The homologous Arabidopsis MRS2/MGT/CorA-type Mg2+ channels, AtMRS2-10 and AtMRS2-1 exhibit different aluminum transport activity.

Magnesium (Mg2+) plays a critical role in many physiological processes. The AtMRS2/MGT family, which consists of nine Arabidopsis genes (and two pseudo-genes) belongs to a eukaryotic subset of the CorA superfamily of divalent cation transporters. AtMRS2-10 and AtMRS2-1 possess the signature GlyMetAsn sequence conserved in the CorA superfamily; however, they have low sequence conservation with CorA. Direct measurement using the fluorescent dye mag-fura-2 revealed that reconstituted AtMRS2-10 and AtMRS2-1 mediated rapid Mg2+ uptake into proteoliposomes. The rapid Mg2+ uptake through AtMRS2-10 was inhibited by aluminum. An assay using the Al-sensitive dye morin indicated Al uptake into the proteoliposomes through AtMRS2-10. AtMRS2-10 also exhibited Ni2+ transport activity but almost no Co2+ transport activity. The rapid Mg2+ uptake through AtMRS2-1 was not inhibited by aluminum. Al uptake into the proteoliposomes through AtMRS2-1 was not observed. The functional complementation assay in Escherichia coli strain TM2 showed that AtMRS2-1 was capable of mediating Mg2+ uptake. Heterologous expression using the E. coli mutant cells also showed that the E. coli cells expressing AtMRS2-1 was more resistant to aluminum than the E. coli cells expressing AtMRS2-10. The results suggested that AtMRS2-10 transported Al into the E. coli cells, and then the transported Al inhibited the growth of E. coli. AtMRS2-1 has been localized to the Arabidopsis tonoplast, indicating that AtMRS2-1 is exposed to much higher concentration of aluminum than AtMRS2-10. Under the conditions, it may be required that the Mg2+ transport of AtMRS2-1 is insensitive to Al inhibition, and AtMRS2-1 is impermeable to Al.