1.A.67 The Membrane Mg2+ Transporter (MMgT) Family
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 (MMgT1) and 123 (MMgT2) amino acids with two predicted transmembrane domains (TMDs). When expressed in Xenopus oocytes, MMgT1 and MMgT2 mediate Mg2+ transport. Uptake is saturable with Michaelis constants of 1.5mM and 0.6mM, respectively. MMgT1-HA and MMgT2-V5 fusion proteins reside in the Golgi complex and post-Golgi vesicles but in separate populations of post-Golgi vesicles. MMgT1 and MMgT2 mRNA increased in kidney epithelial cells cultured in low magnesium media. With the increase in transcripts there was an apparent increase in MMgT1 and MMgT2 protein in the Golgi and post-Golgi vesicles. Thus, MMgT proteins provide regulated pathways for Mg2+ transport in the Golgi and post-Golgi organelles of epithelial-derived cells (Goytain and Quamme, 2007).
Most homologues are ~100-130aas in length. They have 2 TMSs, and are found in all types of eukaryotes. Because there are only 2TMSs, these proteins are presumed to form oligomeric channels (Saier, 2003).
The transport reaction catalyzed by MMgT homologues is: