A soluble 12-kDa protein of the mitochondrial intermembrane space, Mrs11p, is essential for mitochondrial biogenesis and viability of yeast cells.

We have isolated an essential yeast gene termed MRS11, which codes for a soluble protein of the mitochondrial intermembrane space. Interestingly, this new gene shares many similarities with the previously characterized MRS5 gene: when expressed from a multicopy plasmid, MRS11 like MRS5 restores respiration competence to yeast strains defective in the splicing of mitochondrial group II introns. Both genes are essential for viability of yeast cells, as the disruption of either of them is lethal. The proteins encoded by MRS5 and MRS11, which display 35%, sequence identity are both located in the mitochondrial intermembrane space. Depletion of Mrs11p results in a phenotype similar to that observed in Mrs5p-depleted cells: accumulation of the precursor form of mitochondrial hsp60, inability to form spectrophotometrically detectable amounts of cytochromes and changes in the mitochondrial morphology. Although similar in sequence and function, Mrs5p and Mrs11p are not functionally equivalent and neither can substitute for the other, even when overexpressed. Taken together, our data suggest a cooperative mode of action of Mrs11p and Mrs5p in mitochondrial protein import or other related essential mitochondrial processes.