Conserved mechanism for vacuolar magnesium sequestration in yeast and plant cells.

Magnesium (Mg 2+ ) is an essential nutrient for all life forms. In fungal and plant cells, the majority of Mg 2+ is stored in the vacuole but mechanisms for Mg 2+ transport into the vacuolar store are not fully understood. Here we demonstrate that members of ancient conserved domain proteins (ACDPs) from Saccharomyces cerevisiae and Arabidopsis thaliana function in vacuolar Mg 2+ sequestration that enables plant and yeast cells to cope with high levels of external Mg 2+ . We show that the yeast genome (as well as other fungal genomes) harbour a single ACDP homologue, referred to as MAM3, that functions specifically in vacuolar Mg 2+ accumulation and is essential for tolerance to high Mg. In parallel, vacuolar ACDP homologues were identified from Arabidopsis and shown to complement the yeast mutant mam3Δ. An Arabidopsis mutant lacking one of the vacuolar ACDP homologues displayed hypersensitivity to high-Mg conditions and accumulated less Mg in the vacuole compared with the wild type. Taken together, our results suggest that conserved transporters mediate vacuolar Mg 2+ sequestration in fungal and plant cells to maintain cellular Mg 2+ homeostasis in response to fluctuating Mg 2+ levels in the environment.