The mitochondrial outer membrane protein Mas22p is essential for protein import and viability of yeast

We have cloned the gene encoding the protein Mas22p, which spans the outer membrane of yeast mitochondria. Cells that completely lack Mas22p are inviable. The plasmid-borne MAS22 gene suppresses several defects resulting from the deletion of one or more of the mitochondrial protein import receptors. Defects of Mas20p-deficient cells are explained by the reduced level of Mas22p in these mutants. Mas22p has one acidic domain in the cytosol and a second acidic domain in the mitochondrial intermembrane space. We suggest that these domains of Mas22p on either side of the outer membrane function as a relay system for transferring the basic targeting sequences of precursor proteins into the mitochondria.