Plays a role in osmotic stress responses by regulating ion homeostasis and by controlling cell volume via the phosphorylation-mediated inhibition of the chloride channel clh-3 (PubMed:15684092, PubMed:17596296). In addition, increases gpdh-1 translation upon osmotic stress, likely downstream of wnk-1 (PubMed:23076791). Involved in several developmental processes including the tubular formation of the excretory canals, the formation of the intestine and the progression through larval stages (PubMed:20595048). In addition, required for germ line development by controlling meiosis and chromosomal segregation during spermatogenesis. By controlling clh-3 activity, may regulate the development of the excretory canals and fertility (PubMed:18049475). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts (via C-terminus) with clh-3; required for the phosphorylation-mediated inhibition of clh-3 function (PubMed:15684092). Interacts (via C-terminus) with wnk-1; the interaction is direct (PubMed:17596296, PubMed:18049475). Localizes in the nucleus in intestinal cells. Ubiquitously expressed with a higher expression in the excretory cell. Expressed in both male and female germ cells; up-regulated in maturing spermatocytes but absent in mature sperm. Expressed at all stages. Phosphorylated at Thr-280 and Ser-419 probably by wnk-1; phosphorylation results in weak activation (PubMed:18049475). Predominantly autophosphorylated at Thr-32 and Ser-190 and weakly autophosphorylated at Thr-13 and Ser-405 in vitro (PubMed:20595048). Hermaphrodites arrest after the first molt and die after at most 6 days. Most animals appear L2-like with morphologies of gonads and vulva that are out of step with the developmental stage. Mutants have a dilated intestinal lumen with extensive internal folding associated with the accumulation of undigested bacteria and a shortened excretory canal. Some animals have adult-type alae and are sterile with vulva and germ line formation defects (PubMed:20595048). In RNAi-mediated knockdown, impaired survival and slower volume recovery upon hypertonic stress (PubMed:17596296). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.