MCB 2050 FINAL EXAM PRACTICE QUESTIONS WITH
COMPLETE SOLUTIONS
What is the ER?
highly complex network of membrane enclosed, rod-like tubules
and sheet like cisternae- which is the organelle with the greatest
surface area.
Components of the ER?
Lumen: aqueous space inside of the ER tubules and cisternae
Tubules and the cisternae: mediated by reticulons and
they are unique ER integral membrane proteins that possess a
hair-pin V-shaped secondary structure.
They regulate ER membrane curvature.
Why is the Er such a highly dynamic network?
Because the ER tubules and the cisternae are in constant flux
The ER contains multiple subdomains called:
SER- mostly curved tubules lacking ribosomes, involved in
Ca2+ storage and hormone synthesis.
RER- mostly cisterane with bound ribosomes, involved in
protein and membrane phospholipid synthesis.
And more than 20 other subdomains like the nuclear
envelope because the Outer membrane of the NE is merged with
the RER membrane.
,mitochondria and pm associated membranes
ER exit sites
RER
one of the main sites for protein synthesis (translation) in the
cell
1) free ribosomes in the cytoplasm
2) ER membrane bound ribosomes
Fate of the nascent properly folded soluble protein in the RER
remains in the RER or localizes to another ER subdomain.
or localizes to other ER derived organelles
or targets from the ER onto another compartment in the EM
system.
Fate of the nascent properly folded soluble protein in the
cytoplasm
Stays in the cytoplasm
targets post-translationally to the proper intracellular destination
Co-translational translocation of a soluble protein into the RER
lumen.
Step 1&2-
in the cytoplasm, translation of the mRNA on a free ribosome
begins
The N terminus of the nascent, growing polypeptide eventually
emerges from the ribosome.
N terminus will contain a signal sequence.
This is a stretch of 8-15 hydrophobic AAs that serve as an ER
,targeting signal.
the exposed signal sequence is recognized by the signal
recognition particle (SRP)
----> This is a ribonucleoprotein particle that consists of 6
proteins and 1 small RNA.
SRP then binds to the ribosome and stops protein translation.
Step 3- SRP targets the complex to the surface of the ER.
SRP binds to the SRP receptor.
-heterodimeric ER integral membrane protein complex.
-cytoplasmic facing domains of SRP receptor is the docking site
of the incoming SRP
INTERACTION BTN THE SRP AND THE SRP RECEPTOR
is strengthened by both binding to GTP.
Step 4- GTP hydrolysis results in the release of SRP and SRP
receptor, used for another round of import.
Simultaneously, nascent polypeptide and ribosome transferred to
the cytoplasmic side of Sec61 translocon.
Sec61 translocon is a multi-protein complex that consists of
several ER integral membrane protein subunits (sec61 alpha,
beta, and gamma) forming an hour-glass-shaped aqueous
channel.
Transfer of the nascent polypeptide ribosome to Sec61
translocon results in N-terminus of polypeptide being inserted
into the opening of the translocon channel.
Translation resumes and elongating polypeptide chain continues
to pass through translocon channel towards lumen.
passage of the polypeptide through the translocon is driven by
, translation.
Step5 and 6-
As the signal sequence enters the ER lumen it is cleaved by the
signal peptidase and degraded
peptidase will recognize the cleavage sequence at the C-terminal
end of the signal sequence.
Co-translational translocation fo polypeptide into t
Sec 61 translocon
hour glass-shaped translocon channel contains a pore
ring: Which helps serve as a gasket/ gate to seal channel to ions
and small molecules.
Made up of 6 hydrophobic AAs at narrowest pt in the channel
translocon channel is also blocked by a short alpha-helix plug
second gatekeeping mechanism0 maintains ER
compartmentalization.
during peptide translocation over to Sec61, the peptide will
shove away the plug so that its N-terminal is inserted into the
channel
Classes of Integral membrane proteins
Type i (single pass membrane protein):
CT in cytoplasm and NT in nucleus
Type ii (single pass membrane protein)
NT in cytoplasm and CT in nucleus
COMPLETE SOLUTIONS
What is the ER?
highly complex network of membrane enclosed, rod-like tubules
and sheet like cisternae- which is the organelle with the greatest
surface area.
