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J. Cell Biol.,
Volume 145, Number 1, April 5, 1999 69-81
Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, Illinois 60637
Golgi stacks are often located near sites of
"transitional ER" (tER), where COPII transport vesicles are produced. This juxtaposition may indicate that
Golgi cisternae form at tER sites. To explore this idea,
we examined two budding yeasts: Pichia pastoris, which has coherent Golgi stacks, and Saccharomyces cerevisiae, which has a dispersed Golgi. tER structures in the
two yeasts were visualized using fusions between green
fluorescent protein and COPII coat proteins. We also
determined the localization of Sec12p, an ER membrane protein that initiates the COPII vesicle assembly
pathway. In P. pastoris, Golgi stacks are adjacent to discrete tER sites that contain COPII coat proteins as
well as Sec12p. This arrangement of the tER-Golgi
system is independent of microtubules. In S. cerevisiae,
COPII vesicles appear to be present throughout the cytoplasm and Sec12p is distributed throughout
the ER, indicating that COPII vesicles bud from the
entire ER network. We propose that P. pastoris has
discrete tER sites and therefore generates coherent
Golgi stacks, whereas S. cerevisiae has a delocalized tER and therefore generates a dispersed Golgi. These
findings open the way for a molecular genetic analysis
of tER sites.
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