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 d escribed respiration-deficient mrs2-1 mutation when present on a multi copy plasmid, Elevated gene dosage of this new gene, termed MRS5, supp resses also the pet phenotype of a mitochondrial splicing-deficient gr oup II intron mutation M1301. The MRS5 gene product, a 13-kDa protein of low abundance, shows no similarity to other known proteins and is a ssociated with the inner mitochondrial membrane, protruding into the i ntermembrane space, MRS5 codes for an essential protein, as the disrup tion of this gene is lethal even during growth on fermentable carbon s ources, Thus, the Mrs5 protein seems to be involved in mitochondrial k ey functions aside from oxidative energy conservation, which is dispen sable in fermenting yeast cells, Depletion of MRs5p in yeast cells cau ses accumulation of unprocessed precursors of the mitochondrial hsp60 protein and defects in all cytochrome complexes, These findings sugges t an essential role of Mrs5p in mitochondrial biogenesis.