4.4 Changes in Signal Transduction Pathways

When pathways change

Mutations in signaling proteins or chemicals (drugs, toxins) can alter a pathway, changing the cell's response. The result may be harmful or, sometimes, beneficial. A mutation in a receptor might make a cell unable to respond to a signal — or respond when it shouldn't.

Disease and drugs

Faulty signaling is central to many cancers, where cells divide uncontrollably because growth signals are stuck 'on.' Drugs can interfere on purpose: an antagonist blocks a receptor, while an agonist mimics the ligand and activates it. Toxins (like cholera toxin) hijack signaling pathways.

Key terms

Pathway mutation

A change in a signaling protein that alters or blocks the cell's response.

Cancer & signaling

Faulty signaling can leave growth pathways 'on,' causing uncontrolled division.

Antagonist

A molecule that blocks a receptor, preventing signal reception.

Agonist

A molecule that mimics a ligand and activates its receptor.

Toxins & pathways

Some toxins (e.g., cholera) disrupt signaling pathways to cause disease.

Try an AP-style question

A mutation causes a Ras protein to stay bound to GTP permanently. What is the most likely consequence?

1. The receptor stops recognizing and binding the growth factor
2. The cell stops dividing because its GTP supply is used up
3. The growth pathway stays switched on even without a growth factor
4. The growth pathway can never be activated, even with growth factor