Question

when epidermal growth factor (egf) ligands bind to their receptors, the receptors pair up and act as kinases, attaching phosphate groups to one another’s intracellular tails. the activated receptors then trigger a series of events that activate the kinase raf.

which of the following toxins will most likely inhibit the activation of mek?

choose 1 answer:
a a toxin that prevents the dephosphorylation activity of raf
b a toxin that prevents the kinase activity of raf
c a toxin that prevents the transfer of a phosphate group to raf
d a toxin that prevents mek from phosphorylating raf

Explanation:

1. Analyze the signaling pathway: EGF receptor activation leads to Raf activation, and Raf (a kinase) likely phosphorylates MEK to activate it.
2. Evaluate Option A: Preventing Raf dephosphorylation would keep Raf active, promoting MEK activation, not inhibiting it.
3. Evaluate Option B: If Raf's kinase activity is prevented, Raf can't phosphorylate MEK, so MEK activation is inhibited. This matches the requirement.
4. Evaluate Option C: Raf is activated by upstream events (not by phosphate transfer to it in this context), so preventing phosphate transfer to Raf is not relevant to its activation of MEK.
5. Evaluate Option D: MEK phosphorylates downstream targets (like ERK), not Raf. So this is incorrect.

Answer:

B. A toxin that prevents the kinase activity of Raf