Metabolic Labeling, Immunoprecipitation, and Two-Dimensional Tryptic Phosphopeptide Mapping of Human Topoisomerase II

Protein phosphorylation is almost certainly the most important posttranslational mechanism of enzyme regulation in eukaryotic cells (reviewed in 1 ). The equilibrium between phosphorylation by protein kinases and dephosphorylation by protein phosphatases modulates the activity, subcellular localization, or DNA/RNA/protein binding properties of numerous proteins. Indeed, it appears that the majority of intracellular proteins in human cells are phosphorylated to some degree under certain conditions of cell growth. However, interest in protein phosphorylation is more generally directed toward a study of the alterations in phosphorylation status that either accompany a change in cell physiology or are invoked by exposure to an extracellular stimulus (reviewed in 2 ).