Dual specificity protein kinase which may phosphorylate kgb-1 and thereby is an essential component of the JNK pathway composed of mlk-1, mek-1 and kgb-1 (PubMed:15116070, PubMed:18809575, PubMed:20008556). May also have a synergistic role with sek-1 in phosphorylating pmk-1 (PubMed:15256594, PubMed:18809575). Involved in the response to environmental stress including heavy metal ions (Cu(2+) and Cd(2+)), oxidative stress and starvation (PubMed:11013217, PubMed:15116070, PubMed:18832074, PubMed:18809575). In association with sek-1, regulates germline cell apoptosis in response to oxidative, osmotic and heat shock stresses (PubMed:16729024). Involved in resistance to pathogenic bacteria infection (PubMed:15256594). Involved in axon regeneration after injury (PubMed:21670305). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] May be activated by phosphorylation at Ser-221 and Ser-225. Interacts with shc-1; the interaction is independent of mek-1 catalytic activity, is constitutive and may facilitate mlk-1-mediated phosphorylation by bringing mlk-1 and mek-1 together. Expressed in pharyngeal muscles, uterine endothelial cells, intestine and in neurons of ring ganglia, ventral ganglion and ganglia around anus (PubMed:11013217, PubMed:18809575). Expressed also in hypodermis and body muscles (PubMed:18809575). Expressed at the embryonic stage. May be phosphorylated at Ser-221 and/or Ser-225 by mlk-1. Loss of kgb-1 activation (PubMed:15116070, PubMed:20008556). Hypersensitivity to high concentrations of copper (> 50 microM) and cadmium (>5 microM) and to food starvation which is characterized by a failure for most animals to reach adulthood. The few surviving adults appear starved and are defective in egg laying and are infertile (PubMed:11013217, PubMed:15116070). In addition, have moderate susceptibility to pathogenic bacteria infection characterized by a shorter lifespan and a substantial decrease in pmk-1 phosphorylation (PubMed:15256594). In absence of stress, animals have no obvious phenotype (PubMed:11013217, PubMed:15116070). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.