Mrs2p is an essential component of the major electrophoretic Mg²⁺ influx system in mitochondria

Steady-state concentrations of mitochondrial Mg²⁺ previously have been shown to vary with the expression of Mrs2p, a component of the inner mitochondrial membrane with two transmembrane domains. While its structural and functional similarity to the bacterial Mg²⁺ transport protein CorA suggested a role for Mrs2p in Mg²⁺ influx into the organelle, other functions in cation homeostasis could not be excluded. Making use of the fluorescent dye mag-fura 2 to measure free Mg²⁺ concentrations continuously, we describe here a high capacity, rapid Mg²⁺ influx system in isolated yeast mitochondria, driven by the mitochondrial membrane potential ΔΨ and inhibited by cobalt(III)hexaammine. Overexpression of Mrs2p increases influx rates 5-fold, while the deletion of the MRS2 gene abolishes this high capacity Mg²⁺ influx. Mg²⁺ efflux from isolated mitochondria, observed with low ΔΨ only, also requires the presence of Mrs2p. Cross-linking experiments revealed the presence of Mrs2p-containing complexes in the mitochondrial membrane, probably constituting Mrs2p homooligomers. Taken together, these findings characterize Mrs2p as the first molecularly identified metal ion channel protein in the inner mitochondrial membrane.