Human mitochondrial ribosomes can switch structural tRNAs - but when and why?

Importance of conserved hydrophobic pocket region in yeast mitoribosomal mL44 protein for mitotranslation and transcript preference

The mitochondrial ribosome (mitoribosome) is responsible for the synthesis of key oxidative phosphorylation subunits encoded by the mitochondrial genome. Defects in mitoribosomal function therefore can have serious consequences for the bioenergetic capacity of the cell. Mutation of the conserved mitoribosomal mL44 protein has been directly linked to childhood cardiomyopathy and progressive neurophysiology issues. To further explore the functional significance of the mL44 protein in supporting mitochondrial protein synthesis, we have performed a mutagenesis study of the yeast mL44 homolog, the MrpL3/mL44 protein. We specifically investigated the conserved hydrophobic pocket region of the MrpL3/mL44 protein, where the known disease-related residue in the human mL44 protein (L156R) is located. While our findings identify a number of residues in this region critical for MrpL3/mL44's ability to support the assembly of translationally active mitoribosomes, the introduction of the disease-related mutation into the equivalent position in the yeast protein (residue A186) was found to not have a major impact on function. The human and yeast mL44 proteins share many similarities in sequence and structure; however results presented here indicate that these two proteins have diverged somewhat in evolution. Finally, we observed that mutation of the MrpL3/mL44 does not impact the translation of all mitochondrial encoded proteins equally, suggesting the mitochondrial translation system may exhibit a transcript hierarchy and prioritization.

Mutation of the PEBP-like domain of the mitoribosomal MrpL35/mL38 protein results in production of nascent chains with impaired capacity to assemble into OXPHOS complexes

Located in the central protuberance region of the mitoribosome and mitospecific mL38 proteins display homology to PEBP (Phosphatidylethanolamine Binding Protein) proteins, a diverse family of proteins reported to bind anionic substrates/ligands and implicated in cellular signaling and differentiation pathways. In this study, we have performed a mutational analysis of the yeast mitoribosomal protein MrpL35/mL38 and demonstrate that mutation of the PEBP-invariant ligand binding residues Asp(D)232 and Arg(R)288 impacted MrpL35/mL38's ability to support OXPHOS-based growth of the cell. Furthermore, our data indicate these residues exist in a functionally important charged microenvironment, which also includes Asp(D)167 of MrpL35/mL38 and Arg(R)127 of the neighboring Mrp7/bL27m protein. We report that mutation of each of these charged residues resulted in a strong reduction in OXPHOS complex levels that was not attributed to a corresponding inhibition of the mitochondrial translation process. Rather, our findings indicate that a disconnect exists in these mutants between the processes of mitochondrial protein translation and the events required to ensure the competency and/or availability of the newly synthesized proteins to assemble into OXPHOS enzymes. Based on our findings, we postulate that the PEBP-homology domain of MrpL35/mL38, together with its partner Mrp7/bL27m, form a key regulatory region of the mitoribosome.

Diversity and Evolution of Mitochondrial Translation Apparatus

The evolution of mitochondria has proceeded independently in different eukaryotic lines, which is reflected in the diversity of mitochondrial genomes and mechanisms of their expression in eukaryotic species. Mitochondria have lost most of bacterial ancestor genes by transferring them to the nucleus or eliminating them. However, mitochondria of almost all eukaryotic cells still retain relatively small genomes, as well as their replication, transcription, and translation apparatuses. The dependence on the nuclear genome, specific features of mitochondrial transcripts, and synthesis of highly hydrophobic membrane proteins in the mitochondria have led to significant changes in the translation apparatus inherited from the bacterial ancestor, which retained the basic structure necessary for protein synthesis but became more specialized and labile. In this review, we discuss specific properties of translation initiation in the mitochondria and how the evolution of mitochondria affected the functions of main factors initiating protein biosynthesis in these organelles.

