WNK3 and WNK4 Exhibit Opposite Sensitivity with Respect to Cell Volume and Intracellular Chloride Concentration.

Cation-coupled chloride cotransporters (CCC) play a role in modulating intracellular chloride concentration ([Cl-]i) and cell volume. Cell shrinkage and cell swelling are accompanied by an increase or decrease in [Cl-]i, respectively. Cell shrinkage and a decrease in [Cl-]i increases the activity of NKCCs (Na-K-Cl cotransporters: NKCC1, NKCC2 and Na-Cl) and inhibits the activity of KCCs (K-Cl cotransporters: KCC1 to KCC4), while cell swelling and an increase in [Cl-]i activates KCCs and inhibits NKCCs; thus, it is unlikely that the same kinase is responsible for both effects. WNK1 and WNK4 are chloride-sensitive kinases that modulate the activity of CCC in response to changes in [Cl-]i. Here, we showed that WNK3, another member of the serine-threonine kinase WNK family with known effects on CCC, is not sensitive to [Cl-]i, but can be regulated by changes in extracellular tonicity. In contrast, WNK4 is highly sensitive to [Cl-]i but is not regulated by changes in cell volume. The activity of WNK3 towards NaCl cotransporter is not affected by eliminating the chloride-binding site of WNK3, further confirming that the kinase is not sensitive to chloride. Chimeric WNK3-WNK4 proteins were produced, and analysis of the chimeras suggests that sequences within the WNKs carboxy-terminal end may modulate the chloride affinity. We propose that WNK3 is a cell volume-sensitive kinase that translates changes in cell volume into phosphorylation of CCC.