the family. On the other hand, the N-terminal domain is unique in having a six-stranded instead of a five-stranded β sheet to form a complete β barrel. A chloride ion binding site has been identified in the region 320DLG323 of the kinase domain in WNK4. The binding of chloride Cl− in this region inhibits the activation of WNK4. The autoinhibitory domain is a homolog of the RFXV-binding PASK/FRAY homology 2 (PF2) domain. Structural studies have revealed that the autoinhibitory domain consists of three β-strands and two α-helices. Notably, the RFXV‐binding groove is formed by the β3-αA interface of WNK proteins where RFXV peptide ligand interacts directly with residues Phe524, Asp531, and Glu539 of WNK1. The interaction between the RFXV motif and the autoinhibitory domain makes it possible for the C-terminal region of WNK4 to be in close proximity of the kinases domain and subsequently regulate its activity. Function As a typical kinase, WNK4 accomplishes the phosphorylation of its substrate proteins by adding a phosphate moiety in an ATP-dependent manner. This structural modification usually results in functional alterations of downstream substrates. Some currently known substrates of WNK4 includes kinases STE20-serine-proline alanine-rich kinase (SPAK) and oxidative stress response 1 kinase (OSR1), which in turn can phosphorylate and activate the thiazide-sensitive sodium-chloride cotransporter (NCC) (Fig. 2). Similarly, WNK4 activates NKCC1 and deactivate KCC2 through a SPAK-dependent mechanism. The kinase activity of WNK4 has been demonstrated in vitro using the WNK4 kinase domain purified from E. coli. This phosphorylation cascade is critical in regulating sodium and potassium homeostasis dysregulation which is tied to the pathogenesis of PHAII. In addition to NCC, WNK4 also regulates multiple ions channels and cotransporters in the kidney through various mechanisms. These include epithelial Na+ channel (ENaC), renal outer medullary potassium channel (ROMK), transient receptor potential vanilloid member 4 and 5 (TRPV4/5, calcium channels), Na-K-2Cl cotransporter 1 and 2 (NKCC1/2), K+-Cl− cotransporter type 2 (KCC2), and other channels/transporters. WNK4 inhibits the functions of ENaC, ROMK, and TRPV4 by reducing the total and cell surface expression of these channels. WNK4 enhances TRPV5 by increasing its forward trafficking to the plasma membrane in a kinase-dependent manner. The inhibitory effect of WNK4 on ROMK is reversed by serum and glucocorticoid kinase 1 (SGK1) or by a corresponding phosphomimetic S1169D mutation on WNK4. The N-terminal segment of WNK4 containing the kinase domain and acidic motif is required for the WNK4-mediated inhibition of ROMK. The