Questions: Protein Trafficking and Secretory Pathways

Question 1

A researcher engineers a secretory protein with a scrambled signal peptide that SRP cannot recognize. Which outcome is most likely?

• A. The protein is synthesized normally in the cytosol and later imported into the ER post-translationally via a separate receptor.
• B. The protein is synthesized in the cytosol without ER targeting, and will likely misfold or be degraded since it cannot enter the secretory pathway.
• C. The protein enters the ER via COPI-coated vesicles instead of the translocon.
• D. The ERAD pathway detects the missing signal peptide and redirects the protein to the Golgi.

Answer: (B)

SRP recognition of the signal peptide is the essential first step in co-translational translocation. If SRP cannot recognize the signal peptide, the ribosome remains in the cytosol and translation completes there. The protein never enters the ER lumen, cannot be glycosylated or folded by ER chaperones, and cannot enter the secretory pathway. It will likely be degraded by cytosolic proteasomes. Post-translational import does exist for some organelles (mitochondria, peroxisomes) but is not the default alternative for secretory proteins.

Question 2

At which location in the secretory pathway are proteins sorted into routes leading to lysosomes, the plasma membrane, or regulated secretory granules?

• A. At the ER translocon, based on the signal peptide sequence.
• B. In the ER lumen, after N-linked glycosylation is complete.
• C. At the trans-Golgi network, based on sorting signals in the protein's sequence.
• D. At the plasma membrane, after default secretion delivers all proteins there first.

Answer: (C)

Sorting decisions are made at the trans-Golgi network (TGN). This is where proteins are packaged into different vesicle populations based on their sorting signals: lysosomal enzymes carry a mannose-6-phosphate tag that is recognized by receptors directing them to lysosomes; regulated secretory proteins are packaged into dense-core granules for stimulus-triggered release; constitutive secretory proteins are continuously transported to the plasma membrane. The ER and cis/medial Golgi are for processing (folding, glycosylation modification), not final sorting.

Question 3

Translation of secretory proteins is completed in the cytosol before the signal peptide is recognized by SRP and the protein is imported into the ER.

• T. True
• F. False

Answer: False

Translocation into the ER is co-translational: it occurs simultaneously with translation. As soon as the signal peptide emerges from the ribosome (~20 amino acids in), SRP binds it and pauses further translation. The ribosome-SRP complex then docks with the SRP receptor on the rough ER membrane, and translation resumes with the growing polypeptide threaded directly into the translocon as it is synthesized. This co-translational mechanism is critical because fully-synthesized cytosolic proteins cannot be imported into the ER — they would fold prematurely and be too large for the translocon.

Question 4

Misfolded proteins that fail ER quality control are destroyed within the ER lumen by ER-resident proteases.

• T. True
• F. False

Answer: False

The ER has no proteasomes — proteasomes are cytosolic/nuclear. Misfolded proteins that persist despite repeated chaperone-assisted folding attempts are retrotranslocated back into the cytoplasm through the translocon (or associated channels) in a process called ER-associated degradation (ERAD). Once in the cytoplasm, they are ubiquitinated and degraded by the 26S proteasome. This retrotranslocation step is essential — ERAD is a cytoplasmic process, not an ER-lumenal one.

Question 5

What is the role of the Signal Recognition Particle (SRP), and why is it functionally important that it acts co-translationally rather than after translation is complete?

Model answer: SRP is a ribonucleoprotein complex that recognizes the hydrophobic signal peptide as it emerges from the ribosome, pauses translation, and escorts the ribosome-nascent chain complex to the SRP receptor on the rough ER membrane, where translation resumes and the polypeptide is threaded into the translocon. Co-translational action is essential because proteins destined for the ER must be unfolded as they pass through the narrow translocon channel. If translation were allowed to complete first, the protein would fold in the cytosol and could not be translocated. Pause-then-dock ensures the protein is threaded in while it is still an unstructured, elongating chain.

This co-translational mechanism is also energetically efficient: the ribosome's own translation energy drives the polypeptide through the translocon, coupling two processes that would otherwise require separate energy inputs.