Protein kinases: Receptors of external signals

Protein kinases are the largest known protein family. Pour min categories o f kinases exist based on their structure and function. Up to half of all ce llular proteins are reversibly phosphorylated by kinases, and this modifica tion alters protein function within the cell. The kinases are classified no t only by their structure, but by the extracellular ligands that signal to regulate their activity, the proteins that interact with them, and their su b-cellular localization. Inhibitors of kinases are found in nature and can occasionally be of therapeutic value. Inhibitors often provide clues about the cellular function of a newly described kinase. The dephosphorylation of proteins by specific phosphatases provides an additional switch for alteri ng cellular processes.