MCB*2050 - Endomembrane System Questions With
Complete Solutions
Achondrogenesis type 1A Correct Answers A severe disease
that results from mutations in the GMAP-210 gene affecting
chondrocytes.
-Chondrocytes secrete large amounts of matrix material to
facilitate bone formation. This matrix material does not move to
or through the Golgi in organisms deficient of GMAP-210.
ATF6-Mediated UPR Pathway Correct Answers 1. BiP is
released from ATF6 in order to aid in the folding of
accumulating misfolded proteins.
2. 'Active' ATF6 moves from the ER to the Golgi via transporter
vesicles.
3. At the Golgi, cytosolic-facing transcription factor domain of
ATF6 is cleaved off by a Golgi-associated protease and targets
the nucleus.
4. ATF6 transcription factor domain upregulates a number of
genes encoding:
-ER molecular chaperones (assist in proper protein folding)
-ER export components (assist in moving properly folded
proteins out of ER)
-ERAD components (assist in degrading misfolded proteins)
5. ER stress is alleviated OR (if not) cell death occurs.
Biosynthetic pathway Correct Answers Materials are
transported from the ER to the Golgi to endosomes and then to
either lysosomes (referred to as vacuoles in plants and
fungi/yeasts) or in some instances, to the plasma membrane
(PM).
,-Proteins that end up in the lysosome are for lysosome function,
not garbage.
Cap-dependent initiation Correct Answers -All mRNA have 5'
and 3' untranslated regions (UTR), which are important for
regulating the mRNA.
-Ribosomes come together using the 5' cap.
Characteristics of Achondrogenesis type 1A: Correct Answers -
Swollen ER, resulting from stalled transport to the Golgi.
-Disorganized Golgi due to lack of solid interaction with the
cytoskeleton - not filling up with proteins to be processed.
cis Golgi network (CGN) function: Correct Answers Serves as a
sorting station:
-Destination of COPII vesicles from the ERES
-Site of COPI vesicles assembly for retrograde transport to the
ER OR
-Anterograde transport as the CGN matures into the rest of the
Golgi.
cis Golgi network (CGN) structure: Correct Answers -Located
at the cis face of the Golgi complex.
-Consists of an interconnected network of tubules and vesicles
adjacent to the ERES.
Cisternal progression/maturation model (most favoured) -
chemical composition: Correct Answers -While moving
through the complex, the chemical composition of each
cisternae constantly changes.
,-COPI-coated vesicles (budding from the tips of the cisternae)
transport resident Golgi enzymes backwards (retrograde)
through the complex (e.g. a-mannosidase I is continually
transported back from the trans and medial cisternae to the cis
cisternae where it functions during N-linked glycosylation).
Cisternal progression/maturation model (most favoured) - steps:
Correct Answers 1. COPII transport vesicles from the ER
(ERES) carrying newly-synthesized cargo proteins (e.g.
lysosomal-destined proteins) arrive at the cis face of the Golgi
complex and fuse to form a new CGN.
2. Newly formed CGN begins to move forward into the complex
(stack) and progressively matures into the cis cisternae.
3. Each cisternae matures into the next cisternae in the complex.
4. TGN eventually disperses into various types of vesicles.
Different vesicles deliver certain cargo from the TGN to
different compartments in the endomembrane system.
Cisternal progression/maturation model (most favoured) -
vesicles at TGN: Correct Answers Clathrin-coated: transport
cargo to endosomes and/or lysosomes, including M6P-tagged
cargo proteins destined for the lysosome.
Secretory vesicles - transport cargo to the PM and extracellular
space.
Secretory granules - eventually fuse with the PM and release
their cargo in the extracellular space.
Cisternal progression/maturation model (most favoured):
Correct Answers -Golgi subcompartments are considered
dynamic structures. Each subcompartment continually moves
from the cis to the trans side of the Golgi complex.
, -The composition of each subcompartment changes while
moving from cis to trans through the stack (e.g. CGN matures
into the cis cisternae, which matures into the medial cisternae,
etc.)
-The Golgi complex persists overall because transport vesicles
continually move resident Golgi proteins 'back' to the proper
subcompartment.
Clathrin triskelions in receptor-mediated endocytosis: Correct
Answers Clathrin triskelions self-assemble to form the outer
scaffolding (lattice) of the coat on the growing vesicle bud.
Clathrin lattice formation acts as a driving force for membrane
curvature. Cathrin-coated vesicle bud pinches off from the PM
via dynamin.
Co-translation insertion of an integral membrane protein into the
RER - step 1: Correct Answers -N-terminus of the nascent
polypeptide enters the translocon.
-Growing proteins first or only transmembrane domain (TMD)
enters the interior of the translocon, which serves as a stop-
transfer sequence.
-Next step depends on the final topology of the membrane
protein.
Co-translation insertion of an integral membrane protein into the
RER - step 2 - N-cytosolC-lumen: Correct Answers -
Translocon's interior interacts with the protein's TMD to stop
translocation and with several +ve-charged amino acid residues
located just upstream (N-terminal) of the TMD.
-Positively charged amino acids determine the topology of all
types of membrane proteins synthesized at the ER.
