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) (PubMed:20385770). Nucleus when caspase-cleaved. Predominantly expressed in brain and pancreas followed by heart, lung, kidney, skeletal muscle, liver, placenta and testis. 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-309 by PRKCQ (PubMed:14988727). Autophosphorylation at Thr-231 positively regulates its activity (By similarity). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.