Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

TRANSPORT FROM THE ER THROUGH THE GOLGI apparaTUS
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In this section, we focus mainly on the Golgi apparatus (also called the Golgi
complex). It is a major site of carbohydrate synthesis, as well as a sorting and dispatching
station for products of the ER. The cell makes many polysaccharides in
the Golgi apparatus, including the pectin and hemicellulose of the cell wall in
plants and most of the glycosaminoglycans of the extracellular matrix in animals
(discussed in Chapter 19). The Golgi apparatus also lies on the exit route from the
ER, and a large proportion of the carbohydrates that it makes are attached as oligosaccharide
side chains to the many proteins and lipids that the ER sends to it. A
subset of these oligosaccharide groups serve as tags to direct specific proteins into
vesicles that then transport them to lysosomes. But most proteins and lipids, once
they have acquired their appropriate oligosaccharides in the Golgi apparatus, are
recognized in other ways for targeting into the transport vesicles going to other
destinations.
Proteins Leave the ER in COPII-Coated Transport Vesicles
To initiate their journey along the secretory pathway, proteins that have entered
the ER and are destined for the Golgi apparatus or beyond are first packaged into
COPII-coated transport vesicles. These vesicles bud from specialized regions of
the ER called ER exit sites, whose membrane lacks bound ribosomes. Most animal
cells have ER exit sites dispersed throughout the ER network.
Entry into vesicles that leave the ER can be a selective process or can happen by
default. Many membrane proteins are actively recruited into such vesicles, where
they become concentrated. These cargo membrane proteins display exit (transport)
signals on their cytosolic surface that adaptor proteins of the inner COPII
coat recognize (Figure 13–22); some of these components act as cargo receptors
and are recycled back to the ER after they have delivered their cargo to the Golgi
apparatus. Soluble cargo proteins in the ER lumen, by contrast, have exit signals
that attach them to transmembrane cargo receptors. Proteins without exit signals
can also enter transport vesicles, including protein molecules that normally function
in the ER (so-called ER resident proteins), some of which slowly leak out of
the ER and are delivered to the Golgi apparatus. Different cargo proteins enter the
transport vesicles with substantially different rates and efficiencies, which may
result from differences in their folding and oligomerization efficiencies and kinetics,
as well as the factors already discussed. The exit step from the ER is a major
checkpoint at which quality control is exerted on the proteins that a cell secretes
or displays on its surface, as we discussed in Chapter 12.
The exit signals that direct soluble proteins out of the ER for transport to the
Golgi apparatus and beyond are not well understood. Some transmembrane
proteins that serve as cargo receptors for packaging some secretory proteins into
COPII-coated vesicles are lectins that bind to oligosaccharides on the secreted
forming transport vesicle
outer COPII
coat proteins
CYTOSOL
ER LUMEN
resident
ER protein
chaperone proteins bound to
unfolded or misfolded proteins
Sar1-GTP
adaptor
proteins of
inner COPII
coat
exit signal on
cargo receptor
exit signal on
soluble cargo
protein
Figure 13–22 The recruitment of
membrane and soluble cargo molecules
into ER transport vesicles. Membrane
proteins are packaged into budding
transport vesicles through interactions
of exit signals on their cytosolic tails with
adaptor proteins of the inner COPII coat.
Some of these membrane proteins function
as cargo receptors, binding soluble
proteins in the ER lumen and helping to
package them into vesicles. Other proteins
may enter the vesicle by bulk flow. A typical
50 nm transport vesicle contains about 200
membrane proteins, which can be of many
different types. As indicated, unfolded or
incompletely assembled proteins are bound
to chaperones and transiently retained in
the ER compartment.