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

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Abstract
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 homo‐ oligomers. Taken together, these findings characterize Mrs2p as the first molecularly identified metal ion channel protein in the inner mitochondrial membrane.