a gene regulatory protein, A, contains a typical nuclear localization signal but suprizingly 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 ffrom the cytoplasmic fraction and then separate the protein 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.

Gel shows protein A visible in normal cells, without hormone, cytoplasmic fraction and normal cells, with hormone, nuclear fraction. Also in cells lacking protein B, without hormone, nuclear fraction, and cells lacking protein B with hormone, nuclear fraction.

Gell shows protein B in normal cells without hormone and with hormone in cytoplasmic 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.