Journal
AMERICAN JOURNAL OF PHYSIOLOGY-CELL PHYSIOLOGY
Volume 294, Issue 2, Pages C495-C502Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/ajpcell.00238.2007
Keywords
microarray analysis; two-electrode voltage clamp; fluorescence; Xenopus oocytes
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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.
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