Ras is a GTP-binding protein that is often defective in cancer cells. A signal f
ID: 73162 • Letter: R
Question
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. 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?
Explanation / Answer
A. GTP-bound Ras recruits and activates Raf. Rfa then initiates a cascade of protein phosphorylation by phosphorylating MEKs. MEK then activates ERKs by phosphorylation. Activated ERK then diffuses into the nucleus, where it phosphorylates specific transcription factors. Phosphorylation of transcription factors turns on transcription of target gene X, which in turn promotes continuous cell proliferation. Ras has a regulatory pathway in which Ras bound GTP is converted to GDP thus making it inactive. If Ras behaves as thorough it were bound to GTP all the time, then it continuously active the gene necessary for the cell to divide.
B. No, this drug not effective to control the cell division, because even though the drug prevents receptor tyrosine kinase from dimerizing, it cannot stop the Ras which behaves like it were bound to GTP. This Ras constantly keep on activating the genes even without binding to receptor tyrosine kinase.
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