Serine/threonine kinase which plays an important role in the regulation of electrolyte homeostasis, cell signaling, survival and proliferation. Acts as an activator and inhibitor of sodium-coupled chloride cotransporters and potassium-coupled chloride cotransporters respectively (PubMed:16275913, PubMed:16275911, PubMed:16357011). Phosphorylates WNK4. Regulates the phosphorylation of SLC12A1 and SLC12A2. Increases Ca(2+) influx mediated by TRPV5 and TRPV6 by enhancing their membrane expression level via a kinase-dependent pathway (PubMed:18768590). Inhibits the activity of KCNJ1 by decreasing its expression at the cell membrane in a non-catalytic manner. Isoform 1, isoform 2, isoform 3 and isoform 4 stimulate the activity of SLC12A1, SLC12A2 and SLC12A3 and inhibit the activity of SLC12A4, SLC12A5, SLC12A6 and SLC12A7. According to PubMed:19470686, isoform 1 inhibits the activity of SLC12A3. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activation requires autophosphorylation of Ser-308. Phosphorylation of Ser-304 also promotes increased activity (By similarity). Kinase activity is inhibited by WNK4. Interacts with WNK1 and WNK4. Expressed in brain, lung, kidney, liver and pancreas, and in fetal tissues including placenta, fetal brain, lung and kidney. Very low levels of expression were also detected in fetal heart, thymus, liver and spleen. Isoform 1 is brain-specific. Isoform 3 is kidney-specific. Ubiquitinated by the BCR(KLHL2) complex, leading to its degradation. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WNK subfamily. Was named WNK/'with no lysine(K)' because key residues for catalysis, including the lysine involved in ATP binding, are either not conserved or differ compared to the residues described in other kinase family proteins.