Serine/threonine-protein kinase component of the WNK3-SPAK/OSR1 kinase cascade, which plays an important role in the regulation of electrolyte homeostasis and regulatory volume increase in response to hyperosmotic stress (PubMed:16275911, PubMed:16275913, PubMed:16501604, PubMed:22989884, PubMed:36318922). WNK3 mediates regulatory volume increase in response to hyperosmotic stress by acting as a molecular crowding sensor, which senses cell shrinkage and mediates formation of a membraneless compartment by undergoing liquid-liquid phase separation (PubMed:36318922). The membraneless compartment concentrates WNK3 with its substrates, OXSR1/OSR1 and STK39/SPAK, promoting WNK3-dependent phosphorylation and activation of downstream kinases OXSR1/OSR1 and STK39/SPAK (PubMed:22989884). Following activation, OXSR1/OSR1 and STK39/SPAK catalyze phosphorylation of ion cotransporters SLC12A1/NKCC2, SLC12A2/NKCC1, SLC12A3/NCC, SLC12A4/KCC1, SLC12A5/KCC2 or SLC12A6/KCC3, regulating their activity (PubMed:16275911, PubMed:16275913). Phosphorylation of Na-K-Cl cotransporters SLC12A2/NKCC1 and SLC12A2/NKCC1 promote their activation and ion influx; simultaneously, phosphorylation of K-Cl cotransporters SLC12A4/KCC1, SLC12A5/KCC2 and SLC12A6/KCC3 inhibits its activity, blocking ion efflux (PubMed:16275911, PubMed:16275913, PubMed:16357011, PubMed:19470686, PubMed:21613606). Phosphorylates WNK4, possibly regulating the activity of SLC12A3/NCC (PubMed:17975670). May also phosphorylate NEDD4L (PubMed:20525693). Also acts as a scaffold protein independently of its protein kinase activity: negatively regulates cell membrane localization of various transporters and channels, such as KCNJ1 and SLC26A9 (PubMed:16357011, PubMed:17673510). Increases Ca(2+) influx mediated by TRPV5 and TRPV6 by enhancing their membrane expression level via a kinase-dependent pathway (PubMed:18768590).