Complete Solutions
Achondrogenesis type 1A Correct Answers A severe disease
that results from mutations in the GMAP-210 gene affecting
chondrocytes.
-Chondrocytes secrete large amounts of matrix material to
facilitate bone formation. This matrix material does not move to
or through the Golgi in organisms deficient of GMAP-210.
ATF6-Mediated UPR Pathway Correct Answers 1. BiP is
released from ATF6 in order to aid in the folding of
accumulating misfolded proteins.
2. 'Active' ATF6 moves from the ER to the Golgi via transporter
vesicles.
3. At the Golgi, cytosolic-facing transcription factor domain of
ATF6 is cleaved off by a Golgi-associated protease and targets
the nucleus.
4. ATF6 transcription factor domain upregulates a number of
genes encoding:
-ER molecular chaperones (assist in proper protein folding)
-ER export components (assist in moving properly folded
proteins out of ER)
-ERAD components (assist in degrading misfolded proteins)
5. ER stress is alleviated OR (if not) cell death occurs.
Biosynthetic pathway Correct Answers Materials are
transported from the ER to the Golgi to endosomes and then to
either lysosomes (referred to as vacuoles in plants and
fungi/yeasts) or in some instances, to the plasma membrane
(PM).
,-Proteins that end up in the lysosome are for lysosome function,
not garbage.
Cap-dependent initiation Correct Answers -All mRNA have 5'
and 3' untranslated regions (UTR), which are important for
regulating the mRNA.
-Ribosomes come together using the 5' cap.
Characteristics of Achondrogenesis type 1A: Correct Answers -
Swollen ER, resulting from stalled transport to the Golgi.
-Disorganized Golgi due to lack of solid interaction with the
cytoskeleton - not filling up with proteins to be processed.
cis Golgi network (CGN) function: Correct Answers Serves as a
sorting station:
-Destination of COPII vesicles from the ERES
-Site of COPI vesicles assembly for retrograde transport to the
ER OR
-Anterograde transport as the CGN matures into the rest of the
Golgi.
cis Golgi network (CGN) structure: Correct Answers -Located
at the cis face of the Golgi complex.
-Consists of an interconnected network of tubules and vesicles
adjacent to the ERES.
Cisternal progression/maturation model (most favoured) -
chemical composition: Correct Answers -While moving
through the complex, the chemical composition of each
cisternae constantly changes.
,-COPI-coated vesicles (budding from the tips of the cisternae)
transport resident Golgi enzymes backwards (retrograde)
through the complex (e.g. a-mannosidase I is continually
transported back from the trans and medial cisternae to the cis
cisternae where it functions during N-linked glycosylation).
Cisternal progression/maturation model (most favoured) - steps:
Correct Answers 1. COPII transport vesicles from the ER
(ERES) carrying newly-synthesized cargo proteins (e.g.
lysosomal-destined proteins) arrive at the cis face of the Golgi
complex and fuse to form a new CGN.
2. Newly formed CGN begins to move forward into the complex
(stack) and progressively matures into the cis cisternae.
3. Each cisternae matures into the next cisternae in the complex.
4. TGN eventually disperses into various types of vesicles.
Different vesicles deliver certain cargo from the TGN to
different compartments in the endomembrane system.
Cisternal progression/maturation model (most favoured) -
vesicles at TGN: Correct Answers Clathrin-coated: transport
cargo to endosomes and/or lysosomes, including M6P-tagged
cargo proteins destined for the lysosome.
Secretory vesicles - transport cargo to the PM and extracellular
space.
Secretory granules - eventually fuse with the PM and release
their cargo in the extracellular space.
Cisternal progression/maturation model (most favoured):
Correct Answers -Golgi subcompartments are considered
dynamic structures. Each subcompartment continually moves
from the cis to the trans side of the Golgi complex.
, -The composition of each subcompartment changes while
moving from cis to trans through the stack (e.g. CGN matures
into the cis cisternae, which matures into the medial cisternae,
etc.)
-The Golgi complex persists overall because transport vesicles
continually move resident Golgi proteins 'back' to the proper
subcompartment.
Clathrin triskelions in receptor-mediated endocytosis: Correct
Answers Clathrin triskelions self-assemble to form the outer
scaffolding (lattice) of the coat on the growing vesicle bud.
Clathrin lattice formation acts as a driving force for membrane
curvature. Cathrin-coated vesicle bud pinches off from the PM
via dynamin.
Co-translation insertion of an integral membrane protein into the
RER - step 1: Correct Answers -N-terminus of the nascent
polypeptide enters the translocon.
-Growing proteins first or only transmembrane domain (TMD)
enters the interior of the translocon, which serves as a stop-
transfer sequence.
-Next step depends on the final topology of the membrane
protein.
Co-translation insertion of an integral membrane protein into the
RER - step 2 - N-cytosolC-lumen: Correct Answers -
Translocon's interior interacts with the protein's TMD to stop
translocation and with several +ve-charged amino acid residues
located just upstream (N-terminal) of the TMD.
-Positively charged amino acids determine the topology of all
types of membrane proteins synthesized at the ER.
