Cation Interactions and Membrane Potential Induce Conformational Changes in NaPi-IIb

Biophys J. 2016 Sep 6;111(5):973-88. doi: 10.1016/j.bpj.2016.07.025.


Voltage-dependence of Na(+)-coupled phosphate cotransporters of the SLC34 family arises from displacement of charges intrinsic to the protein and the binding/release of one Na(+) ion in response to changes in the transmembrane electric field. Candidate coordination residues for the cation at the Na1 site were previously predicted by structural modeling using the x-ray structure of dicarboxylate transporter VcINDY as template and confirmed by functional studies. Mutations at Na1 resulted in altered steady-state and presteady-state characteristics that should be mirrored in the conformational changes induced by membrane potential changes. To test this hypothesis by functional analysis, double mutants of the flounder SLC34A2 protein were constructed that contain one of the Na1-site perturbing mutations together with a substituted cysteine for fluorophore labeling, as expressed in Xenopus oocytes. The locations of the mutations were mapped onto a homology model of the flounder protein. The effects of the mutagenesis were characterized by steady-state, presteady-state, and fluorometric assays. Changes in fluorescence intensity (ΔF) in response to membrane potential steps were resolved at three previously identified positions. These fluorescence data corroborated the altered presteady-state kinetics upon perturbation of Na1, and furthermore indicated concomitant changes in the microenvironment of the respective fluorophores, as evidenced by changes in the voltage dependence and time course of ΔF. Moreover, iodide quenching experiments indicated that the aqueous nature of the fluorophore microenvironment depended on the membrane potential. These findings provide compelling evidence that membrane potential and cation interactions induce significant large-scale structural rearrangements of the protein.

MeSH terms

  • Animals
  • Cations, Monovalent / metabolism
  • Fish Proteins / chemistry
  • Fish Proteins / genetics
  • Fish Proteins / metabolism
  • Flounder
  • Fluorometry
  • Kinetics
  • Membrane Potentials / physiology*
  • Microscopy, Fluorescence
  • Models, Molecular
  • Mutation
  • Oocytes
  • Patch-Clamp Techniques
  • Protein Conformation
  • Sequence Homology, Amino Acid
  • Sodium / chemistry
  • Sodium / metabolism*
  • Sodium-Phosphate Cotransporter Proteins, Type IIb / chemistry
  • Sodium-Phosphate Cotransporter Proteins, Type IIb / genetics
  • Sodium-Phosphate Cotransporter Proteins, Type IIb / metabolism*
  • Structure-Activity Relationship
  • Water / chemistry
  • Xenopus laevis


  • Cations, Monovalent
  • Fish Proteins
  • Sodium-Phosphate Cotransporter Proteins, Type IIb
  • Water
  • Sodium