May act as a mediator of stress-activated signals. Mediates the inhibition of SLC4A4, SLC26A6 as well as CFTR activities by the WNK scaffolds, probably through phosphorylation. Phosphorylates RELT. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] The phosphorylated form forms a complex with WNK2 (By similarity). Interacts with RELT (By similarity). Interacts with SORL1 (via cytosolic C-terminus) (By similarity). Nucleus when caspase-cleaved. Highly expressed in testis followed by pancreas, kidney, heart and brain. Not expressed in skeletal muscle, liver, lung and spleen. Expressed in the early gut and pancreatic epithelium, at E15 day localized to cells that will eventually become exocrine. Expressed in choroid plexus, developing myocardium, pancreatic epithelium and dorsal root ganglia. PAPA box (proline-alanine repeats) may target the kinase to a specific subcellular location by facilitating interaction with intracellular proteins such as actin or actin-like proteins. Phosphorylated at Ser-318 by PRKCQ. Autophosphorylation at Thr-240 positively regulates its activity. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.