A Gene Regulatory Protein, A, Contains a Typical Nuclear Localization

Essay

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 characteristic bands on the gel.The gel you run is shown in Figure 15-4.
Figure 15-4
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.