WNK1 kinase belongs to a family of serine-threonine protein kinases with an atypical placement of the catalytic lysine. Increased expression of WNK1 causes hypertension and hyperkalemia in humans. WNK1 inhibits renal potassium channel ROMK1 by enhancing its endocytosis, likely contributing to hyperkalemia in affected patients. The domains of WNK1 involved in inhibition of ROMK1 have not been completely elucidated. Here, we reported that an N-terminal proline-rich domain (N-PRD; amino acids 1-119) is necessary and sufficient for WNK1 inhibition of ROMK1. A region (named "NL" for N-linker; amino acids 120-220) located between N-PRD and the kinase domain of WNK1 (amino acids 220-491) antagonized the inhibition of ROMK1 caused by N-PRD. The WNK1 kinase domain reversed the antagonism of NL on N-PRD. Mutagenesis studies revealed that charge-charge interactions between two conserved catalytic residues (Lys-233 and Asp-368) within the kinase domain (not the kinase activity) is critical for kinase domain to reverse the antagonism of NL domain. The WNK1 auto-inhibitory domain (AID; amino acids 491-555) also affected ROMK, presumably by modulating the kinase domain conformation. Thus, multiple intra- and/or intermolecular interactions of WNK1 domains are at play for regulation of ROMK1 by WNK1.