Question

Which one or more of the following statements is/are correct regarding the transport and retention of proteins in different cell organelles?

• A. ( Mannose 6-phosphate residues are involved in targeting proteins to lysosomes. )
• B. ( Transport of proteins into the mitochondrial compartment is aided by positively charged amino acid residues at the N-terminus and internal hydrophobic segments. )
• C. ( The retention of protein in the ER lumen requires the KDEL sequence motif at the C-terminus. )
• D. ( Nuclear proteins are transported in an unfolded conformation and the nuclear localization signal sequence is subsequently cleaved by peptidases in the nucleoplasm.)

Hint:

When studying cellular transport mechanisms, focus on understanding the specific signals and sequences that guide proteins to and maintain them within specific compartments, as these are fundamental to cellular function and homeostasis.

Answer 1

The Correct Option is A

Detailed Explanation of Each Statement: (A) Mannose 6-phosphate residues: This statement is correct. Mannose 6-phosphate tags are critical markers used by the cell to direct lysosome-destined proteins from the Golgi apparatus to the lysosomes. 
(B) Mitochondrial protein transport: This statement is also correct. Mitochondrial targeting sequences typically contain positively charged residues at the N-terminus that help direct the protein to the mitochondria, where it interacts with receptor proteins in the mitochondrial membrane. 
(C) Retention in the ER: Correct. The KDEL sequence (Lys-Asp-Glu-Leu) at the C-terminus of proteins is recognized by KDEL receptors in the ER, which retain these proteins within the ER lumen or recycle them back to the ER if they are transported out. 
(D) Nuclear protein transport: This statement is incorrect. While it is true that nuclear proteins are often transported in an unfolded conformation, the nuclear localization signal is not typically cleaved; it remains intact to facilitate interaction with the nuclear pore complexes. Conclusion:
Statements (A), (B), and (C) correctly describe aspects of protein trafficking and retention in cellular compartments, while statement (D) contains an inaccuracy regarding the fate of nuclear localization signals.