Cell biology — Protein trafficking: Signal sequences are short peptide motifs that primarily function to do what?

Difficulty: Easy

Correct Answer: Target and transport proteins to specific cellular locations (e.g., ER, mitochondria)

Explanation:


Introduction / Context:
Most eukaryotic and prokaryotic proteins must be delivered to the correct cellular compartment to function. Short amino-terminal or internal signal sequences act as postal codes that direct trafficking during or after translation.



Given Data / Assumptions:

  • Question asks for the primary role of signal sequences.
  • Typical signals include ER signal peptides, mitochondrial targeting sequences, nuclear localization signals, and peroxisomal SKL.
  • Folding, termination, and prion refolding are separate processes.


Concept / Approach:
Signal sequences are recognized by receptors (e.g., SRP for ER targeting) or import machineries (TOM/TIM in mitochondria, importins for nucleus). They ensure proteins reach membranes, organelles, or are secreted, enabling proper function and preventing mislocalization.



Step-by-Step Solution:

Identify the role: encode address information for intracellular routing.Map examples: ER signal → co-translational import; NLS → nuclear import; MTS → mitochondrial import.Exclude unrelated functions: translation termination and chaperone-mediated refolding.Select the option describing targeting/transport.


Verification / Alternative check:
Loss-of-function experiments (mutated signals) cause mislocalization; fusing signals to reporters retargets proteins, confirming the function.



Why Other Options Are Wrong:

  • Folding is primarily encoded by sequence but assisted by chaperones; signals do not “force” folding.
  • Termination is driven by stop codons and release factors, not signal peptides.
  • Prion refolding involves chaperones and proteostasis pathways, not generic signal sequences.


Common Pitfalls:
Equating “signal” with “stop” or confusing various targeting signals; note that signal peptides are often cleaved after translocation.



Final Answer:
Target and transport proteins to specific cellular locations (e.g., ER, mitochondria)

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