The small subunit of the mammalian mitochondrial ribosome - Identificationof the full complement of ribosomal proteins present

Identification of all the protein components of the small subunit (28 S) of the mammalian mitochondrial ribosome has been achieved by carrying out pro teolytic digestions of whole 28 S subunits followed by analysis of the resu ltant peptides by liquid chromatography and tandem mass spectrometry (LC/MS /MS). Peptide sequence information was used to search the human EST data ba ses and complete coding sequences of the proteins were assembled. The human mitochondrial ribosome has 29 distinct proteins in the small subunit, Four teen of this group of proteins are homologs of the Escherichia coli 30 S ri bosomal proteins S2, S5, S6, S7, S9, S10, S11, S12, S14, S15, S16, S17, S18 , and S21, All of these proteins have homologs in Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae mitochondrial ribosome s, Surprisingly, three variants of ribosomal protein S18 are found in the m ammalian and D. melanogaster mitochondrial ribosomes while C. elegans has t wo S18 homologs. The S18 homologs tend to be more closely related to chloro plast S18s than to prokaryotic S18s. No mitochondrial homologs to prokaryot ic ribosomal proteins S1, S3, S4, S8, S13, S19, and S20 could be found in t he peptides obtained from the whole 28 S subunit digests or by analysis of the available data bases. The remaining 15 proteins present in mammalian mi tochondrial 28 S subunits (MRP-S22 through MRP-S36) are specific to mitocho ndrial ribosomes, Proteins in this group have no apparent homologs in bacte rial, chloroplast, archaebacterial, or cytosolic ribosomes, All but two of these proteins have a clear homolog in D. melanogaster while all but three can be found in the genome of C. elegans. Five of the mitochondrial. specif ic ribosomal proteins have homologs in S. cerevisiae.