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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Identification and characterization of a novel family of membrane magnesium transporters, MMgT1 and MMgT2

Department of Medicine, University of British Columbia, Vancouver, British Columbia, Canada

Submitted 6 June 2007 ; accepted in final form 28 November 2007

Magnesium is an essential metal, but few selective transporters have been identified at the molecular level. Microarray analysis was used to identify two similar transcripts that are upregulated with low extracellular Mg²⁺. The corresponding cDNAs encode proteins of 131 and 123 amino acids with two predicted transmembrane domains. The two separate gene products comprise the family that we have termed "membrane Mg²⁺ transporters" (MMgTs), because the proteins reside in the membrane and mediate Mg²⁺ transport. When expressed in Xenopus laevis oocytes, MMgT1 and MMgT2 mediate Mg²⁺ transport as determined with two-electrode voltage-clamp analysis and fluorescence measurements. Transport is saturable Mg²⁺ uptake with Michaelis constants of 1.47 ± 0.17 and 0.58 ± 0.07 mM, respectively. Real-time RT-PCR demonstrated that MMgT mRNAs are present in a wide variety of cells. Subcellular localization with immunohistochemistry determined that the MMgT1-hemagglutinin (HA) and MMgT2-V5 fusion proteins reside in the Golgi complex and post-Golgi vesicles, including the early endosomes in COS-7 cells transfected with the respective tagged constructs. Interestingly, MMgT1-HA and MMgT2-V5 were found in separate populations of post-Golgi vesicles. MMgT1 and MMgT2 mRNA increased by about threefold, respectively, in kidney epithelial cells cultured in low-magnesium media relative to normal media and in the kidney cortex of mice maintained on low-magnesium diets compared with those animals consuming normal diets. With the increase in transcripts, there was an apparent increase in MMgT1 and MMgT2 protein in the Golgi and post-Golgi vesicles. These experiments suggest that MMgT proteins may provide regulated pathways for Mg²⁺ transport in the Golgi and post-Golgi organelles of epithelium-derived cells.

microarray analysis; two-electrode voltage clamp; fluorescence; Xenopus oocytes

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