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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Functional characterization of NIPA2, a selective Mg²⁺ transporter

¹Department of Medicine and ²Department of Psychiatry, University of British Columbia, Vancouver, British Columbia, Canada

Submitted 15 February 2008 ; accepted in final form 28 July 2008

We used microarray analysis to identify renal cell transcripts that were upregulated with low magnesium. One transcript, identified as NIPA2 (nonimprinted in Prader-Willi/Angelman syndrome) subtype 2, was increased over twofold relative to cells cultured in normal magnesium. The deduced sequence comprises 129 amino acids with 8 predicted transmembrane regions. As the secondary structure of NIPA2 conformed to a membrane transport protein, we expressed it in Xenopus oocytes and determined that it mediated Mg²⁺ uptake with two-electrode voltage-clamp and fluorescence studies. Mg²⁺ transport was electrogenic, voltage dependent, and saturable, demonstrating a Michaelis affinity constant of 0.31 mM. Unlike other reported Mg²⁺ transporters, NIPA2 was very selective for the Mg²⁺ cation. NIPA2 mRNA is found in many tissues but particularly abundant in renal cells. With the use of immunofluorescence, it was shown that NIPA2 protein was normally localized to the early endosomes and plasma membrane and was recruited to the plasma membrane in response to low extracellular magnesium. We conclude that NIPA2 plays a role in magnesium metabolism and regulation of renal magnesium conservation.

nonimprinted in Prader-Willi/Angelman syndrome