text: The MNR2 gene encodes a protein closely related to the Alr proteins, but includes conserved features that define a distinct subgroup of CorA proteins in fungal genomes, suggesting a distinct role in Mg homeostasis. Like an alr1 mutant, growth of an mnr2 mutant was sensitive to Mg-deficient conditions, but the mnr2 mutant was observed to accumulate more Mg than a wild-type strain under these conditions. These phenotypes suggested that Mnr2 may regulate Mg storage within an intracellular compartment. Consistent with this interpretation, the Mnr2 protein was localized to the membrane of the vacuole, an internal compartment implicated in the storage of excess mineral nutrients by yeast. A direct role of Mnr2 in Mg transport was suggested by the observation that increased Mnr2 expression, which redirected some Mnr2 protein to the cell surface, also suppressed the Mg-requirement of an alr1 alr2 double mutant strain. The mnr2 mutation also altered accumulation of other divalent cations, suggesting this mutation may increase Alr gene expression or protein activity. Recent work  supported this model, by showing that Alr1 activity was increased in an mnr2 mutant strain, and that the mutation was associated with induction of Alr1 activity at a higher external Mg concentration than was observed for an Mnr2 wild-type strain. These effects were observed without any change in Alr1 protein accumulation, again indicating that Alr1 activity may be regulated directly by the Mg concentration within the cell.