Structural Model Of The Human Sodium-Phosphate Cotransporter Napi-Ii

Homeostasis of inorganic phosphate (Pi) in vertebrates is maintained by control of intestinal absorption, storage and release in bones, and renal excretion. These processes depend on tightly regulated expression of Na+-coupled Pi transporters of the SLC34 solute carrier family (NaPi-II). Their crucial role in Pi homeostasis is underscored by pathologies resulting from naturally occurring SLC34 mutations and SLC34 knock-out animals. SLC34 isoforms have been extensively studied with respect to transport mechanism and structure-function relationships; however, the 3-dimensional structure is unknown. All SLC34 transporters share a duplicated motif comprising a glutamine followed by a stretch of threonine or serine residues, suggesting the presence of structural repeats as found in other transporter families. Nevertheless, standard bioinformatic approaches fail to clearly identify a suitable template for molecular modeling. Here, we used hydrophobicity profiles and hidden Markov Models to first define a structural repeat common to all SLC34 isoforms. Similar approaches identify a relationship with the core regions in a crystal structure of Vibrio cholerae Na+-dicarboxylate transporter VcINDY, from which we generated a homology model of human NaPi-IIa. The aforementioned SLC34 motifs in each repeat localize to the center of the model, and were predicted to form Na+ and Pi coordination sites. Functional relevance of key amino acids was confirmed by biochemical and electrophysiological analysis of expressed, mutated transporters. Moreover, the validity of the predicted architecture is corroborated by extensive published structure-function studies. The NaPi-IIa model provides a firm foundation for a molecular understanding of Na+-coupled Pi uptake by SLC34 transporters.