Schematic illustration of the basic functioning of MAPK pathways. Receptors sense specific extracellular stimuli (denoted by A, B, and C) and activate MAPKKKs. This step usually involves other factors, such as G proteins or protein kinases. Activation of MAPKKK results in phosphorylation and activation of MAPKK. Some MAPKKKs are also capable of activating unrelated protein kinases (PK) of other pathways. MAPKKs activate MAPKs. MAPKs target different substrates, including transcription factors, cytoskeletal proteins, or other kinases and phosphatases, which are then directly or indirectly responsible for the responses. Arrows indicate activation of targets.

Schematic illustration of the possible functions of the Arabidopsis ANP1 and tobacco NPK1 MAPK pathways. (A) Cold and H₂O₂ activate the AtMPK3 and AtMPK6 pathway through either an unknown MAPKKK or ANP1, respectively (4). ANP1 activation results in induction of stress-inducible genes and inhibition of auxin-inducible genes. Auxin stimulates auxin-inducible gene expression by an independent pathway. (B) Mitotic regulation of the cell cycle is mediated by the NPK1 pathway, which also consists of the MAPKK Q and MAPK R (13). NACK1, a kinesin-like protein, is an upstream activator of NPK1 (13).