Pathways of Intracellular Signal Transduction

A: Incorrect. Many kinases are themselves activated by other kinases.

B: Incorrect. Many kinases are downstream targets of G protein-coupled receptors.

C: Correct! Most kinases can either phosphorylate serines and threonines or tyrosines, but MEK can phosphorylate both a threonine and a tyrosine residue on ERK2.

D: Incorrect. Many kinases function within a cascade of kinases, whereby one phosphorylates, and thus activates, the next, and so on."