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Mammalian Mitochondrial Ribosomal Proteins: Revisited

Mitochondria are responsible for providing 90% of the energy in eukaryotic organisms by oxidative phosphorylation (OXPHOS). Mitochondrially encoded components of OXPHOS complexes are all essential and synthesized by a specialized translation system containing 55S ribosomes in mitochondria. Mammalian 55S ribosomes are composed of ~80 nuclear encoded mitochondrial ribosomal proteins (MRPs) and about half of these proteins have distinct homologs in ribosomes from other species. In the initial identification of MRPs, however, it was difficult to assign MRPs with no bacterial homolog unambiguously to the mitochondrial ribosome. Further, there were concerns that one or more ribosomal proteins were not identified due to the limited availability of ESTs from different organisms. With the improvement in genome sequencing and sensitivity of mass spectrometry (MS)‐based technologies, we have identified four additional MRPs; coiled‐coil‐helix‐coiled‐coil‐helix domain containing protein 1 (CHCH1), pentatricopeptide repeat‐containing protein 3 (PTCD3), immature colon carcinoma transcript 1 protein (ICT1), and growth arrest and DNA‐damage‐inducible proteins‐interacting protein 1 (CRIF‐1) as new mitochondrial ribosomal proteins present in highly purified 55S ribosomes. The association of PTCD3 and ICT1 with mitochondrial ribosome was demonstrated previously. Here, we report CHCH1 and CRIF1 as bona fide MRPs and their subunit assignments by a combination of MS‐based proteomics and immunoblotting analysis.