Published 31 July 2006. doi:10.1083/jcb.200603044
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 174, Number 3, 359-368
mBet3p is required for homotypic COPII vesicle tethering in mammalian cells
Sidney Yu1,2,
Ayano Satoh2,
Marc Pypaert2,
Karl Mullen3,
Jesse C. Hay3, and
Susan Ferro-Novick1,2
1 Howard Hughes Medical Institute and 2 Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06519
3 Division of Biological Sciences, The University of Montana, Missoula, MT 59812
Correspondence to Susan Ferro-Novick: susan.ferronovick{at}yale.edu
TRAPPI is a large complex that mediates the tethering of COPII vesicles to the Golgi (heterotypic tethering) in the yeast Saccharomyces cerevisiae. In mammalian cells, COPII vesicles derived from the transitional endoplasmic reticulum (tER) do not tether directly to the Golgi, instead, they appear to tether to each other (homotypic tethering) to form vesicular tubular clusters (VTCs). We show that mammalian Bet3p (mBet3p), which is the most highly conserved TRAPP subunit, resides on the tER and adjacent VTCs. The inactivation of mBet3p results in the accumulation of cargo in membranes that colocalize with the COPII coat. Furthermore, using an assay that reconstitutes VTC biogenesis in vitro, we demonstrate that mBet3p is required for the tethering and fusion of COPII vesicles to each other. Consistent with the proposal that mBet3p is required for VTC biogenesis, we find that ERGIC-53 (VTC marker) and Golgi architecture are disrupted in siRNA-treated mBet3p-depleted cells. These findings imply that the TRAPPI complex is essential for VTC biogenesis.
Abbreviations used in this paper: MVB, multivesicular body; tER, transitional ER; VTC, vesicular tubular cluster.

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