Activates the JUN N-terminal pathway. Essential for pronephros and cement gland development. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimerization via the leucine zipper domains is required for autophosphorylation and subsequent activation. Homodimer. Binds to the GTPase rac1 but not cdc42 or rhoA. Interacts (via kinase domain) with pak1 (via kinase domain). Interacts with the ubiquitin-conjugating enzyme ube2d4. In adults, strongly expressed in the brain and spleen with lower levels in pancreas, heart, muscle and kidney (at protein level). In the developing embryo, expressed at stage 22 in the cement gland. Weakly expressed in the pronephros from stage 24 or 25, with expression increasing in strength by stage 30 and continuing at least until stage 37. Expression in the developing pronephros correlates with epithelialization of the proximal pronephric tubules. Expressed zygotically from stages 12 to 14 (late gastrula to early neurula), increasing in concentration up to stages 40 to 45 (late tadpole). Expression continues through to adults. Autophosphorylation on serine and threonine residues within the activation loop plays a role in enzyme activation. Mono- and poly-ubiquitinated. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.