How MAP Kinases Are Regulated

The closely related MAP kinases,1(1) extracellular signal-regulated protein kinases 1 and 2 (ERK1 and ERK2), are ubiquitous components of signal transduction pathways. ERK1 and ERK2 are activated by diverse extracellular stimuli and by protooncogene products that induce proliferation or enhance differentiation (reviewed in Refs. 1 and 2). MAP kinase phosphorylations have an impact on processes in the cytoplasm, the nucleus, the cytoskeleton, and the membrane. The variety of signals that conscript the MAP kinase pathway demonstrates that this cascade serves a myriad of purposes, and the consequences of its activation will depend on cellular context.