Question

1.. Protein X has a single N-terminal ER signal sequence, while protein Y has a single internal ER signal sequence. Which of these two proteins will be found in the ER lumen (Answer "X" or "Y")?

2. Protein A has an N-terminal ER signal sequence and one stop transfer sequence, while Protein B has an internal ER signal sequence and one stop transfer sequence. Which of these two proteins will have two transmembrane domains once its translocation is complete (Answer "A" or "B")?

3. What is the difference between an N-terminal ER signal sequence (SS) and an N-terminal mitochondrial SS?

a) the ER SS is an alpha helix, while the mito SS is a beta strand

b) the ER SS is a beta strand, while the mito SS is an alpha helix

c) the ER SS is an amphipathic alpha helix, while the mito SS is a hydrophobic alpha helix

d) the ER SS is a hydrophobic alpha helix, while the mito SS is an amphipathic alpha helix

4. True or False: ER stop transfer sequences and mitochondrial stop transfer sequences have similar characteristics.

5. True or False: Unlike translocation of proteins into mitochondria, translocation of proteins into the ER does not require energy.

Answer 1

1. Protein X will be found in the ER lumen. These kind of proteins destined to be secreted within an organelle like golgi bodies, ER have a single N-terminal signal sequence which is cleaved inside the lumen of the organelle.

2. Protein B is likely to have two transmembrane domains. Protein A is expected to be inside ER lumen as it has a N-terminal sequence.

3. Neither of the signal sequence are beta strands, so options a and b are not correct.

ER has an hydrophobic alpha helix while mitochondria has amphipathic alpha helix in the signal peptide. Alpha helix structure facilitates spanning of the domaid in the membrane.

So, out of the options c and d, option d is the answer.

4. Stop transfer sequence is specific for an organelle. So ER and mitochondria do not have similar Transfer sequence. Therefore answea is False.