A gene regulatory protein, A, contains a typical nuclear localization signal but
ID: 266789 • Letter: A
Question
A gene regulatory protein, A, contains a typical nuclear localization signal but surprisingly is usually found in the cytosol. When the cell is exposed to hormones, protein A moves from the cytosol into the nucleus, where it turns on genes involved in cell division. When you purify protein A from cells that have not been treated with hormones, you find that protein B is always complexed with it. To determine the function of protein B, you engineer cells lacking the gene for protein B. You compare normal and defective cells by using differential centrifugation to separate the nuclear fraction from the cytoplasmic fraction, and then separating the proteins in these fractions by gel electrophoresis. You identify the presence of protein A and protein B by looking for their risbanon the ge. The gel you nun is shown in the figure normal cells cells lacking protein B C N C N C N C N protein A protein B +with hormone -=without hormone C cytoplasmic fraction N nuclear fraction On the basis of these results, what is the function of protein B? Explain your conclusion and propose a mechanism for how protein B works in several sentencesExplanation / Answer
Case I (Normal cells):
It is given that protein A is complexed with protein B and protein A has nuclear localization signal (NLS).
In the absence of hormone, the proteins A and B are exclusively present in cytoplasmic fraction and absent from the nuclear fraction.In the presence of hormone, protein A is present in nuclear fraction while protein B is present in cytoplasmic fraction.
It follows that the hormone binding initiates a signalling cascade which may cause conformational change in protein B resulting in its dissociation from protein A. The protein A then moves to nucleus.
This can be explained on the basis that in absence of hormonal signalling, the protein B masks the NLS of protein resulting in its localization in the cytoplasm. When hormone binding initiates signalling, it brings about conformational change in protein B, resulting in its dissociation from protein A. This unmasks the NLS of protein A which then moves to nucleus and turns on genes involved in cell division.
Case II (Cells lacking protein B):
In this case protein A is found in the nuclear fraction either in presence or absence of hormone. As protein B is not available in the cytoplasm, the NLS of protein will always be exposed resulting in its localization in nucleus.
Thus, we conclude that the protein B functions by masking the NLS of protein A.
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