Linear Density Sucrose Gradients to Study Mitoribosomal Biogenesis in Tissue-Specific Knockout Mice

Like bacterial and cytoplasmic ribosomes, mitoribosomes are large ribonucleoprotein complexes with molecular weights in the range of several million Daltons. Traditionally, studying the assembly of such high molecular weight complexes is done using ultracentrifugation through linear density gradients, which remains the method of choice due to its versatility and superior resolving power in the high molecular weight range. Here, we present a protocol for the analysis of mitoribosomal assembly in heart mitochondrial extracts using linear density sucrose gradients that we have previously employed to characterize the essential role of different mitochondrial proteins in mitoribosomal biogenesis. This protocol details in a stepwise manner a typical mitoribosomal assembly analysis starting with isolation of mitochondria, preparation and ultracentrifugation of the gradients, fractionation and ending with SDS-PAGE, and immunoblotting of the gradient fractions. Even though we provide an example with heart mitochondria, this protocol can be directly applied to virtually all mouse tissues, as well as cultured cells, with little to no modifications.

The Diseased Mitoribosome

Mitochondria control life and death in eukaryotic cells. Harboring a unique circular genome, a by-product of an ancient endosymbiotic event, mitochondria maintains a specialized and evolutionary divergent protein synthesis machinery, the mitoribosome. Mitoribosome biogenesis depends on elements encoded in both the mitochondrial genome (the RNA components) and the nuclear genome (all ribosomal proteins and assembly factors). Recent cryo-EM structures of mammalian mitoribosomes have illuminated their composition and provided hints regarding their assembly and elusive mitochondrial translation mechanisms. A growing body of literature involves the mitoribosome in inherited primary mitochondrial disorders. Mutations in genes encoding mitoribosomal RNAs, proteins, and assembly factors impede mitoribosome biogenesis, causing protein synthesis defects that lead to respiratory chain failure and mitochondrial disorders such as encephalo- and cardiomyopathy, deafness, neuropathy, and developmental delays. In this article, we review the current fundamental understanding of mitoribosome assembly and function, and the clinical landscape of mitochondrial disorders driven by mutations in mitoribosome components and assembly factors, to portray how basic and clinical studies combined help us better understand both mitochondrial biology and medicine.

Application of Cryo-EM for Visualization of Mitoribosomes

Mitochondrial ribosomes (mitoribosomes) are specialized machineries that carry out the synthesis of a limited number of proteins encoded in the mitochondrial genome, including components of the oxidative phosphorylation pathway. They have incorporated several structural features distinguishing them from bacterial and eukaryotic cytosolic counterparts. Our current understanding of the assembly and functioning of mitoribosomes is limited, and recent developments in cryo-EM provide promising directions for detailed investigation. Here we describe methods to purify mitoribosomes from human embryonic kidney cells for cryo-EM studies.

Accretion history of large ribosomal subunits deduced from theoretical minimal RNA rings is congruent with histories derived from phylogenetic and structural methods

Accretions of tRNAs presumably formed the large complex ribosomal RNA structures. Similarities of tRNA secondary structures with rRNA secondary structures increase with the integration order of their cognate amino acid in the genetic code, indicating tRNA evolution towards rRNA-like structures. Here analyses rank secondary structure subelements of three large ribosomal RNAs (Prokaryota: Archaea: Thermus thermophilus; Bacteria: Escherichia coli; Eukaryota: Saccharomyces cerevisiae) in relation to their similarities with secondary structures formed by presumed proto-tRNAs, represented by 25 theoretical minimal RNA rings. These ranks are compared to those derived from two independent methods (ranks provide a relative evolutionary age to the rRNA substructure), (a) cladistic phylogenetic analyses and (b) 3D-crystallography where core subelements are presumed ancient and peripheral ones recent. Comparisons of rRNA secondary structure subelements with RNA ring secondary structures show congruence between ranks deduced by this method and both (a) and (b) (more with (a) than (b)), especially for RNA rings with predicted ancient cognate amino acid. Reconstruction of accretion histories of large rRNAs will gain from adequately integrating information from independent methods. Theoretical minimal RNA rings, sequences deterministically designed in silico according to specific coding constraints, might produce adequate scales for prebiotic and early life molecular evolution.

