Regulation of protein kinase cascades by protein phosphatase 2A

Many protein kinases themselves are regulated by reversible phosphorylation , Upon cell stimulation, specific kinases are transiently phosphorylated an d activated. Several of these protein kinases are substrates for protein ph osphatase 2A (PP2A), and PP2A appears to be the major kinase phosphatase in eukaryotic cells that downregulates activated protein kinases. This idea i s substantiated by the observation that some viral proteins and naturally o ccurring toxins target PP2A and modulate its activity. There is increasing evidence that PP2A activity is regulated by extracellular signals and durin g the cell cycle. Thus, PP2A is likely to play an important role in determi ning the activation kinetics of protein kinase cascades.