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 which has a single N- terminal ER signal sequence will be found in the ER lumen. This is due to the fact that proteins which reside in the lumen of the Endoplasmic Reticulum use the translocation pathway which is co-translational in nature. The pathway begins essentially with the recognition of the N-terminal signal sequence of the protein by a signal recognition particle. The protein is then transferred to translocon which is a conducting channel comprising of membrane bound proteins. For some proteins, the signal peptide is cleaved off after it has been translocated to ER while for some, the peptide signal sequence remains which further serve as signal anchor sequences.