Imagine you have engineered a set of genes, each encoding aprotein with a pair of conflicting signal sequences that specifydifferent compartments. If the genes were expressed in a cell, predict which signal would win out for the following combinations. Explain your reasoning.
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 mitochondria and retention in ER (ie, the KDEL signal for retrieval from Golgi to ER). (In this case, you are to assume that we have specially engineered the protein to have a KDEL sequence, and a mitochondrial import sequence, but noamino-terminal signal sequence.)

a) into the ER

b) into the ER

c) into mitochondria

Explanation:

a) This signal is located at the N-terminus of the protein and acts before the internal signal is synthesized for import into the nucleus. When the protein enters the ER, the sequence of the import signal in the nucleus cannot work because the protein would be prevented from interacting with the cytosolic receptors.

b) The signal that is imported into the peroxisome cannot function once the protein was sequestered in the ER.

c) In order for the protein to be retained in the ER, the protein must first be imported into the ER. Without there being no signal for that import of the protein into the ER, that retention signal could not work.