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Encoding a protein with a pair of conflicting signal sequences

Imagine you have engineered a set of genes, each encoding a protein with a pair of conflicting signal sequences that specify different compartments. If the genes wer expressed in a cell, predict which signal would win out for the following combinations. Explain.

a. Signals for import into the nucleus and import into the ER.
b. Signals for import into peroxisomes and import into the ER.
c. Signals for import into the mitochondria and retention in the ER
(i.e. the KDEL signal for retrieval from Golgi to ER?

Solution Preview

PROTEIN SORTING AND SIGNAL SEQUENCES

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Question:

Imagine you have engineered a set of genes, each encoding a protein with a pair of conflicting signal sequences that specify different compartments. If the genes wer expressed in a cell, predict which signal would win out for the following combinations. Explain.

a. Signals for import into the nucleus and import into the ER.
b. Signals for import into peroxisomes and import into the ER.
c. Signals for import into the mitochondria and retention in the ER (i.e. the KDEL signal for retrieval from Golgi to ER?

Response:

This question isn't really that hard once you have a basic overview of the different mechanisms that the cell uses to target proteins to their respective organelles. Essentially, proteins that are destined to go to the nucleus, mitochondrion, and peroxisome are all synthesized on free ribosomes in the cytosol. After protein synthesis is complete, the proteins then make their way to their respective organelle. In other words, these proteins undergo what is called "post-translational import."

On the other hand, although proteins destined ...

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