Identification of 12 new yeast mitochondrial ribosomal proteins including 6 that have no prokaryotic homologues

Mitochondrial ribosomal proteins were studied best in yeast, where the smal l subunit was shown to contain about 35 proteins. Yet, genetic and biochemi cal studies identified only 14 proteins, half of which were predictable by sequence homology with prokaryotic ribosomal components of the small subuni t. Using a recently described affinity purification technique and tagged ve rsions of yeast Ykl155c and Mrp1, we isolated this mitochondrial ribosomal subunit and identified a total of 20 proteins, of which 12 are new. For a s ubset of the newly described ribosomal proteins, we showed that they are lo calized in mitochondria and are required for the respiratory competency of the yeast cells. This brings to 26 the total number of proteins described a s components of the mitochondrial small ribosomal subunit. Remarkably, almo st half of the previously and newly identified mitochondrial ribosomal comp onents showed no similarity to any known ribosomal protein. Homologues coul d be found, however, in predicted protein sequences from Schizosaccharomyce s pombe. In more distant species, putative homologues were detected for Ykl 155c, which shares conserved motifs with uncharacterized proteins of higher eukaryotes including humans. Another newly identified ribosomal protein, Y gl129c, was previously shown to be a member of the DAP-3 family of mitochon drial apoptosis mediators.