Cell Biology Exam 3 PASSED Actual Exam
Questions and CORRECT Answers
Some secretory proteins and some integral membrane proteins must pass through a
membrane. A ribonucleoprotein known as the signal recognition particle (SRP) guides these
proteins either to the plasma membrane (in bacteria) or the endoplasmic reticulum (in
eukaryotes). What are the steps of the membrane translocation of eukaryotic secretory
proteins? - CORRECT ANSWER✔✔- 1. The signal peptide exits the ribosome during
translation.
2. SRP binds to the signal peptide and translation stops.
3. SRP moves the ribosome to the ER membrane.
4. Ribosome docks to a translocon.
5. Translation resumes.
6. The growing polypeptide crosses the ER membrane through translocon.
7. Signal peptidase removes the signal peptide from the growing polypeptide.
8. The remainder of the polypeptide is moved into the ER lumen.
9. Protein is transported out of the ER.
Protein X is an unknown membrane protein found in erythrocytes that can be extracted
primarily from plasma membranes using a concentrated salt solution. What type of
membrane protein is protein X? - CORRECT ANSWER✔✔- Since protein X is extracted
from the membrane by altering the ionic strength of the extraction solution, it is a peripheral
membrane protein.
To determine the location of protein X in the plasma membrane, erythrocytes were subjected
to two different treatments. First, intact erythrocytes were treated with proteolytic enzymes.
After the cells were lysed and membrane components extracted, protein X was intact. In the
second experiment, the erythrocyte cells were permeabilized prior to treatment with
proteolytic enzymes. When the plasma membrane components were then extracted, protein X
was extensively fragmented. What do these observations suggest about the location of protein
X in the plasma membrane? - CORRECT ANSWER✔✔- Since proteolytic enzymes cannot
fragment protein X in intact erythrocytes, protein X is located inside the cell and interacts
with the inner surface of the membrane.
Proteins imported to what organelle do NOT have their signal sequence cleaved? -
CORRECT ANSWER✔✔- Peroxisomes
, In what way does peroxisomal protein import differ from mitochondrial protein import? -
CORRECT ANSWER✔✔- Ubiquitination is a key step in receptor recycling.
Where would you find the precursor of a mitochondrial matrix protein in the following types
of mitochondrial mutants:
(a) a mutation in the Tom22 signal receptor;
(b) a mutation in the Tom70 signal receptor;
(c) a mutation in the matrix Hsp70;
(d) a mutation in the matrix signal peptidase? - CORRECT ANSWER✔✔- (a) cytosol;
(b) cytosol;
(c) inner membrane;
(d) matrix
What is true of secretory protein translocation? - CORRECT ANSWER✔✔- 1. Type I
membrane proteins contain two signal sequences; one targets the protein to the ER and one is
a stop transfer anchor sequence.
2. Type IV membrane proteins contain a mixture of both stop transfer anchor sequences and
signal anchor sequences, the order of which determines the protein's orientation in the
membrane.
3. Hydropathy profiles are used to computationally identify secreted proteins.
Which of the following are required for cotranslational translocation of a secretory protein
into the ER? - CORRECT ANSWER✔✔- 1. hydrophobic signal sequence
2. signal recognition particle
3. signal recognition particle receptor
4. GTP
5. translocon
What is the source of energy needed for unidirectional translocation across the membrane in
(a) cotranslational translocation into the endoplasmic reticulum (ER); (b) post-translational
translocation into the ER; and
Questions and CORRECT Answers
Some secretory proteins and some integral membrane proteins must pass through a
membrane. A ribonucleoprotein known as the signal recognition particle (SRP) guides these
proteins either to the plasma membrane (in bacteria) or the endoplasmic reticulum (in
eukaryotes). What are the steps of the membrane translocation of eukaryotic secretory
proteins? - CORRECT ANSWER✔✔- 1. The signal peptide exits the ribosome during
translation.
2. SRP binds to the signal peptide and translation stops.
3. SRP moves the ribosome to the ER membrane.
4. Ribosome docks to a translocon.
5. Translation resumes.
6. The growing polypeptide crosses the ER membrane through translocon.
7. Signal peptidase removes the signal peptide from the growing polypeptide.
8. The remainder of the polypeptide is moved into the ER lumen.
9. Protein is transported out of the ER.
Protein X is an unknown membrane protein found in erythrocytes that can be extracted
primarily from plasma membranes using a concentrated salt solution. What type of
membrane protein is protein X? - CORRECT ANSWER✔✔- Since protein X is extracted
from the membrane by altering the ionic strength of the extraction solution, it is a peripheral
membrane protein.
To determine the location of protein X in the plasma membrane, erythrocytes were subjected
to two different treatments. First, intact erythrocytes were treated with proteolytic enzymes.
After the cells were lysed and membrane components extracted, protein X was intact. In the
second experiment, the erythrocyte cells were permeabilized prior to treatment with
proteolytic enzymes. When the plasma membrane components were then extracted, protein X
was extensively fragmented. What do these observations suggest about the location of protein
X in the plasma membrane? - CORRECT ANSWER✔✔- Since proteolytic enzymes cannot
fragment protein X in intact erythrocytes, protein X is located inside the cell and interacts
with the inner surface of the membrane.
Proteins imported to what organelle do NOT have their signal sequence cleaved? -
CORRECT ANSWER✔✔- Peroxisomes
, In what way does peroxisomal protein import differ from mitochondrial protein import? -
CORRECT ANSWER✔✔- Ubiquitination is a key step in receptor recycling.
Where would you find the precursor of a mitochondrial matrix protein in the following types
of mitochondrial mutants:
(a) a mutation in the Tom22 signal receptor;
(b) a mutation in the Tom70 signal receptor;
(c) a mutation in the matrix Hsp70;
(d) a mutation in the matrix signal peptidase? - CORRECT ANSWER✔✔- (a) cytosol;
(b) cytosol;
(c) inner membrane;
(d) matrix
What is true of secretory protein translocation? - CORRECT ANSWER✔✔- 1. Type I
membrane proteins contain two signal sequences; one targets the protein to the ER and one is
a stop transfer anchor sequence.
2. Type IV membrane proteins contain a mixture of both stop transfer anchor sequences and
signal anchor sequences, the order of which determines the protein's orientation in the
membrane.
3. Hydropathy profiles are used to computationally identify secreted proteins.
Which of the following are required for cotranslational translocation of a secretory protein
into the ER? - CORRECT ANSWER✔✔- 1. hydrophobic signal sequence
2. signal recognition particle
3. signal recognition particle receptor
4. GTP
5. translocon
What is the source of energy needed for unidirectional translocation across the membrane in
(a) cotranslational translocation into the endoplasmic reticulum (ER); (b) post-translational
translocation into the ER; and
