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BIO130 Section Two Guide (3B)

5 Pages
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Department
Biology
Course Code
BIO130H1
Professor
Kenneth Yip

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Chapter 13 Intracellular Vesicular Traffic
Lecture 5+6: (Pg. 766-769, 779-783)
Transport From the ER Through the Golgi Apparatus
-Golgi apparatus major site for carbohydrate synthesis, sorting & dispatching
station for products of the ER
oThe cell makes many polysaccharides in the Golgi
oLies on the exit route from the ER many carbohydrates it makes are
attached as oligosaccharide side chains to many proteins and lipids that the
ER sends to it
Proteins Leave the ER in COPII-Coated Transport Vesicles
-Proteins that enter the ER and are destined for the Golgi or beyond are first
packaged into small COPII-coated transport vesicles these bud from ER exit
sites, whose membrane lacks bound ribosomes
-Entry into vesicles leaving the ER is selective; cargo proteins display exit signals on
their cytosolic surface that COPII can recognize (these coat components act as cargo
receptors and are recycled back to the ER after they deliver to the Golgi
-Soluble cargo proteins in the ER lumen have exit signals that attach them to
transmembrane cargo receptors, which in turn bind through exit signals in their
cytoplasmic tails to components of the COPII coat
-At a lower rate, proteins without exit signals enter transport vesicles (ER resident
proteins) and leak out of the ER and are delivered to the Golgi
-Secretory proteins made in high concentrations can leave the ER without signals
Only Proteins That Are Properly Folded and Assembled Can Leave the ER
- Misfolded proteins remain in the ER, bound to chaperone proteins like BiP or
calnexin
oChaperones cover up exit signals or somehow anchor the protein in the ER
-Failed proteins are transported to the
cytosol, and degraded by proteasomes
-90% of newly synthesized subunits of T
cell receptors are destroyed without ever
reaching the cell surface, where they
function
-Continual degradation of these proteins
provide an early warning system to alter
the immune system of a virus infection
Using specialized ABC-type
transporters, the ER imports peptide
fragments of viral proteins produced by
proteases in the proteasome
oForeign peptides are loaded onto MHC in the ER lumen and proteins
transported to the cell surface
oT lymphocytes recognizes the peptides as non-self antigens and kill infected
cells
www.notesolution.com
- Mutations that cause cystic fibrosis result in the production of a misfolded form of a
plasma membrane protein important for Cl- transport. Although the mutant protein
would function perfectly normally if it reached the plasma membrane, it remains in
the ER.
oThe disease thus results not because the mutation inactivates the protein but
because the active protein is discarded before it reaches the plasma
membrane.
Vesicular Tubular Clusters Mediate Transport from the ER to the Golgi
Apparatus
-Once transport vesicles bud off, and shed their coat, they fuse with one another
oHomotypic/heterotypic fusion (similar/different membranes)
-In both types of fusion, a set of matching SNAREs is required
oSNARE proteins catalyze the membrane fusion reactions in vesicular
transport
oThe interaction is symmetrical (both membranes contribute v- and t-
SNAREs
-Vesicular tubular clusters - Structures formed when vesicles fuse
oclusters are separate from the ER and lack many proteins that function in
the ER
oFunction as
transport
containers from the
ER to the Golgi
oMove along
microtubules to the
Golgi, then fuse to
deliver contents
-As soon as these clusters form, they begin to bud off transport vesicles of their own
oThese vesicles are COPI-coated and they carry ER resident proteins that
have escaped back to the ER, as well as proteins like cargo receptors
-The retrieval (retrograde) transport continues as the vesicular tubular clusters
move towards the Golgi
Transport From the Trans Golgi Network to Lysosomes
-The trans Golgi network sorts proteins that pass through the Golgi apparatus
according to their final destination (except resident proteins)
Lysosomes Are the Principal Sites of Intracellular Digestion
-Lysosomes membrane-enclosed compartments filled with soluble hydrolytic
enzymes that control intracellular digestion of macromolecules
oContain 40 types of hydrolytic enzymes that are all acid hydrolases
ofor optimal activity, they need to be activated by proteolytic cleavage and
require acidic environments (4.5-5.0)
www.notesolution.com

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Description
Chapter 13 Intracellular Vesicular Traffic Lecture 5+6: (Pg. 766-769, 779-783) Transport From the ER Through the Golgi Apparatus - Golgi apparatus major site for carbohydrate synthesis, sorting & dispatching station for products of the ER o The cell makes many polysaccharides in the Golgi o Lies on the exit route from the ER many carbohydrates it makes are attached as oligosaccharide side chains to many proteins and lipids that the ER sends to it Proteins Leave the ER in COPII- Coated Transport Vesicles - Proteins that enter the ER and are destined for the Golgi or beyond are first packaged into small COPII-coated transport vesicles these bud from ER exit sites, whose membrane lacks bound ribosomes - Entry into vesicles leaving the ER is selective; cargo proteins display exit signals on their cytosolic surface that COPII can recognize (these coat components act as cargo receptors and are recycled back to the ER after they deliver to the Golgi - Soluble cargo proteins in the ER lumen have exit signals that attach them to transmembrane cargo receptors, which in turn bind through exit signals in their cytoplasmic tails to components of the COPII coat - At a lower rate, proteins without exit signals enter transport vesicles (ER resident proteins) and leak out of the ER and are delivered to the Golgi - Secretory proteins made in high concentrations can leave the ER without signals Only Proteins That Are Properly Folded and Assembled Can Leave the ER - Misfolded proteins remain in the ER, bound to chaperone proteins like BiP or calnexin o Chaperones cover up exit signals or somehow anchor the protein in the ER - Failed proteins are transported to the cytosol, and degraded by proteasomes - 90% of newly synthesized subunits of T cell receptors are destroyed without ever reaching the cell surface, where they function - Continual degradation of these proteins provide an early warning system to alter the immune system of a virus infection Using specialized ABC-type transporters, the ER imports peptide fragments of viral proteins produced by proteases in the proteasome o Foreign peptides are loaded onto MHC in the ER lumen and proteins transported to the cell surface o T lymphocytes recognizes the peptides as non-self antigens and kill infected cells www.notesolution.com
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