Mitogen-activated protein kinase

MAPK cascades relay signals through sequential phosphorylation, creating both amplification and regulatory checkpoints. A MAP3K, often activated by a small GTPase, receptor-associated complex, or cellular stress, phosphorylates a MAP2K on two serine or threonine residues. The MAP2K then phosphorylates the MAPK on a conserved activation-loop motif, commonly Thr–X–Tyr, allowing the MAPK to phosphorylate substrates in the cytoplasm or nucleus.¹

Signal duration, intensity, location, and scaffold organization help determine the outcome. Protein phosphatases reverse activating phosphorylations, while scaffold proteins can assemble selected pathway components and restrict cross-talk. MAPKs recognize substrate docking features as well as catalytic phosphorylation sites, so pathway specificity depends on more than the simple order of kinases.²

The four best-established mammalian MAPK groups are ERK1/2, JNK, p38, and ERK5. ERK1/2 is commonly stimulated by growth factors through receptor tyrosine kinases, RAS, RAF, and MEK, and frequently promotes proliferation, differentiation, or survival. JNK and p38 respond prominently to inflammatory cytokines, osmotic or oxidative stress, ultraviolet radiation, and other damaging conditions, although their functions overlap and depend strongly on cell type and timing.³

ERK5 forms a distinct growth-factor-responsive branch with specialized structural and transcriptional features. The same stimulus can activate several branches, and one branch can produce different effects in different tissues. In plants, fungi, and animals, related MAPK modules have independently become central signaling architectures, illustrating both their deep evolutionary conservation and their functional diversification.⁴

MAPKs coordinate ordinary processes including embryonic development, immune responses, synaptic plasticity, cell-cycle entry, migration, and programmed cell death. Their outputs arise from phosphorylation of transcription factors, cytoskeletal proteins, metabolic enzymes, and other kinases, with nuclear translocation often linking short-lived signals to longer-term gene-expression programs.²

Persistent or misregulated MAPK signaling contributes to cancer, inflammatory disease, fibrosis, and developmental disorders. Mutations or amplifications affecting RAS, RAF, MEK, or related regulators can lock growth pathways into abnormal activity. This biology has produced targeted therapies, including inhibitors of BRAF, MEK, and ERK in selected cancers, but resistance commonly emerges through pathway reactivation, parallel signaling, or altered drug dependence.

MAPK signaling is shaped as much by timing and geography as by kinase identity. Oscillations, pulses, and sustained activation can lead to different transcriptional outcomes, while endosomes, mitochondria, the cytoskeleton, and the nucleus can act as distinct signaling locales. Scaffold proteins such as KSR and JIP-family proteins organize particular kinase combinations, but scaffolding can also insulate pathways from one another rather than simply increase signal strength.²

MAPKs also participate in feedback loops: ERK can phosphorylate upstream pathway components, and induced phosphatases can dampen signaling after activation. Experimental interpretation therefore depends on stimulus duration, cell state, subcellular location, and assay choice. A measured increase in phosphorylated MAPK is not by itself proof of a particular biological outcome, because the same phosphorylation event may accompany distinct responses in different contexts.³

MAP3K

MAP kinase kinase kinase; the upstream tier that activates a MAP2K.

MAP2K

MAP kinase kinase; the dual-specificity kinase that activates a MAPK.

Dual-specificity kinase

A kinase capable of phosphorylating both serine or threonine and tyrosine residues.

ERK

Extracellular signal-regulated kinase, a MAPK group often associated with growth-factor signaling.

JNK

c-Jun N-terminal kinase, a MAPK group responsive to stress and inflammatory signals.

p38

A stress-responsive MAPK group with major roles in inflammation and environmental-stress responses.