A growth factor binds an RTK and activates the Ras→Raf→MEK→ERK cascade. A drug irreversibly inhibits MEK. Which downstream effect is most directly observed within minutes of drug exposure?
- A
Inability of Ras to exchange GDP for GTP at the membrane
- B
Defective dimerization and autophosphorylation of the RTK
- Ccheck_circle
Loss of ERK phosphorylation despite continued ligand binding
- D
Failure of the growth factor to bind the receptor extracellular domain
Explanation
MEK is the kinase that phosphorylates ERK. Inhibiting MEK leaves the steps upstream (binding, dimerization, Ras-GTP) intact but blocks ERK activation directly.