The large subunit of the mammalian mitochondrial ribosome - Analysis of the complement of ribosomal proteins present

Identification of all the protein components of the large subunit (39 S) of the mammalian mitochondrial ribosome has been achieved by carrying out pro teolytic digestions of whole 39 S subunits followed by analysis of the resu ltant peptides by liquid chromatography and mass spectrometry. Peptide sequ ence information was used to search the human EST data bases and complete c oding sequences were assembled. The human mitochondrial 39 S subunit has 48 distinct proteins. Twenty eight of these are homologs of the Escherichia c oli 50 S ribosomal proteins L1, L2, L3, L4, L7/L12, L9, L10, L11, L13, L14, L15, L16, L17, L18, L19, L20, L21, L22, L23, L24, L27, L28, L30, L32, L33, L34, L35, and L36. Almost all of these proteins have homologs in Drosophil a melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae mitoch ondrial ribosomes. No mitochondrial homologs to prokaryotic ribosomal prote ins L5, L6, L25, L29, and L31 could be found either in the peptides obtaine d or by analysis of the available data bases. The remaining 20 proteins pre sent in the 39 S subunits are specific to mitochondrial ribosomes. Proteins in this group have no apparent homologs in bacterial, chloroplast, archaeb acterial, or cytosolic ribosomes. All but two of the proteins has a clear h omolog in D. melanogaster while all can be found in the genome of C. elegan s. Ten of the 20 mitochondrial specific 39 S proteins have homologs in S. c erevisiae. Homologs of 2 of these new classes of ribosomal proteins could b e identified in the Arabidopsis thaliana genome.