Structural Basis for Potassium Inhibition of WNK Kinases

WNK kinases are chloride- and osmotic-stress-regulated protein kinases recently shown to be controlled by potassium. Prior studies demonstrated the direct binding of chloride and osmotic stress-related water in WNK kinase regulation. Here, we probe potassium binding and regulation of WNK kinases via crystallography coupled with mutagenic analysis of WNK kinase autophosphorylation and activity. Crystals of unphosphorylated WNK1 grown in cesium formate, a surrogate for potassium, yielded nonsulfur scattering peaks at 5.75 keV. Mutations were introduced into amino acids flanking the anomalous diffraction peaks. Mutations in WNK1/E388 and the corresponding WNK3/E314, probing a peak close to WNK1/I384, led to reduced inhibition by potassium while maintaining kinase autophosphorylation and substrate phosphorylation activity. Other peaks probed by mutagenesis either did not bear out as potassium regulatory sites or were not validated due to the inactivity of the mutants synthesized. Previously synthesized chloride- and water-binding mutants demonstrate correlated sensitivity to chloride and potassium. Potassium, chloride, and water are all WNK inhibitors that share a common mechanism binding the same low-activity asymmetric dimer of WNK1 kinase domains.