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

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:

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)