Protein targeting signals Which protein would typically NOT require a localization (targeting) signal peptide or sequence to reach its final destination in the cell?

Introduction / Context:
Proteins are synthesized on ribosomes and must be targeted to correct cellular compartments. Signal sequences or localization motifs direct proteins to the nucleus, ER, mitochondria, peroxisomes, and other destinations. Understanding which proteins do and do not require such signals is crucial for cell biology and biotechnology.

Given Data / Assumptions:

• Cytosolic proteins function in the cytosol where they are synthesized.
• Targeting can be co-translational (to ER via signal peptide/SRP) or post-translational (e.g., NLS to nucleus).
• Signals vary: NLS, signal peptide, transit peptides, peroxisomal PTS1/PTS2, and sorting tags like M6P for lysosomes.

Concept / Approach:

Proteins remaining in the cytosol generally lack dedicated signal sequences; default localization is the cytosol. In contrast, nuclear proteins require nuclear localization signals (NLS). Secreted and membrane proteins need an N-terminal signal peptide to enter the ER, followed by trafficking. Lysosomal enzymes acquire mannose-6-phosphate tags in the Golgi for targeting to lysosomes. ER membrane proteins possess topogenic sequences (signal-anchor, stop-transfer) to insert into membranes.

Step-by-Step Solution:

Verification / Alternative check:

Expression of signal-less GFP remains cytosolic; adding NLS drives nuclear import; adding signal peptide targets to ER—experimental evidence for signal dependency.

Why Other Options Are Wrong:

• Nuclear proteins: require NLS for import via importins.
• ER-bound and cell-surface proteins: require ER signal sequences and membrane topogenic signals.
• Lysosomal hydrolases: require M6P tagging for proper sorting.

Common Pitfalls:

• Assuming diffusion suffices for nuclear entry; macromolecules need active transport and NLS.

Final Answer:

A cytosolic protein