MCB 2050 (Part 2) Questions With Complete Solutions
1. Misfolded proteins in the ER are translocated back into the
cytosol via the translocon (retro-translocation)
2. In cytosol oligosaccharide chain is removed and misfolded
protein is poly-ubiquinated (serves as signal for protein
degradation)
3. Poly-ubiquinated protein binds to the lid of the proteasome,
Ub chain is removed, and protein is threaded into the
proteasome and degraded (AAs are recycled)
How does the ER associated degradation (ERAD) pathway
work?
ER stress signals are generated when misfolded protein
accumulation in the ER is to high for the ERAD pathway alone.
Stress signals activate UPR pathways.
1. Stress conditions cause BiP to be released from PERK.
2. PERK sensors dimerize and become active.
3. Cytosolic facing kinase domains of activated PERK dimer
phosphorylate and inhibit TF eIF2α (responsible for the
initiation of protein synthesis)
4. Protein synthesis decreases and chaperone proteins focus on
misfolded proteins
ER stress is either alleviated or cell death occurs
Explain how the PERK mediated unfolded protein response
(UPR) pathway works.
,ER stress signals are generated when misfolded protein
accumulation in the ER is to high for the ERAD pathway alone.
Stress signals activate UPR pathways.
1. Stress conditions cause BiP to be released from ATF6.
2. Activated ATF6 moves from ER to golgi.
3. At golgi cytosolic facing, TF domain of ATF6 is cleaved and
targeted to nucleus
4. ATF transcription factor upregulates genes encoding
molecular chaperone proteins (assist in folding), ER export
components (moves properly folded proteins out of the ER),
ERAD components (assist in misfolded protein degradation)
ER stress is either alleviated or cell death occurs
Explain how the ATF6 mediated unfolded protein response
(UPR) pathway works.
Unique subdomain of the ER involved in the formation of
transport vesicles destined for the golgi. Enriched with proteins
responsible for the proper formation and packaging of these
vesicles.
What are ER exit sites (ERES)?
Functions:
1. Recognize and concentrate specific protein and lipid
components that will be incorporated into the budding vesicle
2. Mediate ERES membrane curvature and formation of the
budding vesicle
, Classes:
1. COPII: anterograde transport from ERES to golgi
2. COPI: retrograde transport within golgi network and from
golgi to ER.
3. Clathrin: transport from golgi or plasma membrane to
endosomes
What are the 2 main functions of coat proteins (COP) in vesicle
transport? What are the 3 major classes of coat proteins?
1. Sar1-GDP is recruited to the surface of the ERES where a
guanine exchange factor (GEF ~ catalyzes the exchange of GDP
and GTP), exchanges it for Sar1-GTP.
2. Sar1-GTP integrates itself into the outer membrane at the
ERES which results in membrane curvature.
3. Sar1-GTP recruits Sec23 & 24 to the ERES surface. At the
ERES surface Sec23 & 24 form a dimer which promotes further
membrane bending. Sec24 also binds the cytosolic domains of
membrane cargo proteins (proteins destined for golgi),
membrane cargo protein receptors (bind to soluble proteins
destined for golgi), and membrane receptor proteins (SNAREs).
4. Additional soluble COPII components (Sec 13 and Sec31) are
recruited to the surface of the growing vesicle (act as
scaffolding).
5. CopII coat assembly causes the vesicle to pinch off from the
ERES. After pinching off the COPII coat dissembles (Sar1-GTP
converted back to Sar1-GDP and dissembles with rest of coat
proteins). .
Explain the process of transport vesicle assembly at the ERES.
1. Misfolded proteins in the ER are translocated back into the
cytosol via the translocon (retro-translocation)
2. In cytosol oligosaccharide chain is removed and misfolded
protein is poly-ubiquinated (serves as signal for protein
degradation)
3. Poly-ubiquinated protein binds to the lid of the proteasome,
Ub chain is removed, and protein is threaded into the
proteasome and degraded (AAs are recycled)
How does the ER associated degradation (ERAD) pathway
work?
ER stress signals are generated when misfolded protein
accumulation in the ER is to high for the ERAD pathway alone.
Stress signals activate UPR pathways.
1. Stress conditions cause BiP to be released from PERK.
2. PERK sensors dimerize and become active.
3. Cytosolic facing kinase domains of activated PERK dimer
phosphorylate and inhibit TF eIF2α (responsible for the
initiation of protein synthesis)
4. Protein synthesis decreases and chaperone proteins focus on
misfolded proteins
ER stress is either alleviated or cell death occurs
Explain how the PERK mediated unfolded protein response
(UPR) pathway works.
,ER stress signals are generated when misfolded protein
accumulation in the ER is to high for the ERAD pathway alone.
Stress signals activate UPR pathways.
1. Stress conditions cause BiP to be released from ATF6.
2. Activated ATF6 moves from ER to golgi.
3. At golgi cytosolic facing, TF domain of ATF6 is cleaved and
targeted to nucleus
4. ATF transcription factor upregulates genes encoding
molecular chaperone proteins (assist in folding), ER export
components (moves properly folded proteins out of the ER),
ERAD components (assist in misfolded protein degradation)
ER stress is either alleviated or cell death occurs
Explain how the ATF6 mediated unfolded protein response
(UPR) pathway works.
Unique subdomain of the ER involved in the formation of
transport vesicles destined for the golgi. Enriched with proteins
responsible for the proper formation and packaging of these
vesicles.
What are ER exit sites (ERES)?
Functions:
1. Recognize and concentrate specific protein and lipid
components that will be incorporated into the budding vesicle
2. Mediate ERES membrane curvature and formation of the
budding vesicle
, Classes:
1. COPII: anterograde transport from ERES to golgi
2. COPI: retrograde transport within golgi network and from
golgi to ER.
3. Clathrin: transport from golgi or plasma membrane to
endosomes
What are the 2 main functions of coat proteins (COP) in vesicle
transport? What are the 3 major classes of coat proteins?
1. Sar1-GDP is recruited to the surface of the ERES where a
guanine exchange factor (GEF ~ catalyzes the exchange of GDP
and GTP), exchanges it for Sar1-GTP.
2. Sar1-GTP integrates itself into the outer membrane at the
ERES which results in membrane curvature.
3. Sar1-GTP recruits Sec23 & 24 to the ERES surface. At the
ERES surface Sec23 & 24 form a dimer which promotes further
membrane bending. Sec24 also binds the cytosolic domains of
membrane cargo proteins (proteins destined for golgi),
membrane cargo protein receptors (bind to soluble proteins
destined for golgi), and membrane receptor proteins (SNAREs).
4. Additional soluble COPII components (Sec 13 and Sec31) are
recruited to the surface of the growing vesicle (act as
scaffolding).
5. CopII coat assembly causes the vesicle to pinch off from the
ERES. After pinching off the COPII coat dissembles (Sar1-GTP
converted back to Sar1-GDP and dissembles with rest of coat
proteins). .
Explain the process of transport vesicle assembly at the ERES.
