Structure of MAPKs

Mitogen-activated protein kinases (MAPKs) are protein-serine/threonine kinases activated by signaling pathways triggered by developmental stages, cell-surface receptors, cell stresses and other environmental cues. The MAPK family includes the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) and a splice variant of each, at least two ERK3 isoforms, ERK5, ERK7, four p38 MAP kinases (p38α, β, γ, and δ), and three c-Jun-N-terminal kinases/stress-activated protein kinases (JNK1–3/SAPKα, β, and γ), each with multiple splice variants (1 ,2 ). These kinases are often categorized based on their most efficacious activators, although all are regulated by numerous overlapping stimuli. ERK1/2 are major targets of Ras-dependent signals and are usually most strongly activated by growth factors and proliferative stimuli. The p38 MAPKs and the JNK/SAPKs are recognized as stress sensors and, in some cases, promote apoptosis. ERK5 is significantly activated by growth factors and stresses and does not fits easily into either of these categories. Signals that activate ERK3 and ERK7 have not been determined.