Mutations in an unrecognized internal NPT2A PDZ motif disrupt phosphate transport causing congenital hypophosphatemia

The Na ⁺ -dependent phosphate cotransporter-2A (NPT2A, SLC34A1) is a primary regulator of extracellular phosphate homeostasis. Its most prominent structural element is a carboxy-terminal PDZ ligand that binds Na ⁺ /H ⁺ Exchanger Regulatory Factor-1 (NHERF1, SLC9A3R1). NHERF1, a multidomain PDZ protein,establishes NPT2A membrane localization and is required for hormone-sensitive phosphate transport. NPT2A also possesses an uncharacterized internal PDZ ligand. Two recent clinical reports describe congenital hypophosphatemia in children harboring Arg ⁴⁹⁵ His or Arg ⁴⁹⁵ Cys variants within the internal PDZ motif. The wild-type internal ⁴⁹⁴ TRL ⁴⁹⁶ PDZ ligand binds NHERF1 PDZ2, which we consider a regulatory domain. Ablating the internal PDZ ligand with a ⁴⁹⁴ AAA ⁴⁹⁶ substitution blocked hormone-sensitive phosphate transport. Complementary approaches, including CRISPR/Cas9 technology, site-directed mutagenesis, confocal microscopy, and modeling, showed that NPT2A Arg ⁴⁹⁵ His or Arg ⁴⁹⁵ Cys variants do not support PTH or FGF23 action on phosphate transport. Coimmunoprecipitation experiments indicate that both variants bind NHERF1 similarly to WT NPT2A. However, in contrast to WT NPT2A, NPT2A Arg ⁴⁹⁵ His or Arg ⁴⁹⁵ Cys variants remain at the apical membrane and are not internalized in response to PTH. We predict that Cys or His substitution of the charged Arg ⁴⁹⁵ changes the electrostatics, preventing phosphorylation of the upstream Thr ⁴⁹⁴ , interfering with phosphate uptake in response to hormone action, and inhibiting NPT2A trafficking. We advance a model wherein the carboxyterminal PDZ ligand defines apical localization NPT2A, while the internal PDZ ligand is essential for hormone-triggered phosphate transport.