METTL15 introduces N4-methylcytidine into human mitochondrial 12S rRNA and is required for mitoribosome biogenesis

Post-transcriptional RNA modifications, the epitranscriptome, play important roles in modulating the functions of RNA species. Modifications of rRNA are key for ribosome production and function. Identification and characterization of enzymes involved in epitranscriptome shaping is instrumental for the elucidation of the functional roles of specific RNA modifications. Ten modified sites have been thus far identified in the mammalian mitochondrial rRNA. Enzymes responsible for two of these modifications have not been characterized. Here, we identify METTL15, show that it is the main N4-methylcytidine (m⁴C) methyltransferase in human cells and demonstrate that it is responsible for the methylation of position C839 in mitochondrial 12S rRNA. We show that the lack of METTL15 results in a reduction of the mitochondrial de novo protein synthesis and decreased steady-state levels of protein components of the oxidative phosphorylation system. Without functional METTL15, the assembly of the mitochondrial ribosome is decreased, with the late assembly components being unable to be incorporated efficiently into the small subunit. We speculate that m⁴C839 is involved in the stabilization of 12S rRNA folding, therefore facilitating the assembly of the mitochondrial small ribosomal subunits. Taken together our data show that METTL15 is a novel protein necessary for efficient translation in human mitochondria.

Structural insights into dimethylation of 12S rRNA by TFB1M: indispensable role in translation of mitochondrial genes and mitochondrial function

Mitochondria are essential molecular machinery for the maintenance of cellular energy supply by the oxidative phosphorylation system (OXPHOS). Mitochondrial transcription factor B1 (TFB1M) is a dimethyltransferase that maintains mitochondrial homeostasis by catalyzing dimethylation of two adjacent adenines located in helix45 (h45) of 12S rRNA. This m⁶₂A modification is indispensable for the assembly and maturation of human mitochondrial ribosomes. However, both the mechanism of TFB1M catalysis and the precise function of TFB1M in mitochondrial homeostasis are unknown. Here we report the crystal structures of a ternary complex of human (hs) TFB1M–h45–S-adenosyl-methionine and a binary complex hsTFB1M–h45. The structures revealed a distinct mode of hsTFB1M interaction with its rRNA substrate and with the initial enzymatic state involved in m⁶₂A modification. The suppression of hsTFB1M protein level or the overexpression of inactive hsTFB1M mutants resulted in decreased ATP production and reduced expression of components of the mitochondrial OXPHOS without affecting transcription of the corresponding genes and their localization to the mitochondria. Therefore, hsTFB1M regulated the translation of mitochondrial genes rather than their transcription via m⁶₂A modification in h45.

The Uroboros Theory of Life's Origin: 22-Nucleotide Theoretical Minimal RNA Rings Reflect Evolution of Genetic Code and tRNA-rRNA Translation Machineries

Theoretical minimal RNA rings attempt to mimick life's primitive RNAs. At most 25 22-nucleotide-long RNA rings code once for each biotic amino acid, a start and a stop codon and form a stem-loop hairpin, resembling consensus tRNAs. We calculated, for each RNA ring's 22 potential splicing positions, similarities of predicted secondary structures with tRNA vs. rRNA secondary structures. Assuming rRNAs partly derived from tRNA accretions, we predict positive associations between relative secondary structure similarities with rRNAs over tRNAs and genetic code integration orders of RNA ring anticodon cognate amino acids. Analyses consider for each secondary structure all nucleotide triplets as potential anticodon. Anticodons for ancient, chemically inert cognate amino acids are most frequent in the 25 RNA rings. For RNA rings with primordial cognate amino acids according to tRNA-homology-derived anticodons, tRNA-homology and coding sequences coincide, these are separate for predicted cognate amino acids that presumably integrated late the genetic code. RNA ring secondary structure similarity with rRNA over tRNA secondary structures associates best with genetic code integration orders of anticodon cognate amino acids when assuming split anticodons (one and two nucleotides at the spliced RNA ring 5' and 3' extremities, respectively), and at predicted anticodon location in the spliced RNA ring's midst. Results confirm RNA ring homologies with tRNAs and CDs, ancestral status of tRNA half genes split at anticodons, the tRNA-rRNA axis of RNA evolution, and that single theoretical minimal RNA rings potentially produce near-complete proto-tRNA sets. Hence genetic code pre-existence determines 25 short circular gene- and tRNA-like RNAs. Accounting for each potential splicing position, each RNA ring potentially translates most amino acids, realistically mimicks evolution of the tRNA-rRNA translation machinery. These RNA rings 'of creation' remind the uroboros' (snake biting its tail) symbolism for creative regeneration.

