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Conferring electrogenicity to the electroneutral phosphate cotransporter NaPi-IIc (SLC34A3) reveals an internal cation release step

  • Ion channels, Receptors and Transporters
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Abstract

The SLC34 family of Na+-dependent inorganic phosphate cotransporters comprises two electrogenic isoforms (NaPi-IIa, NaPi-IIb) and an electroneutral isoform (NaPi-IIc). Both fulfill essential physiological roles in mammalian phosphate homeostasis. By substitution of three conserved amino acids, found in all electrogenic isoforms, at corresponding sites in NaPi-IIc, electrogenicity was re-established and the Na+/P i stoichiometry increased from 2:1 to 3:1. However, this engineered electrogenic construct (AAD-IIc) had a reduced apparent P i affinity and different presteady-state kinetics from the wild-type NaPi-IIa/b. We investigated AAD-IIc using electrophysiology and voltage clamp fluorometry to elucidate the compromised behavior. The activation energy for cotransport was threefold higher than for NaPi-IIc and 1.5-fold higher than for NaPi-IIa and the temperature dependence of presteady-state charge displacements suggested that the large activation energy was associated with the empty carrier reorientation. AAD-IIc shows a weak interaction of external Na+ ions with the electric field, and thus retains the electroneutral cooperative interaction of two Na+ ions preceding external P i binding of NaPi-IIc. Most of the presteady-state charge movement was accounted for by the empty carrier (in the absence of external P i ), and the cytosolic release of one Na+ ion (in the presence of P i ). Simulations using a kinetic model recapitulated the presteady-state and steady-state behavior and allowed identification of two critical partial reactions: the final release of Na+ to the cytosol and external P i binding. Fluorometric recordings from AAD-IIc mutants with Cys substituted at functionally important sites established that AAD-IIc undergoes substrate- and voltage-dependent conformational changes that correlated qualitatively with its presteady-state kinetics.

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Notes

  1. Standard procedures to minimize the contamination by endogenous Ca2+-dependent Cl− currents, such as preincubation of oocytes in BAPTA-AM, or replacement of external Ca2+ with Ba2+ did not fully suppress these currents.

  2. We also previously reported evidence of a leak mode for AAD-IIc based on the electrogenic response to application of the blocker phosphonoformic acid (PFA) [4]. A detailed characterisation of this mode was not undertaken in the present study due to the low apparent affinity of AAD-IIc for Pi and PFA that would result incomplete suppression of the leak current with the substrate concentrations used.

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Acknowledgments

We gratefully acknowledge Eva Hänsenberger for oocyte preparation. Special thanks to Dr Anne-Kristine Meinild (UZH) and Dr Donald D.F. Loo (UCLA) for their insightful comments. This work was supported by the Swiss National Science Foundation grant to ICF and Hartmann Müller-Stiftung grant to CG.

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Authors and Affiliations

  1. Institute of Physiology and Zurich Center for Integrative Human Physiology (ZIHP), University of Zurich, Winterthurerstrasse 190, 8057, Zurich, Switzerland

    Monica Patti, Chiara Ghezzi & Ian C. Forster

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  1. Monica Patti
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  2. Chiara Ghezzi
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  3. Ian C. Forster
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Correspondence to Ian C. Forster.

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Monica Patti and Chiara Ghezzi contributed equally to this work.

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Patti, M., Ghezzi, C. & Forster, I.C. Conferring electrogenicity to the electroneutral phosphate cotransporter NaPi-IIc (SLC34A3) reveals an internal cation release step. Pflugers Arch - Eur J Physiol 465, 1261–1279 (2013). https://doi.org/10.1007/s00424-013-1261-9

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