Essential Cell Biology 5th edition

522 CHAPTER 15 Intracellular Compartments and Protein Transport
membrane with newly made lipids and proteins (Movie 15.9), enabling
the plasma membrane to expand prior to cell division and refreshing old
lipids and proteins in nonproliferating cells. The constitutive pathway
also carries soluble proteins to the cell surface to be released to the outside,
a process called secretion. Some of these proteins remain attached
to the cell surface; some are incorporated into the extracellular matrix;
still others diffuse into the extracellular fluid to nourish or signal other
cells. Entry into the constitutive pathway does not require a particular
signal sequence like those that direct proteins to endosomes or back to
the ER.
QUESTION 15–7
What would you expect to happen
in cells that secrete large amounts
of protein through the regulated
secretory pathway if the ionic
conditions in the ER lumen could be
changed to resemble those in the
lumen of the trans Golgi network?
In addition to the constitutive exocytosis pathway, which operates continually
in all eukaryotic cells, there is a regulated exocytosis pathway, which
operates only in cells that are specialized for secretion. Each specialized
secretory cell produces large quantities of a particular product—such as
a hormone, mucus, or digestive enzymes—which is stored in secretory
vesicles for later release. These vesicles, which are part of the endomembrane
system, bud off from the trans Golgi network and accumulate near
the plasma membrane. There they wait for an extracellular signal that
will stimulate them to fuse with the plasma membrane and release their
contents to the cell exterior by exocytosis (Figure 15−30). An increase in
blood glucose, for example, signals insulin-producing endocrine cells in
the pancreas to secrete the hormone (Figure 15−31).
Proteins destined for regulated secretion are sorted and packaged in the
trans Golgi network. Proteins that travel by this pathway have special
surface properties that cause them to aggregate with one another under
the ionic conditions (acidic pH and high Ca 2+ ) that prevail in the trans
Golgi network. The aggregated proteins are packaged into secretory vesicles,
which pinch off from the network and await a signal instructing
them to fuse with the plasma membrane. Proteins secreted by the constitutive
pathway, on the other hand, do not aggregate and are therefore
carried automatically to the plasma membrane by the transport vesicles
of the constitutive pathway. Selective aggregation has another function:
it allows secretory proteins to be packaged into secretory vesicles at
concentrations much higher than the concentration of the unaggregated
Figure 15−30 In secretory cells, the
regulated and constitutive pathways of
exocytosis diverge in the trans Golgi
network. Many soluble proteins are
continually secreted from the cell by the
constitutive secretory pathway (blue arrows),
which operates in all eukaryotic cells
(Movie 15.10). This pathway also continually
supplies the plasma membrane with newly
synthesized lipids and proteins. Specialized
secretory cells have, in addition, a regulated
exocytosis pathway (red arrows) by which
selected proteins in the trans Golgi network
are diverted into secretory vesicles, where
the proteins are concentrated and stored
until an extracellular signal stimulates their
secretion. It is unclear how these special
aggregates of secretory proteins (red )
are segregated into secretory vesicles.
Secretory vesicles have unique proteins in
their membranes; perhaps some of these
proteins act as receptors for secretory
protein aggregates in the trans Golgi
network.
newly
synthesized
soluble proteins
for constitutive
secretion
Golgi apparatus
newly synthesized plasma
membrane lipids
trans
Golgi
network
newly synthesized plasma
membrane protein
signal
transduction
secretory vesicle
storing concentrated
secretory proteins
transport
vesicle
unregulated
exocytosis
regulated
exocytosis
CYTOSOL
CONSTITUTIVE
SECRETION
plasma membrane
extracellular
signal such as
hormone or
neurotransmitter
REGULATED
SECRETION
EXTRACELLULAR SPACE