Identification of four proteins from the small subunit of the mammalian mitochondrial ribosome using a proteomics approach

Proteins in the small subunit of the mammalian mitochondrial ribosome were separated by two-dimensional polyacrylamide gel electrophoresis. Four indiv idual proteins were subjected to in-gel Endoprotease Lys-C digestion. The s equences of selected proteolytic peptides were obtained by electrospray tan dem mass spectrometry. Peptide sequences obtained from in-gel digestion of individual spots were used to screen human, mouse, and rat expressed sequen ce tag databases, and complete consensus cDNAs for these species were deduc ed in silico. The corresponding protein sequences were characterized by com parison to known ribosomal proteins in protein databases. Four different cl asses of mammalian mitochondrial small subunit ribosomal proteins were iden tified. Only two of these proteins have significant sequence similarities t o ribosomal proteins from prokaryotes, These proteins are homologs to Esche richia coli S9 and S5 proteins. The presence of these newly identified mito chondrial ribosomal proteins are also investigated in the Drosophila melano gaster, Caenorhabdtitis elegans, and in the genomes of several fungi.