Striking Diversity of Mitochondria-Specific Translation Processes across Eukaryotes

Mitochondria are essential organelles that act as energy conversion powerhouses and metabolic hubs. Their gene expression machineries combine traits inherited from prokaryote ancestors and specific features acquired during eukaryote evolution. Mitochondrial research has wide implications ranging from human health to agronomy. We highlight recent advances in mitochondrial translation. Functional, biochemical, and structural data have revealed an unexpected diversity of mitochondrial translation systems, particularly of their key players, the mitochondrial ribosomes (mitoribosomes). Ribosome assembly and translation mechanisms, such as initiation, are discussed and put in perspective with the prevalence of eukaryote-specific families of mitochondrial translation factors such as pentatricopeptide repeat (PPR) proteins.

Mammalian mitochondrial translation - revealing consequences of divergent evolution

Mitochondria are ubiquitous organelles present in the cytoplasm of all nucleated eukaryotic cells. These organelles are described as arising from a common ancestor but a comparison of numerous aspects of mitochondria between different organisms provides remarkable examples of divergent evolution. In humans, these organelles are of dual genetic origin, comprising ∼1500 nuclear-encoded proteins and thirteen that are encoded by the mitochondrial genome. Of the various functions that these organelles perform, it is only oxidative phosphorylation, which provides ATP as a source of chemical energy, that is dependent on synthesis of these thirteen mitochondrially encoded proteins. A prerequisite for this process of translation are the mitoribosomes. The recent revolution in cryo-electron microscopy has generated high-resolution mitoribosome structures and has undoubtedly revealed some of the most distinctive molecular aspects of the mitoribosomes from different organisms. However, we still lack a complete understanding of the mechanistic aspects of this process and many of the factors involved in post-transcriptional gene expression in mitochondria. This review reflects on the current knowledge and illustrates some of the striking differences that have been identified between mitochondria from a range of organisms.

Small is big in Arabidopsis mitochondrial ribosome

Mitochondria are responsible for energy production through aerobic respiration, and represent the powerhouse of eukaryotic cells. Their metabolism and gene expression processes combine bacterial-like features and traits that evolved in eukaryotes. Among mitochondrial gene expression processes, translation remains the most elusive. In plants, while numerous pentatricopeptide repeat (PPR) proteins are involved in all steps of gene expression, their function in mitochondrial translation remains unclear. Here we present the biochemical characterization of Arabidopsis mitochondrial ribosomes and identify their protein subunit composition. Complementary biochemical approaches identified 19 plant-specific mitoribosome proteins, of which ten are PPR proteins. The knockout mutations of ribosomal PPR (rPPR) genes result in distinct macroscopic phenotypes, including lethality and severe growth delay. The molecular analysis of rppr1 mutants using ribosome profiling, as well as the analysis of mitochondrial protein levels, demonstrate rPPR1 to be a generic translation factor that is a novel function for PPR proteins. Finally, single-particle cryo-electron microscopy (cryo-EM) reveals the unique structural architecture of Arabidopsis mitoribosomes, characterized by a very large small ribosomal subunit, larger than the large subunit, bearing an additional RNA domain grafted onto the head. Overall, our results show that Arabidopsis mitoribosomes are substantially divergent from bacterial and other eukaryote mitoribosomes, in terms of both structure and protein content.

