Question

6. Specific structural features tag proteins for transport to various intracellular and extracellular destinations. Several examples were described in chapter 12, including (1) the short peptide Lys-Asp-Glu-Leu, (2) characteristic hydrophobic membrane-spanning domains, and (3) mannose-6-phosphate residues attached to oligosaccharide side chains. For each structural feature, answer the following questions with detail:

a. Where in the cell is the tag incorporated into the protein?

b. How does the tag ensure that the protein reaches its destination?

c. Where would the protein likely go if you were to remove the tag?

Answer 1

a) For the peptide chain, the signal sequence is usually present in the N-terminal site and very rarely in the C-terminal site or middle of the peptide chain. As the tag is usually present on the N-terminal site, just after the translation of 14 to 20 residues the signal sequence is emerged and can be recognized by the signal receptor proteins or SRPs.

b) For each specific type of tag, specific signal receptor proteins or SRPs are present which after the translation of the signal residues binds to the signal tag. Then the SRP bind to the specific SRP receptor. The SRP receptor is usually colocalized with a translocon channel that facilitates the entry of the peptide chain into the organelle. This generalized mechanism may vary a little depending on the organism and cell organelle.

c) If the signal tag is removed, the protein should usually by default stay in the cytoplasm as the proteins are translated or synthesized in the cytoplasm. (In rare cases, if this protein is non specifically recognized by any SRPs then it can get inside to that SRP specific cell organelles. )