Published 6 December 2004. doi:10.1083/jcb.200408001
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 167, Number 5, 875-887
Functional specialization within a vesicle tethering complex
:
bypass of a subset of exocyst deletion mutants by Sec1p or Sec4p
Andreas Wiederkehr1,
Johan-Owen De Craene1,
Susan Ferro-Novick2, and
Peter Novick1
1 Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510
2 Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, CT 06510
Correspondence to Peter Novick: peter.novick{at}yale.edu
The exocyst is an octameric protein complex required to tether secretory vesicles to exocytic sites and to retain ER tubules at the apical tip of budded cells. Unlike the other five exocyst genes, SEC3, SEC5, and EXO70 are not essential for growth or secretion when either the upstream activator rab, Sec4p, or the downstream SNARE-binding component, Sec1p, are overproduced. Analysis of the suppressed sec3
, sec5
, and exo70
strains demonstrates that the corresponding proteins confer differential effects on vesicle targeting and ER inheritance. Sec3p and Sec5p are more critical than Exo70p for ER inheritance. Although nonessential under these conditions, Sec3p, Sec5p, and Exo70p are still important for tethering, as in their absence the exocyst is only partially assembled. Sec1p overproduction results in increased SNARE complex levels, indicating a role in assembly or stabilization of SNARE complexes. Furthermore, a fraction of Sec1p can be coprecipitated with the exoycst. Our results suggest that Sec1p couples exocyst-mediated vesicle tethering with SNARE-mediated docking and fusion.
Abbreviations used in this paper: 5FOA, 5-fluoroorotic acid; DIC, differential interference contrast; SC, synthetic complete; SM, Sec1p/Munc18-like proteins.

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