A stuck-on step doesn't need the signal above it
- The idea
- Predict
- See it
- Why
- Check
The idea
“Blocking the signal or the receptor stops every step below it, even one that's stuck on.”
On paper a pathway is a chain of arrows, and cutting the top stops everything below. A stuck-on step doesn't wait for the arrow above it.
The short version
The mistake: Blocking the signal or the receptor stops every step below it, even one that's stuck on.
What's actually true: A protein locked in its active form passes the signal on by itself, so blocking a step above it won't turn the response off.
- In a signaling pathway, each protein is switched on by the one above it and switches on the one below.
- Normally each step shuts off again. Ras hydrolyzes its GTP to GDP, and phosphatases remove the phosphates kinases added. So the response ends soon after the signal.
- A protein locked in its active form keeps signaling on its own, so the steps above it lose control.
- That's why tumors with locked-on Ras often resist drugs that block the growth factor receptor.
On the exam: Find the step that's stuck on. Only a block there or below it stops the response.