Serine/threonine-protein kinase, which phosphorylates mlk-1. Involved in the stress response to heavy metals by activating the mlk-1/mek-1/kgb-1 pathway (PubMed:20008556). In ventral cord commissural motoneurons, required for dorsal axon guidance downstream of unc-6/netrin repulsion receptor unc-5 and probably of Rho GTPases ced-10 and mig-2. Plays a redundant role with mig-10 in orientating axonal growth of HSN neurons (PubMed:18499456). Plays a redundant role with pak-1 in P neuroblast migration and in distal tip cell (DTC)-mediated guidance of gonad elongation probably downstream of Rho GTPases (PubMed:17050621, PubMed:19023419, PubMed:23390595). In association with pak-2, plays a role in embryogenesis. In association with pak-1, may be involved in spermatogenesis (PubMed:23390595). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with mlk-1; the interaction is independent of max-2 and mlk-1 kinase activities. Interacts with mig-2 (GTP-bound form). Expressed ubiquitously in early embryo. At the comma stage and at later embryonic stages expression is mainly restricted to the anterior region of the embryo and to the ventral cord. At the 1.5-fold embryonic stage, expression starts in pharynx and in several head and tail neurons and is maintained in adults. During L1 larval stage, expressed in ALM and PLM neurons and later in PVD and AVM neurons. Isoforms a and c: Partial sensitivity to the heavy metal Cu(2+), which is characterized by a failure of larvae to reach adulthood. Partial reduction in basal kgb-1 phosphorylation and after Cu(2+) treatment (PubMed:20008556). Several DD and DC motoneuron axons fail to reach the dorsal cord. RNAi-mediated knockdown causes a similar phenotype but less severe. In max-2 and pak-1 double mutants, defect in axonal guidance is more severe. Animals are also uncoordinated, defective in egg laying and in distal tip cell (DTC) migration, morphology and guidance, and exhibit ventral enclosure defects (PubMed:17050621). A more severe phenotype has been reported where double mutant animals have embryonic morphogenesis defects resulting in lethality. The few surviving adults are sterile and often have truncated gonads which, in some cases, lack sperm. RNAi-mediated knockdown rescues embryonic lethality of pak-1 and pak-2 double mutants (PubMed:23390595). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.