Multiple Pathways for Protein Phosphatase 1 (PP1) Regulation of Na-K-2Cl Cotransporter (NKCC1) Function THE N-TERMINAL TAIL OF THE Na-K-2Cl COTRANSPORTER SERVES AS A REGULATORY SCAFFOLD FOR Ste20-RELATED PROLINE/ALANINE-RICH KINASE (SPAK) AND PP1

The Na-K-2Cl cotransporter (NKCC1) participates in epithelial transport and in cell volume maintenance by mediating the movement of ions and water across plasma membranes. Functional studies have previously demonstrated that NKCC1 activity is stimulated by protein phosphatase 1 (PP1) inhibitors. In this study, we utilized both in vivo (heterologous cRNA expression in Xenopus laevis oocytes) and in vitro (P-32-phosphorylation assays with glutathione S-transferase fusion proteins) experiments to determine whether PP1 exerts its inhibitory effect directly on the cotransporter, or indirectly by affecting the activating kinase. We found that PP1 reduced NKCC1 activity in oocytes under both isotonic and hypertonic conditions to the same level as in water-injected controls. Interestingly, mutation of key residues in the PP1 binding motif located in the N-terminal tail of NKCC1 significantly reduced the inhibitory effect of PP1. In vitro experiments performed with recombinant PP1, SPAK (Ste20-related proline/alanine-rich kinase, which activates NKCC1), and the N terminus of NKCC1 fused to glutathione S-transferase demonstrated that PP1 dephosphorylated both the kinase and the cotransporter in a time-dependent manner. More importantly, PP1 dephosphorylation of SPAK was significantly greater when protein-protein interaction between the kinase and the N-terminal tail of NKCC1 was present in the reaction, indicating the necessity of scaffolding the phosphatase and kinase in proximity to one another. Taken together, our data are consistent with PP1 inhibiting NKCC1 activity directly by dephosphorylating the cotransporter and indirectly by dephosphorylating SPAK.