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Protein Targeting Quiz
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Which motif is a classic signal for import of a soluble protein into the peroxisomal matrix?
- A A nuclear localisation signal composed of basic residues You picked ✕
- B Post-translational phosphorylation of the protein in the cytosol You picked ✕
- C An N-terminal signal peptide rich in hydrophobic amino acids You picked ✕
- D A signal anchor sequence that inserts the protein into the endoplasmic reticulum membrane You picked ✕
- E A C-terminal tripeptide sequence of serine-lysine-leucine You picked ✓
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The answer is E: A C-terminal tripeptide sequence of serine-lysine-leucine
Explanation
Why E is correct
See Proteins targeted to peroxisomes. Option E is correct. Peroxisomes are small organelles that carry out oxidation reactions and break down certain fatty acids. They have no DNA of their own, so every peroxisomal protein is made on free ribosomes in the cytosol and then imported after it has been made. Soluble proteins that go to the peroxisomal matrix (the space inside) carry a short targeting sequence at the extreme C-terminal end of the chain, the peroxisomal targeting signal type 1 (PTS1). Its prototype is the tripeptide serine-lysine-leucine (SKL in the one-letter amino acid code), although many variants exist. A cytosolic receptor, PEX5, recognises the PTS1, binds the cargo and brings it to the peroxisome, where it imports folded proteins across the membrane. Remember that each destination uses its own address: SKL for the peroxisome, a basic nuclear localisation signal for the nucleus, an N-terminal hydrophobic signal sequence for the ER and KDEL for retrieval to the ER.
- Signal: the peroxisomal targeting signal type 1 (PTS1) is a tripeptide at the extreme C-terminal end.
- Sequence: its prototype is serine-lysine-leucine (SKL).
- Receptor: PEX5 binds the PTS1 in the cytosol and brings the cargo to the peroxisome.
Why the other options are incorrect
- A. A nuclear localisation signal composed of basic residues: A nuclear localisation signal made of basic residues directs a protein to the nucleus through the nuclear pore. It would be the answer to a question about nuclear import. Your answer
- B. Post-translational phosphorylation of the protein in the cytosol: Phosphorylation is a reversible modification that changes a protein's activity, and it is not the targeting signal for the peroxisome. Targeting depends on a sequence within the polypeptide. Your answer
- C. An N-terminal signal peptide rich in hydrophobic amino acids: An N-terminal signal peptide rich in hydrophobic amino acids sends a protein to the endoplasmic reticulum, which is the secretory route. It is not the signal for entry into the peroxisome matrix. Your answer
- D. A signal anchor sequence that inserts the protein into the endoplasmic reticulum membrane: A signal anchor sequence inserts a protein into the ER membrane. Peroxisomal matrix proteins are imported from the cytosol instead, after they have been made on free ribosomes. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which molecular tag is added in the Golgi apparatus to direct many soluble lysosomal enzymes towards lysosomes?
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2
A newly synthesised polypeptide is destined for secretion from the cell. Which event most directly ensures that translation becomes coupled to translocation into the endoplasmic reticulum lumen?
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