You are working in a lab that studies protein signaling You

You are working in a lab that studies protein signaling. You are helping a graduate student purify a novel protein complex involved in cellular responses to a specific drug that stimulates cells to grow and divide. You isolate the protein complex from two sets of cells: 1) one set where the cells have been exposed to the drug and 2) one set where the cells have NOT been treated with the drug. When you run the protein complexes on the gel, the 90kD protein consistently migrates slightly slower in the gel, with a band just above the 90kD marker ONLY when isolated from cells that have been exposed to the drug. Assuming you have correctly prepared your SDS-PAGE, provide a reasonable BIOLOGICAL explanation for this observation. [Note: technical problems with the gel should NOT be considered].

Solution

The 90 kDa protein which is novel in nature, probably is involved in the growth and division of the cells in response to the drug.

In absence of the drug, the protein is not activated probably to stimulate growth and division.

Now, it is stated that the 90kD protein consistently migrates slightly slower in the gel, with a band just above the 90kD marker ONLY when isolated from cells that have been exposed to the drug. Hence there is a modification in the protein- such as phosphorylation or methylation- which activates the protein & also increases the molecular weight of the protein slightly. Nevertheless, this modification of the protein, in presence of the drug, might be necessary to activate the protein and thereby, enhance the growth of the cells.