Question

Q5: Using genetic engineering techniques, you have created a set of proteins that contain two (and only two) conflicting signal sequences that specify different compartments. Predict which signal would win out for the following combinations. Explain your answers.

A. Signals for import into the nucleus and extracellular matrix.

2. Signals for export from the nucleus and import into the peroxisome.
3. Signals for import into mitochondria and retention in the Golgi complex.

Signals for import into mitochondria and retention in the Golgi complex.

Answer 1

Sorting of a protein to a membrane-sealed organelle involves two main steps: targeting to the organelle and interaction with its surface receptors, and translocation through the organellar membranes by specific import machinery.

Dual targeting of proteins can undergo transmembrane translocation into a membrane-sealed compartment such as mitochondria, chloroplasts, the endoplasmic reticulum (ER) or peroxisomes. The nucleus is not included among these membrane-sealed compartments because nucleocytoplasmic transport takes place through the huge nuclear pore complexes that do not inflict the same constraints on protein movement into or out of the organelle. And also, nuclear import signals function only after a protein is fully synthesized and fully folded.

In this case,

1. Signals for import into the nucleus and extracellular matrix.

Answer: The protein would stay in the extracellular matrix itself, because nuclear import signals function only after a protein is fully synthesized and fully folded and for that the protein should first enter the cell.

Hence, the signal for import into extracellular matrix would win

2. Signals for export from the nucleus and import into the peroxisome.

Answer: The protein would enter the peroxisome. For a nuclear export signal to work, the protein would have to reach the nucleus first and it needs a nuclear import signal.

Hence, the signal for import into the peroxisome would win.

3. Signals for import into mitochondria and retention in the Golgi complex.

Answer: The protein would enter the mitochondria. There is no signal for Golgi complex import, so the Golgi complex retention signal would not function.

Hence, the signal for import into mitochondria would win