Given below are a few statements about intracellular protein transport:

A. Proteins that are destined for the lysosome are tagged with a mannose-6-phosphate (M6P) group in the Golgi apparatus, which is recognized by the M6P receptor in the trans-Golgi network.
B. Signal recognition particle directly mediates the insertion of proteins into the mitochondrial membrane.
C. The KDEL receptor in the ER and Golgi apparatus works by retrieving soluble ER resident proteins that have accidentally moved to the Golgi.
D. Cargo proteins that need to be exported from the ER are packaged into COPII vesicles based on the presence of an ER export signal in their cytosolic tail.
E. Clathrin-coated vesicles are primarily involved in vesicle trafficking between the Golgi apparatus and the ER.

Choose the option that has all correct statements:

Correct option is A

Sol.

• A is correct: Lysosomal proteins receive M6P tags in the Golgi, recognized by M6P receptors in the trans-Golgi for sorting.

• B is incorrect: Signal Recognition Particle (SRP) directs proteins to the ER membrane, not mitochondria. Mitochondrial protein import uses different targeting signals and translocases.

• C is correct: The KDEL receptor retrieves ER-resident soluble proteins mistakenly transported to the Golgi, returning them to the ER.

• D is correct: COPII vesicles mediate export from ER, packaging cargo proteins based on ER export signals.

• E is incorrect: Clathrin-coated vesicles mediate trafficking between the Golgi and plasma membrane or endosomes, but COPI vesicles handle Golgi-to-ER retrograde transport.

Thus, statements A, C, and D are correct.

Information Booster:

• M6P tagging is a key signal for targeting lysosomal enzymes, discovered as a classical mechanism of protein sorting (Source: Molecular Biology of the Cell, Alberts et al.).

• SRP guides nascent polypeptides to the ER membrane, where cotranslational translocation occurs; mitochondrial protein import is independent of SRP.

• KDEL receptor functions as a retrieval receptor ensuring ER lumenal proteins are retained.

• COPII vesicles are responsible for anterograde transport from ER to Golgi; COPI vesicles mediate retrograde Golgi-to-ER transport.

• Clathrin-coated vesicles are involved mainly in post-Golgi trafficking to endosomes and plasma membrane.