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4 Cards in this Set

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Nuclear localization signals are not cleaved off after transport into the nucleus, whereas the signal sequences for import into other organelles are often removed after import. Why is it critical that nuclear localization signals remain attached to their proteins?
At each mitosis, the contents of the nucleus and the cytosol mix when the
nuclear envelope disassembles. When the nucleus reassembles, the nuclear
proteins must be selectively reimported. If the nuclear localization signals
were removed upon import, the proteins would be trapped in the cytosol after
the next mitosis.

My answer: NLS are used for both import and export of proteins in and out of the nucleus, if NLS are cleaved export (or re-import) of proteins would not occur.
You have made a peptide that contains a functional mitochondrial import signal. Would you expect the addition of an excess of this peptide to affect the import of normal mitochondrial proteins? Why or why not?
Only 1 signal sequence is necessary for transport into mitochondria.
Peptides with mitochondrial import signals would be expected to compete
with mitochondrial proteins for binding to the translocation machinery.
Thus, an excess of such peptides should reduce or abolish import of mitochondrial
proteins.
Why do mitochondria need a translocator to import proteins across the outer membrane when the outer membranes already have pores formed by porins?
The porins are freely permeable to inorganic ions and metabolites but are impermeable to most proteins.
State whether the following proteins are being translated on ribosomes attached to the ER, and explain your answer.

a. A protein that resides in the mitochondria.

b. A protein that resides in an endosome.

c. A protein that will be secreted into the extracellular space.

d. A nuclear protein.

e. A protein linked to the plasma membrane by a GPI anchor.

f. A protein that resides between the inner and outer nuclear membranes.
a. A protein that resides in the mitochondria.
Translated in the cytosol, post translational on free ribosomes
b. A protein that resides in an endosome.
Translated in the cytosol, post translational on free ribosomes
A protein that resides in an endosome will be translated on rough ER, because transit to the endosome occurs via the endoplasmic reticulum.
c. A protein that will be secreted into the extracellular space.
Translated in the cytosol, post translational on free ribosomes
A protein that will be secreted into the extracellular space will be translated on rough ER and will enter the ER lumen to be sent, via the Golgi, to the extracellular space.
d. A nuclear protein.
Translated in the cytosol, post translational on free ribosomes
e. A protein linked to the plasma membrane by a GPI anchor.
Co-translational, membrane bound ribosomes

f. A protein that resides between the inner and outer nuclear membranes.
Co-translational, membrane bound ribosomes