Question 1 of 5
Why does SRP arrest translation after it binds to the signal sequence and only the first 70 amino acids of the protein have been synthesized?
A. Most secretory proteins are only 70 amino acids long (7 kDa) so translation is completed by the time SRP binds.
B. SRP arrests translation to allow the exposed signal sequence to be cleaved from the protein before it enters the translocon.
C. SRP arrests translation so the signal sequence is not misfolded within the interior of the protein.
D. SRP does not arrest translation. The SRP receptor arrests translation after SRP docks the ribosome and newly synthesized protein on the ER.

Question 2 of 5

What is the purpose of GTP hydrolysis when SRP binds to the SRP receptor?
A. It's a source of energy to allow protein synthesis to resume.
B. It causes SRP to dissociate from the signal sequence, ribosome and SRP receptor.
C. It "hands off" the ribosome to the translocon
D. (a) & (b)
E. (b) & (c)

Question 3 of 5
How does a single P54SRP bind to diverse ER signal sequences that do not have identical amino acid sequences?
A. Multiple D & E on P54SRP form electrostatic bonds with "core" K & R present on all signal sequences.
B. Multiple K & R on P54SRP form electrostatic bonds with "core" D & E present on all signal sequences.
C. Multiple polar amino acids on P54SRP form hydrogen bonds with "core" polar amino acids present on all signal sequences.
D. Cells have multiple isoforms of P54SRP that interact specifically with distinct ER signal sequences.
E. Multiple hydrophobic amino acids on P54SRP form van der Waals interactions with "core" hydrophobic amino acids present on all signal sequences.

Question 4 of 5
The membrane spanning α-helix of a type I integral membrane protein such as the LDL receptor or Glycophorin A:
A. Is referred to as an "internal signal anchor" and ensures that the N-terminal portion of the protein that precedes it will be inserted into the ER lumen and the C-terminal portion of the protein that follows it will remain in the cytosol.
B. Is referred to as an "internal signal anchor" and ensures that the N-terminal portion of the protein that precedes it will remain in the cytosol and the C-terminal portion of the protein that follows it will be inserted into the ER lumen.
C. Is referred to as an "stop-transfer anchor" and ensures that the N-terminal portion of the protein that precedes it will be inserted into the ER lumen and the C-terminal portion of the protein that follows it will remain in the cytosol.
D. Is referred to as an "stop-transfer anchor" and ensures that the N-terminal portion of the protein that precedes it will remain in the cytosol and the C-terminal portion of the protein that follows it will be inserted into the ER lumen.

Question 5 of 5
You're studying the Na+/K+ ATPase in a certain cell line. (Remember, it's present in virtually all cells so the particular cell type doesn't matter here). You have 4 samples of these cells that have identical #s of Na+/K+ ATPases.

Sample 1 are the control cells that are not treated with anything. They retain 100% Na+/K+ ATPase activity throughout the experiment.

Sample 2 is treated continuously with a sufficiently high dose of the cardiac glycoside oubain that you see 100% inhibition of Na+/K+ ATPase activity 6 hours later in the oubain treated cells. 100% Na+/K+ ATPase activity is still inhibited 12 hours later.

Sample 3 is treated continuously with a sufficiently high dose of an siRNA that completely degrades the mRNA encoding the α subunit of the Na+/K+ ATPase in less than 30 minutes. However, you observe that only 50% of Na+/K+ ATPase activity is inhibited 6 hours later in the siRNA-treated cells even though all the α subunit mRNA has been degraded. Only 25% of Na+/K+ ATPase activity remains 12 hrs after siRNA treatment.

Sample 4 is treated continuously with a sufficiently high dose of the general protein synthesis inhibitor, cycloheximide. However, you observe that only 50% of Na+/K+ ATPase activity is inhibited 6 hours later in the cycloheximide-treated cells even though all protein synthesis has been inhibited. Only 25% of Na+/K+ ATPase activity remains 12 hrs after cycloheximide treatment.

What can you conclude from this experiment?
A. siRNA and cycloheximide only partially inhibit Na+/K+ ATPase activity.
B. oubain completely inhibits both Na+/K+ ATPase activity and its synthesis.
C. Oubain completely inhibits Na+/K+ ATPase synthesis but not its activity.
D. The half-life of the α subunit protein is 6 hrs.
E. You cannot conclude anything from this experiment.