mediate vesicle transfer

Presenter:	Kang, Byung-Ho       Contact Presenter
Authors	Kang, Byung-Ho  (A)   Staehelin, L. Andrew  (A)
Affiliations:	(A): MCD Biology University of Colorado at Boulder, Boulder, CO 80309

Plant Golgi stacks are mobile, traveling along actin tracks at speeds of up to ~4 μm/sec. How COPII vesicles are transferred from ER export sites to the moving Golgi stacks is not understood. We have examined COPII vesicle transfer in high-pressure frozen/freeze-substituted plant root tip cells and suspension culture cells by electron tomography. Formation of each COPII vesicle is accompanied by the assembly of a ribosome-excluding scaffold that extends ~40 nm beyond the COPII coat. These COPII scaffolds capture Golgi stacks by binding to the cis-side of the Golgi matrix, thereby forcing the Golgi stacks to stop at active ER export sites. The COPII vesicles are then transferred together with their scaffolds to the Golgi matrix. In root meristem cells, Golgi stacks docked to ER export sites via the scaffolding structures had an average of 4.1 COPII vesicles, whereas non-docked Golgi stacks had 1.5 COPII vesicles. The COPII scaffold contains AtUso1, a plant homolog of the Saccharomyces cerevisiae COPII vesicle-tethering factor, ScUso1p. In time-lapse imaging, AtUso1-GFP is associated mostly with wiggling Golgi stacks and not with fast moving ones. Our data demonstrate that the AtUso1-containing COPII scaffold mediates ER-to-Golgi COPII vesicle transfer by first capturing Golgi stacks and then by fusing with the Golgi matrix. We also postulate that the docked, wiggling Golgi stacks harvest COPII vesicles by a plucking mechanism.