Properties of Human Mitochondrial Ribosomes

Abstract

SummaryMammalian mitochondrial ribosomes (55S) differ unexpectedly from bacterial (70S) and cytoplasmic ribosomes (80S), as well as other kinds of mitochondrial ribosomes. Typical of mammalian mitochondrial ribosomes, the bovine mitochondrial ribosome has been developed as a model system for the study of human mitochondrial ribosomes, to address several questions related to the structure, function, biosynthesis and evolution of these interesting ribosomes. Bovine mitochondrial ribosomal proteins (MRPs) from each subunit have been identified and characterized with respect to individuality and electrophoretic properties, amino acid sequence, topographic disposition, RNA binding properties, evolutionary relationships and reaction with affinity probes of ribosomal functional domains. Several distinctive properties of these ribosomes are being elucidated, including their antibiotic susceptibility and composition. Human mitochondrial ribosomes lack several of the major RNA stem structures of bacterial ribosomes but they contain a correspondingly higher protein content (as many as 80 proteins), suggesting a model where proteins have replaced RNA structural elements during the evolution of these ribosomes. Despite their lower RNA content they are physically larger than bacterial ribosomes, because of the 'extra' proteins they contain. The extra proteins in mitochondrial ribosomes are 'new' in the sense that they are not homologous to proteins in bacterial or cytoplasmic ribosomes. Some of the new proteins appear to be bifunctional. All of the mammalian MRPs are encoded in nuclear genes (a separate set from those encoding cytoplasmic ribosomal proteins) which are evolving more rapidly than those encoding cytoplasmic ribosomal proteins. The MRPs are imported into mitochondria where they assemble coordinately with mitochondrially transcribed rRNAs into ribosomes that are responsible for translating the 13 mRNAs for essential proteins of the oxidative phosphorylation system. IUBMB Life, 55: 505-513, 2003

Proteomics and electron microscopic characterization of the unusual mitochondrial ribosome-related 45S complex in Leishmania tarentolae

“…Most of the identified small subunit ribosomal proteins were highly basic (pI > 9), as are their homologs in other sources, reflecting their interactions with the ribosomal RNA [32][33][34][35]. Mitochondrial ribosomal proteins, as a rule, are significantly longer than their eubacterial counterparts [31][32][33][34][35][36][37][38].…”

“…Mitochondrial ribosomal proteins, as a rule, are significantly longer than their eubacterial counterparts [31][32][33][34][35][36][37][38]. This difference is due to the presence of N-or C-terminal extensions which were proposed to serve structural and/or functional compensation for the missing stemloop structures of the ribosomal RNAs [36], although this idea was not supported by the cryo-EM analysis of bovine mitoribosomes [31].…”

Evolution of the protein stoichiometry in the L12 stalk of bacterial and organellar ribosomes

“…With the same computational approach as described above, we analysed the length of helix a8 in the mitochondrial homologue of L10, MRPL10, and in L10 from chloroplasts using all sequences that could be unambiguously assigned as organellar L10 in eukaryotic genomes. In general, mitochondrial ribosomes are quite different from bacterial ones, as they contain many more proteins and less rRNA 32 . The fact that the closest MRPL10 orthologue is L10 from alphaproteobacteria reflects the alphaproteobacterial origin of mitochondria 33 .…”

Terminating human mitochondrial protein synthesis: A shift in our thinking

“…Although the 55S and bacterial 70S particles have a similar mass, those of the mammalian organelle are larger and have a more porous structure. 29 Further, on isolation the mammalian mitoribosome large subunit has a strong tendency to retain P-site tRNA, which is in contrast to E-site tRNA that co-purifies with bacterial ribosomes. This together with the fact that the mammalian 16S mt-rRNA has lost regions corresponding to the rRNA of the bacterial 70S ribosome that would contact the E-site tRNA, has led to the suggestion that these mitoribosomes lack an E-site (reviewed in ref.…”

“…Despite more than two decades of frustration, the field still lacks reliable and widely accepted methods of organellar transfection in intact cells and in vitro translation systems that would render us capable of asking more groups have documented just how significantly different this particle is, not only when compared to the bacterial 70S and the eukaryotic 80S ribosome but also in comparison to other mitochondrial ribosomes. [26][27][28][29] The normal ratio of ∼70% RNA to 30% protein is reversed, and this increased protein content affects the buoyant density and sedimentation value generating a 55S mitoribosome. Although the 55S and bacterial 70S particles have a similar mass, those of the mammalian organelle are larger and have a more porous structure.…”