Components of the ER?
Lumen: aqueous space inside of the ER tubules and cisternae
Tubules and the cisternae: mediated by reticulons and
they are unique ER integral membrane proteins that possess a
hair-pin V-shaped secondary structure.
They regulate ER membrane curvature.
Why is the Er such a highly dynamic network?
Because the ER tubules and the cisternae are in constant flux
The ER contains multiple subdomains called:
SER- mostly curved tubules lacking ribosomes, involved in
Ca2+ storage and hormone synthesis.
RER- mostly cisterane with bound ribosomes, involved in
protein and membrane phospholipid synthesis.
And more than 20 other subdomains like the nuclear
envelope because the Outer membrane of the NE is merged with
the RER membrane.
,mitochondria and pm associated membranes
ER exit sites
RER
one of the main sites for protein synthesis (translation) in the
cell
1) free ribosomes in the cytoplasm
2) ER membrane bound ribosomes
Fate of the nascent properly folded soluble protein in the RER
remains in the RER or localizes to another ER subdomain.
or localizes to other ER derived organelles
or targets from the ER onto another compartment in the EM
system.
Fate of the nascent properly folded soluble protein in the
cytoplasm
Stays in the cytoplasm
targets post-translationally to the proper intracellular destination
Co-translational translocation of a soluble protein into the RER
lumen.
Step 1&2-
in the cytoplasm, translation of the mRNA on a free ribosome
begins
The N terminus of the nascent, growing polypeptide eventually
emerges from the ribosome.
N terminus will contain a signal sequence.
This is a stretch of 8-15 hydrophobic AAs that serve as an ER
,targeting signal.
the exposed signal sequence is recognized by the signal
recognition particle (SRP)
----> This is a ribonucleoprotein particle that consists of 6
proteins and 1 small RNA.
SRP then binds to the ribosome and stops protein translation.
Step 3- SRP targets the complex to the surface of the ER.
SRP binds to the SRP receptor.
-heterodimeric ER integral membrane protein complex.
-cytoplasmic facing domains of SRP receptor is the docking site
of the incoming SRP
INTERACTION BTN THE SRP AND THE SRP RECEPTOR
is strengthened by both binding to GTP.
Step 4- GTP hydrolysis results in the release of SRP and SRP
receptor, used for another round of import.
Simultaneously, nascent polypeptide and ribosome transferred to
the cytoplasmic side of Sec61 translocon.
Sec61 translocon is a multi-protein complex that consists of
several ER integral membrane protein subunits (sec61 alpha,
beta, and gamma) forming an hour-glass-shaped aqueous
channel.
Transfer of the nascent polypeptide ribosome to Sec61
translocon results in N-terminus of polypeptide being inserted
into the opening of the translocon channel.
Translation resumes and elongating polypeptide chain continues
to pass through translocon channel towards lumen.
passage of the polypeptide through the translocon is driven by
, translation.
Step5 and 6-
As the signal sequence enters the ER lumen it is cleaved by the
signal peptidase and degraded
peptidase will recognize the cleavage sequence at the C-terminal
end of the signal sequence.
Co-translational translocation fo polypeptide into t
Sec 61 translocon
hour glass-shaped translocon channel contains a pore
ring: Which helps serve as a gasket/ gate to seal channel to ions
and small molecules.
Made up of 6 hydrophobic AAs at narrowest pt in the channel
translocon channel is also blocked by a short alpha-helix plug
second gatekeeping mechanism0 maintains ER
compartmentalization.
during peptide translocation over to Sec61, the peptide will
shove away the plug so that its N-terminal is inserted into the
channel
Classes of Integral membrane proteins
Type i (single pass membrane protein):
CT in cytoplasm and NT in nucleus
Type ii (single pass membrane protein)
NT in cytoplasm and CT in nucleus
