Questions: Protein Translocation and Signal Sequences

Question 1

A mutation deletes the N-terminal signal sequence from a protein normally secreted into the ER lumen. What is the most likely consequence?

• A. The protein is degraded immediately by the proteasome since it lacks a zip code
• B. The protein is synthesized normally and accumulates in the cytosol, unable to enter the ER
• C. SRP recognizes the next hydrophobic segment and targets the ribosome to the ER as a backup
• D. The protein is redirected to the mitochondria by default

Answer: (B)

Without the N-terminal signal sequence, SRP cannot recognize and halt translation. Translation continues normally on free cytosolic ribosomes, and the protein is released into the cytosol. It cannot enter the ER post-translationally (unlike mitochondrial proteins) because the co-translational machinery requires the signal sequence to be recognized while the ribosome is still active. The protein accumulates in the cytosol and is typically non-functional or eventually degraded.

Question 2

Why do proteins destined for the mitochondrial matrix require cytosolic chaperones (like Hsp70) during import, while ER-targeted proteins do not need the same chaperone assistance?

• A. Mitochondrial proteins are larger and require more energy to thread through the membrane
• B. Mitochondrial targeting sequences are cleaved earlier, leaving the protein without a membrane anchor
• C. Mitochondrial proteins are fully synthesized before import begins, so chaperones must prevent premature folding that would block import; ER-targeted proteins are threaded through the translocon as they are made
• D. Hsp70 is only present in the cytosol and cannot reach the ER membrane

Answer: (C)

The key distinction is timing. ER proteins undergo co-translational translocation: SRP halts translation, docks the ribosome at the Sec61 translocon, and the growing polypeptide is threaded through the channel as it is synthesized — never having a chance to fold in the cytosol. Mitochondrial proteins are post-translationally imported: translation is complete before import begins, and the protein must remain in an extended, unfolded conformation to thread through the TOM/TIM channels. Cytosolic Hsp70 prevents premature folding that would otherwise make the protein import-incompetent.

Question 3

After signal sequence cleavage by signal peptidase in the ER lumen, the mature secretory protein retains a copy of its signal sequence at its new C-terminus to maintain ER retention.

• T. True
• F. False

Answer: False

Signal peptidase cleaves the signal sequence from the N-terminus on the lumenal side of the ER membrane, and the cleaved signal sequence is degraded. The mature protein does NOT retain the signal sequence — that is the point. The zip code is a disposable label: it directs delivery but is removed from the final product. ER retention of resident proteins is instead achieved by separate retrieval signals (e.g., KDEL sequences) recognized by receptor-mediated retrieval from later compartments.

Question 4

Both ER-targeted and mitochondrially targeted proteins must be in an unfolded or partially unfolded conformation to pass through their respective membrane translocon channels.

• T. True
• F. False

Answer: True

Folded proteins cannot pass through the narrow protein-conducting channels (Sec61 for ER, TOM/TIM for mitochondria) — the channels are too narrow to accommodate folded tertiary structure. For ER proteins, this is ensured by co-translational translocation: the nascent chain is threaded before it can fold. For mitochondrial proteins, cytosolic chaperones (Hsp70) actively prevent premature folding after synthesis, keeping the polypeptide in an extended conformation competent for import. This requirement is why timing and chaperone biology differ between the two pathways.

Question 5

What is the fundamental conceptual difference between co-translational and post-translational protein translocation, and why does this difference necessitate different strategies to prevent premature protein folding?

Model answer: Co-translational translocation (ER pathway) occurs while the protein is still being synthesized: SRP recognizes the signal sequence as it emerges from the ribosome, halts translation, and docks the ribosome at the Sec61 translocon. The polypeptide is threaded into the ER lumen as each amino acid is added — never spending time in the cytosol as a complete chain and never having opportunity to fold. Post-translational translocation (mitochondrial pathway) occurs after synthesis is complete: the full-length polypeptide must be imported into the organelle after being released from the ribosome. Since complete polypeptides spontaneously fold, cytosolic chaperones (Hsp70) must actively hold the protein in an extended, import-competent conformation until it can be threaded through the TOM/TIM channels.

The timing difference is the conceptual core of this topic. ER import elegantly solves the folding problem by coupling synthesis to translocation. Mitochondrial import cannot use this strategy (because mitochondria have their own ribosomes only for a subset of proteins and the cytosolic ribosomes are not docked at the mitochondrial surface), so it requires an active chaperone system as a workaround.