Questions: Protein Targeting and Subcellular Localization

Question 1

A mutation deletes the N-terminal signal peptide from a protein that is normally secreted from the cell. Where will this protein most likely accumulate?

• A. In the ER lumen, but more slowly than usual
• B. In the nucleus, because the absence of one signal causes re-routing to another
• C. In the cytosol, because without the signal peptide the SRP cannot recognize it and redirect it to the translocon
• D. In the mitochondrial matrix, because hydrophobic sequences are recognized by TOM complexes

Answer: (C)

The N-terminal signal peptide is the address label that the signal recognition particle (SRP) recognizes to redirect the ribosome to the ER membrane. Without it, SRP cannot dock the ribosome at the translocon, and translation continues in the cytosol. The protein will be released into the cytoplasm and cannot enter the secretory pathway. Cells do not randomly re-route proteins with missing signals — if a targeting sequence is absent, the default destination is the cytosol. This is illustrated by I-cell disease, where improperly tagged lysosomal enzymes are secreted instead.

Question 2

What is the key mechanistic difference between how ER-targeted proteins and mitochondria-targeted proteins are translocated?

• A. ER translocation requires ATP; mitochondrial import is driven entirely by the membrane potential
• B. ER translocation is co-translational — the protein enters the ER while still being synthesized; mitochondrial import is post-translational — the completed protein is imported after release from the ribosome
• C. Only mitochondrial import uses targeting sequences; ER proteins are recognized by the lipid composition of the membrane
• D. ER proteins use the TOM complex; mitochondrial proteins use the Sec61 translocon

Answer: (B)

The timing difference is fundamental. ER translocation is co-translational: as the signal peptide emerges from the ribosome, SRP binds it, halts translation, and docks the ribosome at the ER membrane so the growing polypeptide threads through the translocon while it is still being made. Mitochondrial import is post-translational: the protein is fully synthesized in the cytosol (held unfolded by chaperones like Hsp70) and then imported through the TOM/TIM complexes after release. The TOM/TIM vs. translocon (Sec61) distinction also marks different machinery for different compartments.

Question 3

The signal peptide that directs a protein to the ER remains attached to the mature protein and serves as a permanent membrane anchor after translocation is complete.

• T. True
• F. False

Answer: False

Signal peptides are transient address labels, not permanent components. After the protein is threaded through the Sec61 translocon into the ER lumen, the signal peptide is cleaved off by signal peptidase, an enzyme in the ER membrane. The mature, secreted or luminal protein does not retain its signal peptide. This cleavage is one reason signal peptides were historically difficult to identify — they disappear from the final protein product.

Question 4

Mitochondrial precursor proteins synthesized in the cytosol must be kept unfolded by chaperones before import so they can thread through the narrow TOM and TIM channel complexes.

• T. True
• F. False

Answer: True

Unlike ER translocation (which is co-translational and threads the protein through the translocon as it emerges from the ribosome), mitochondrial import is post-translational — the protein has been fully synthesized. Fully folded proteins cannot thread through the narrow import channels. Cytosolic chaperones like Hsp70 bind the mitochondrial precursor protein and keep it in an unfolded, import-competent conformation until it can engage the TOM complex at the outer mitochondrial membrane.

Question 5

Why must the cell use completely different targeting machinery for routing proteins to the ER versus mitochondria, rather than a single universal import system?

Model answer: The ER and mitochondria have fundamentally different membrane compositions, internal environments, and functional requirements. ER translocation must be co-translational to handle membrane proteins efficiently and because the ER lumen is oxidizing (important for disulfide bond formation). Mitochondria use a post-translational pathway because they must import thousands of different proteins encoded by nuclear genes — many of which need to reach the matrix, inner membrane, or intermembrane space, each requiring different TIM complexes. Different signal sequences (hydrophobic signal peptides vs. amphipathic helical MTS) are recognized by machinery matched to those specific compartments. A universal system would not be able to discriminate between hundreds of different destination signals with the required fidelity.

The diversity of targeting machinery reflects the evolutionary origin of the cell's compartments and the functional requirements of each. Mistargeting is catastrophic — as seen in I-cell disease — which is why the specificity of each targeting pathway is maintained separately rather than collapsed into a single system.