Cheat Sheet Test 3

Cheat Sheet Test 3 - Where will a protein lacking any...

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Where will a protein lacking any signal peptides be localized (this addresses what the “default fate” of a protein is)? What about an artificially designed peptide containing both an ER signal peptide and a NLS? Explain. A protein without any signal sequence will simply be translated by ribosomes in the cytosol and will remain there. A protein with an ER signal will be transported into the ER in spite of a NLS. This is because of ER transport typically being a co-translational process; the ER signal will be recognized and the ribosome will dock to the ER. The fully formed peptide will be in the ER and not have any access to the nucleus or the nuclear import machinery. Why would the expression of another acyl transferase having an ER signal peptide (this enzyme catalyzes the addition of glycerol 3-phosphate to fatty acids in the membrane) still not allow for even membrane growth in the absence of scramblase? The fatty acids and head group modifying enzymes are all available on the cytosolic side of the ER. Targeting the acyl transferase to the inside of the ER will not help matters because of the lack of substrates to work with. [This is the same idea behind why plasma membrane proteins are only glycosylated on the extra cellular side.] If the N-terminal half of PhyB is localized to the cytosol does that indicate the presence of signaling peptides? Does the C-terminal half of PhyB localizing to the nucleus indicate the presence of a NLS? Explain. It is likely that a protein lacking a NLS will remain in the cytosol, so the N- terminal localization does not tell us much. Likewise, it is not clear that even if the C-terminal portion of PhyB localizes to the nucleus that it has a NLS. This half of the protein could be binding to or being bound by another protein that has the NLS, allowing for import of the PhyB fragment. How might suppression of calnexin or calreticulin allow lung tissue to get around one of the main issues of mutant CFTR proteins? Why would this avenue of treatment for cystic fibrosis be a “bad idea?” One of the more common problems with CFTR in cystic fibrosis is that it is recognized as misfolded, retained in the ER, and exported to the cytosol for destruction by the proteosome, even if the misformed CFTR could still have some function. If calnexin and calreticulin were suppressed, this could theoretically allow these malformed but functional CFTR proteins to escape the ER and travel to the plasma membrane to function. However, this is not really a good idea because suppression of these chaperone proteins would allow the forward progress of many other misfolded proteins that could damage the cell through aggregation or simple lack of functioning proteins. What ensures that Sar1 and Rab proteins will associate with specific membranes? (In another way to think about it, why won’t Sar1 or Rabs randomly associate with any membrane after activation?)
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Cheat Sheet Test 3 - Where will a protein lacking any...

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