Rapid Isolation of the Mitoribosome from HEK Cells

The human mitochondria possess a dedicated set of ribosomes (mitoribosomes) that translate 13 essential protein components of the oxidative phosphorylation complexes encoded by the mitochondrial genome. Since all proteins synthesized by human mitoribosomes are integral membrane proteins, human mitoribosomes are tethered to the mitochondrial inner membrane during translation. Compared to the cytosolic ribosome the mitoribosome has a sedimentation coefficient of 55S, half the rRNA content, no 5S rRNA and 36 additional proteins. Therefore, a higher protein-to-RNA ratio and an atypical structure make the human mitoribosome substantially distinct from its cytosolic counterpart. Despite the central importance of the mitoribosome to life, no protocols were available to purify the intact complex from human cell lines. Traditionally, mitoribosomes were isolated from mitochondria-rich animal tissues that required kilograms of starting material. We reasoned that mitochondria in dividing HEK293-derived human cells grown in nutrient-rich expression medium would have an active mitochondrial translation, and, therefore, could be a suitable source of material for the structural and biochemical studies of the mitoribosome. To investigate its structure, we developed a protocol for large-scale purification of intact mitoribosomes from HEK cells. Herein, we introduce nitrogen cavitation method as a faster, less labor-intensive and more efficient alternative to traditional mechanical shear-based methods for cell lysis. This resulted in preparations of the mitoribosome that allowed for its structural determination to high resolution, revealing the composition of the intact human mitoribosome and its assembly intermediates. Here, we follow up on this work and present an optimized and more cost-effective method requiring only ~10¹⁰ cultured HEK cells. The method can be employed to purify human mitoribosomal translating complexes, mutants, quality control assemblies and mitoribosomal subunits intermediates. The purification can be linearly scaled up tenfold if needed, and also applied to other types of cells.

Anti-replicative recombinant 5S rRNA molecules can modulate the mtDNA heteroplasmy in a glucose-dependent manner

Mutations in mitochondrial DNA are an important source of severe and incurable human diseases. The vast majority of these mutations are heteroplasmic, meaning that mutant and wild-type genomes are present simultaneously in the same cell. Only a very high proportion of mutant mitochondrial DNA (heteroplasmy level) leads to pathological consequences. We previously demonstrated that mitochondrial targeting of small RNAs designed to anneal with mutant mtDNA can decrease the heteroplasmy level by specific inhibition of mutant mtDNA replication, thus representing a potential therapy. We have also shown that 5S ribosomal RNA, partially imported into human mitochondria, can be used as a vector to deliver anti-replicative oligoribonucleotides into human mitochondria. So far, the efficiency of cellular expression of recombinant 5S rRNA molecules bearing therapeutic insertions remained very low. In the present study, we designed new versions of anti-replicative recombinant 5S rRNA targeting a large deletion in mitochondrial DNA which causes the KSS syndrome, analyzed their specific annealing to KSS mitochondrial DNA and demonstrated their import into mitochondria of cultured human cells. To obtain an increased level of the recombinant 5S rRNA stable expression, we created transmitochondrial cybrid cell line bearing a site for Flp-recombinase and used this system for the recombinase-mediated integration of genes coding for the anti-replicative recombinant 5S rRNAs into nuclear genome. We demonstrated that stable expression of anti-replicative 5S rRNA versions in human transmitochondrial cybrid cells can induce a shift in heteroplasmy level of KSS mutation in mtDNA. This shift was directly dependent on the level of the recombinant 5S rRNA expression and the sequence of the anti-replicative insertion. Quantification of mtDNA copy number in transfected cells revealed the absence of a non-specific effect on wild type mtDNA replication, indicating that the decreased proportion between mutant and wild type mtDNA molecules is not a consequence of a random repopulation of depleted pool of mtDNA genomes. The heteroplasmy change could be also modulated by cell growth conditions, namely increased by cells culturing in a carbohydrate-free medium, thus forcing them to use oxidative phosphorylation and providing a selective advantage for cells with improved respiration capacities. We discuss the advantages and limitations of this approach and propose further development of the anti-replicative strategy based on the RNA import into human mitochondria.

Mitochondrial Genome Engineering: The Revolution May Not Be CRISPR-Ized

In recent years mitochondrial DNA (mtDNA) has transitioned to greater prominence across diverse areas of biology and medicine. The recognition of mitochondria as a major biochemical hub, contributions of mitochondrial dysfunction to various diseases, and several high-profile attempts to prevent hereditary mtDNA disease through mitochondrial replacement therapy have roused interest in the organellar genome. Subsequently, attempts to manipulate mtDNA have been galvanized, although with few robust advances and much controversy. Re-engineered protein-only nucleases such as mtZFN and mitoTALEN function effectively in mammalian mitochondria, although efficient delivery of nucleic acids into the organelle remains elusive. Such an achievement, in concert with a mitochondria-adapted CRISPR/Cas9 platform, could prompt a revolution in mitochondrial genome engineering and biological understanding. However, the existence of an endogenous mechanism for nucleic acid import into mammalian mitochondria, a prerequisite for mitochondrial CRISPR/Cas9 gene editing, remains controversial.