Exocytosis

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Lecture 37 Exocytosis reading 757-765
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• Proteins lacking a signal sequence take the
  default pathway in vesicles destined for the
  plasma membrane.
• Destinations for regulated pathways from the
  Trans Golgi involve signal sequence or patch
• Late endosome - signal patch directing M6P tag
• Secretory vesicles - signal unknown

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Secretory vesicles concentrate and store
products. Secreted products can be either small
molecules or proteins. Proteins originate at the
ER. In the Golgi, these proteins aggregate and
are packaged into transport vesicles.
Black dots mark immuno-gold staining of clathrin.
White arrow identifies an immature secretory
vesicle.
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Insulin is a good example of a protein that is
stored in secretory vesicles until a cell
receives an signal to secrete the insulin.
Removal of the Pre-sequence (not shown), folding
and disulfide bond formation occur in ER.
Processing to the final form occurs in the
secretory vesicle.
This is an example of a protein that you would
not want to treat with mercaptoethanol because
reduction of disulfide bonds would inactivate the
protein.
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Insulin secretion is triggered in pancreatic ß
cells when blood glucose levels are high. 1.
Glucose enters ß cells and causes a rise in
ATP. 2. ATP triggers closing of ATP-sensitive K
channels. 3. Closing of K channels causes a
slight depolarization of membrane. 4.
Voltage-gated Ca2 channel opens. 5. Rise in
cytosolic Ca2 activates SNARES to cause
vesicle-PM fusion and release of insulin. 6.
Insulin causes muscle and adipocytes to take up
blood glucose and liver to reduce glucose
synthesis.
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(Glucose Uniporter)
Fig. 15-7 from Lodishs Molecular Cell Biology,
5th edition
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Some proteins are processed in secretory vesicles
into multiple small polypeptides. One
explanation for this strategy is that the small
polypeptides are too short to be
co-translationally transported into the ER
(ß-endorphin is 30 amino acids long).
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Secretion of the contents of a secretory vesicle
occurs in response to specific signals. In many
cases, binding of a ligand to a cell surface
receptor triggers secretion. In the nerve, the
action potential triggers secretion.
In presynaptic cells, secretory vesicles
(synaptic vesicles) containing neurotransmitters
are docked at the plasma membrane by SNAREs. The
arrival of an action potential causes
voltage-gated Ca channels to open and the
influx of Ca activates the SNAREs to fuse the
membranes. Neurotoxins that cause tetanus and
botulism incapacitate the fusion process by
entering the cell and selectively cleaving the
SNAREs.
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Small neurotransmitters like acetylcholine, GABA,
and glycine are not proteins. They are
synthesized in the cytosol and loaded into
synaptic vesicles that are derived by endocytosis
of the plasma membrane.
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Cells like this epithelial cell are said to be
polarized because different parts of the plasma
membrane are functionally distinct. Tight
junctions located between cells keep the apical
and basolateral domains of the plasma membrane
separate.
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Nerve cells are another example of a polarized
cell. For example, dendrites will have the
neurotransmitter-gated channels whereas the nerve
terminals will have the t-SNARE that anchors the
synaptic vesicles. The organization of the
cytoskeleton in the axon hillock keeps the two
regions of the plasma membrane from mixing.
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Two mechanisms appear to be used. Direct sorting
relies on signal sequences that function in the
golgi to sort proteins into vesicles destined for
a specific part of the plasma membrane. Indirect
sorting uses transcytosis to deliver proteins to
the correct membrane. In this case, sorting
occurs in the endosome after the proteins have
been retrieved from the plasma membrane by
endocytosis.
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The Na - K ATPase appears to get trapped in the
basal lateral domain by association with
cytoskeletal components that are located only at
the basolateral domain. Transcytosis delivers Na
- K ATPases that happen to insert in the apical
domain back to the basolateral region.
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Direct sorting of proteins to the apical membrane
appears to depend on proteins concentrating in
lipid rafts that form in the Golgi.
Saturated glycolipids and cholesterol
self-associate in the Golgi. The unique
composition of lipids increases the thickness of
the membrane, which in turn causes proteins with
long transmembrane domains to concentrate in the
lipid rafts. The rafts then bud from the trans
Golgi network into transport vesicles that go to
the apical plasma membrane.