Question

the location of proteins as they move through a cell can be tracked using special molecular tags. scientists can also use this tagging approach to identify changes to the protein sorting process affect the final location of the protein. an example of a protein tagging experiment is provided below. the results indicate the localization of a membrane protein over time in a normal cell and a mutant cell that cannot properly move proteins through the endomembrane system:
normal mutant
5 min cytoplasm cytoplasm
20 min rough er cytoplasm
40 min golgi cytoplasm
60 min cell membrane cytoplasm
based on these data, what could explain the observations seen in this mutant cell? select all that apply.
the mutant cell is unable to add sugars in the golgi.
the mutant cells membrane protein does not have a signal sequence.
the mutant cell has a defective kinesin protein.
the mutant cell lacks the rna - protein complex that binds to the signal sequence.

Explanation:

1. In normal cells, membrane - proteins follow a pathway from cytoplasm to rough ER, then to Golgi and finally to cell membrane. In mutant cells, the protein stays in the cytoplasm throughout.
2. A lack of signal sequence on the membrane - protein would prevent it from being targeted to the endomembrane system for further processing and transport, keeping it in the cytoplasm.
3. The RNA - protein complex (signal recognition particle, SRP) that binds to the signal sequence is crucial for guiding the ribosome - nascent protein complex to the rough ER. If it's lacking, the protein won't enter the endomembrane system and will remain in the cytoplasm.
4. Kinesin is mainly involved in long - range transport along microtubules, and its defect is not the most likely cause for the initial failure of the protein to enter the endomembrane system.
5. The inability to add sugars in the Golgi would affect the modification of the protein but not its initial targeting to the endomembrane system.

Answer:

The mutant cell's membrane protein does not have a signal sequence.
The mutant cell lacks the RNA - protein complex that binds to the signal sequence.