The following images represent simplified microscopic views of cells in which a
ID: 301841 • Letter: T
Question
The following images represent simplified microscopic views of cells in which a copy of the DNA polymerase has been fused to a green fluorescent protein. The amino acid sequence of a portion of the nuclear localization sequence of the polymerase is shown below each image. normal (wild type) mutant # mutant 2 mart #3 mutant 84 0 A. What characteristic common to many signal sequences do these data demonstrate? (2pt) B. In a follow up experiment you took the sequence used for mutation #3 and inserted it into the wildtype protein without removing the wildtype signal. Do you expect the outcome to look like the wildtype, like mutant #3, or some combination of the two? Why? (2pt)Explanation / Answer
A.
Available signal sequences show the common characteristic of simple karyophilic clusters of arginines and lysines important in all nucleus-targeted proteins.
These signal sequences help in anchoring of these proteins to specialized transporter molecules called importins found on the nuclear pore complex. These peptides have been termed nuclear localization signals (NLS).
B
In the follow-up experiment if I took the signal sequence of the mutant #3 that is given to be -Pro-Lys-Thr-Lys-Arg-Lys- and inserted it into a wild-type protein which has its nucleur-localization signal sequence intact, then what phenotype will I seen in the cells.
Research has shown that active Nuclear Localization Signal sequence has to be exposed to the protein surface in the process of protein-folding, in the cell hidden or cryptic sequences are revealed to the surface by mechanisms of protein phosphorylation, dephosphorylation, dissociation of an inhibitory subunit that masks the NLS, processing of a larger precursor, and binding of a hormone to regulate the nuclear import of a protein (Boulikas 1993). Hence depending on which signal sequence out of the two (wild type or mutant#3 sequence) is expressed on the surface of the final active protein will determine the localization of the protein, it can either be in nucleus or in the cytoplasm.
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