Mrs5p, an essential protein of the mitochondrial intermembrane space, affects protein import into yeast mitochondria.

We have isolated a yeast nuclear gene that suppresses the previously described respiration-deficient mrs2-1 mutation when present on a multicopy plasmid. Elevated gene dosage of this new gene, termed MRS5, suppresses also the pet phenotype of a mitochondrial splicing-deficient group II intron mutation M1301. The MRS5 gene product, a 13-kDa protein of low abundance, shows no similarity to other known proteins and is associated with the inner mitochondrial membrane, protruding into the intermembrane space. MRS5 codes for an essential protein, as the disruption of this gene is lethal even during growth on fermentable carbon sources. Thus, the Mrs5 protein seems to be involved in mitochondrial key functions aside from oxidative energy conservation, which is dispensable in fermenting yeast cells. Depletion of Mrs5p in yeast cells causes accumulation of unprocessed precursors of the mitochondrial hsp60 protein and defects in all cytochrome complexes. These findings suggest an essential role of Mrs5p in mitochondrial biogenesis.