NRBP1 pseudokinase binds to and activates the WNK pathway in response to osmotic stress

WNK family kinases are regulated by osmotic stress and control ion homeostasis by activating SPAK and OXSR1 kinases. Using a proximity ligation approach, we found that osmotic stress promotes the association of WNK1 with the NRBP1 pseudokinase and TSC22D2/4 adaptor proteins, results that are confirmed by immunoprecipitation and mass spectrometry and immunoblotting studies. NRBP1 pseudokinase is closely related to WNK isoforms and contains a RΦ-motif binding conserved C-terminal (CCT) domain, similar to the CCT domains in WNKs, SPAK and OXSR1. Knockdown or knock-out of NRBP1 markedly inhibited sorbitol-induced activation of WNK1 and downstream components. We demonstrate recombinant NRBP1 can directly induce the activation of WNK4 in vitro . AlphaFold-3 modelling predicts that WNK1, SPAK, NRBP1, and TSC22D4 form a complex, in which two TSC22D4 RΦ-motifs interact with the CCTL1 domain of WNK1 and the CCT domain of NRBP1. Our data indicates NRBP1 functions as an upstream activator of the WNK pathway.