Ras is a GTP-binding protein that is often defective in cancer cells. A signal from a growth factor through a receptor tyrosine kinase often stimulates normal cells to divide

Question

When the receptor tyrosine kinase binds the growth factor, Ras is stimulated to bind GTP. Ras in turn activates proteins that promote cell proliferation. A common mutation in cancerous cells causes Ras to behave as though it were bound to GTP all the time.
A. Why is this mutation advantageous to cancerous cells?
B. Your friend decides that the signaling pathway involving the Ras protein is a good target for drug design, because the Ras protein is often defective in cancer cells. Your friend designs a drug that will turn off the receptor tyrosine kinase by preventing it from dimerizing. Do you think that this drug will affect cells that have a defective Ras protein that acts as if it were always bound to GTP? Why or why not?

Answer 1

A. A Ras mutation that causes Ras to behave as though it were bound to GTP all the time is advantageous to cancer cells because Ras is then activated and turns on the activities of proteins required for cell proliferation. If the cell-proliferation proteins are always turned on, the cancer cell will be able to proliferate at an unregulated rate, outgrowing its normal neighbors. The ability to proliferate in a signal-independent fashion is one of the hallmarks of a cancer cell.
B. Unfortunately, a drug that blocks activation of the receptor that activates Ras will be unlikely to have a useful effect on a cell containing mutant Ras protein that behaves as though it were constantly activated. Because Ras acts downstream of the receptor, the activating mutation makes its effect felt regardless of the state of the receptor on which Ras activation would normally depend: mutant Ras that is always active is no longer dependent on the receptor for activation. Therefore, blocking the ability of the receptor to dimerize and activate Ras will probably not affect the cells containing the mutant Ras protein.