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J. Cell Biol.,
Volume 144, Number 6, March 22, 1999 1135-1149

* Laboratory for Three-Dimensional Fine Structure and Three-dimensional reconstructions of portions of the Golgi complex from cryofixed, freeze-substituted normal rat kidney cells have been made by
dual-axis, high-voltage EM tomography at ~7-nm resolution. The reconstruction shown here (~1 × 1 × 4 µm3) contains two stacks of seven cisternae separated
by a noncompact region across which bridges connect
some cisternae at equivalent levels, but none at nonequivalent levels. The rest of the noncompact region is
filled with both vesicles and polymorphic membranous elements. All cisternae are fenestrated and display
coated buds. They all have about the same surface area,
but they differ in volume by as much as 50%. The trans-most cisterna produces exclusively clathrin-coated
buds, whereas the others display only nonclathrin coated buds. This finding challenges traditional views of
where sorting occurs within the Golgi complex. Tubules
with budding profiles extend from the margins of both
cis and trans cisternae. They pass beyond neighboring
cisternae, suggesting that these tubules contribute to
traffic to and/or from the Golgi. Vesicle-filled "wells"
open to both the cis and lateral sides of the stacks. The
stacks of cisternae are positioned between two types of
ER, cis and trans. The cis ER lies adjacent to the ER-Golgi intermediate compartment, which consists of discrete polymorphic membranous elements layered in
front of the cis-most Golgi cisterna. The extensive trans
ER forms close contacts with the two trans-most cisternae; this apposition may permit direct transfer of lipids between ER and Golgi membranes. Within 0.2 µm of
the cisternae studied, there are 394 vesicles (8 clathrin
coated, 190 nonclathrin coated, and 196 noncoated), indicating considerable vesicular traffic in this Golgi region. Our data place structural constraints on models of
trafficking to, through, and from the Golgi complex.
Department of Molecular, Cellular, and Developmental Biology,
University of Colorado, Boulder, Colorado 80309-0347; and § Department of Cellular and Structural Biology University of
Colorado School of Medicine, Denver, Colorado 80262-1111
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