In the context of phosphorylation-based signal transduction, what is the primary role of activated protein kinases in a cell?

• A. They remove phosphate groups from phosphorylated proteins to terminate signaling and restore baseline activity.
• B. They synthesize $ATP$ from $ADP$ and inorganic phosphate during oxidative phosphorylation.
• C. They bind DNA and directly regulate transcription by acting as sequence-specific transcription factors.
• D. They catalyze the transfer of a phosphate group from $ATP$ to specific amino acid residues on target proteins, thereby altering the targets' activity, interactions, or localization.

Answer: (D)