Essential Cell Biology 5th edition

Secretory Pathways
519
cis
Golgi
network
transport
vesicle
vacuole
cis
cisterna
medial
cisterna
trans
cisterna
trans
Golgi
network
(A)
Figure 15−26 The Golgi apparatus consists of a stack of flattened,
membrane-enclosed sacs. (A) A three-dimensional model of a Golgi stack
reconstructed from a sequential series of electron micrographs of the Golgi
apparatus in a secretory animal cell. To see how such models are assembled,
watch Movie 15.8. (B) Electron micrograph of a Golgi stack from a plant cell,
where the Golgi apparatus is especially distinct; the stack is oriented as in (A).
(C) A pita-bread model of the Golgi apparatus. Vesicles are shown as olives.
(B, courtesy of George Palade.)
(B)
(C)
200 nm
The proteins travel through the cisternae in sequence ECB5 in two E15.26/15.26
ways: (1) by
means of transport vesicles that bud from one cisterna and fuse with the
next; and (2) by a maturation process in which the Golgi cisternae themselves
migrate through the Golgi stack. Proteins finally exit from the trans
Golgi network in transport vesicles destined for either the cell surface or
another organelle of the endomembrane system (see Figure 15−19).
Both the cis and trans Golgi networks are thought to be important for
protein sorting: proteins entering the cis Golgi network can either move
onward through the Golgi stack or, if they contain an ER retention signal,
be returned to the ER; proteins exiting from the trans Golgi network
are sorted according to whether they are destined for lysosomes (via
endosomes) or for the cell surface. We discuss some examples of sorting
by the trans Golgi network later, and we present some of the methods for
tracking proteins through the secretory pathways of the cell in How We
Know, pp. 520–521.
Many of the oligosaccharide chains that are added to proteins in the ER
(see Figure 15–24) undergo further modifications in the Golgi apparatus.
On some proteins, for example, more complex oligosaccharide chains
are created by a highly ordered process in which sugars are added and
removed by a series of enzymes that act in a rigidly determined sequence
as the protein passes through the Golgi stack. As would be expected, the
enzymes that act early in the chain of processing events are located in
cisternae close to the cis face, while enzymes that act late are located
in cisternae near the trans face.
Secretory Proteins Are Released from the Cell by
Exocytosis
In all eukaryotic cells, a steady stream of vesicles buds from the trans
Golgi network and fuses with the plasma membrane in the process of
exocytosis. This constitutive exocytosis pathway supplies the plasma