Identification of a novel KLHL3-interacting motif in the C-terminal region of WNK4

Mutations in with-no-lysine [K] kinase 4 (WNK4) and kelch-like 3 (KLHL3) are linked to pseudohypoaldosteronism type 2 (PHAII, also known as familial hyperkalemic hypertension or Gordon's syndrome). WNK4 is degraded by a ubiquitin E3 ligase with KLHL3 as the substrate adaptor for WNK4. Several PHAII-causing mutations, e.g. those in the acidic motif (AM) of WNK4 and in the Kelch domain of KLHL3, impair the binding between WNK4 and KLHL3. This results in a reduction in WNK4 degradation and an increase in WNK4 activity, leading to PHAII. Although the AM is important in interacting with KLHL3, it is unclear whether this is the only motif in WNK4 responsible for KLHL3-interacting. In this study, a novel motif of WNK4 that is capable of mediating the degradation of the protein by KLHL3 was identified. This C-terminal motif (termed as CM) is located in amino acids 1051-1075 of WNK4 and is rich in negatively charged residues. Both AM and CM responded to the PHAII mutations in the Kelch domain of KLHL3 in a similar manner, but AM is dominant among the two motifs. The presence of this motif likely allows WNK4 protein to respond to the KLHL3-mediated degradation when the AM is dysfunctional due to a PHAII mutation. This may be one of the reasons why PHAII is less severe when WNK4 is mutated compared to KLHL3 is mutated.