Structural insights into unique features of the human mitochondrial ribosome recycling

Translation termination in human mitochondria - substrate specificity of mitochondrial release factors

Mitochondria are the essential players in eukaryotic ATP production by oxidative phosphorylation, which relies on the maintenance and accurate expression of the mitochondrial genome. Even though the basic principles of translation are conserved due to the descendance from a bacterial ancestor, some deviations regarding translation factors as well as mRNA characteristics and the applied genetic code are present in human mitochondria. Together, these features are certain challenges during translation the mitochondrion has to handle. Here, we discuss the current knowledge regarding mitochondrial translation focusing on the termination process and the associated quality control mechanisms. We describe how mtRF1a resembles bacterial RF1 mechanistically and summarize in vitro and recent in vivo data leading to the conclusion of mtRF1a being the major mitochondrial release factor. On the other hand, we discuss the ongoing debate about the function of the second codon-dependent mitochondrial release factor mtRF1 regarding its role as a specialized termination factor. Finally, we link defects in mitochondrial translation termination to the activation of mitochondrial rescue mechanisms highlighting the importance of ribosome-associated quality control for sufficient respiratory function and therefore for human health.

Translation in Mitochondrial Ribosomes

Mitochondrial protein synthesis is essential for the life of aerobic eukaryotes. Without it, oxidative phosphorylation cannot be coupled. Evolution has shaped a battery of factors and machinery that are key to production of just a handful of critical proteins. In this general concept chapter, we attempt to briefly summarize our current knowledge of the overall process in mitochondria from a variety of species, breaking this down to the four parts of translation: initiation, elongation, termination, and recycling. Where appropriate, we highlight differences between species and emphasize gaps in our understanding. Excitingly, with the current revolution in cryoelectron microscopy and mitochondrial genome editing, it is highly likely that many of these gaps will be resolved in the near future. However, the absence of a faithful in vitro reconstituted system to study mitochondrial translation is still problematic.

Evolution: Mitochondrial Ribosomes Across Species

The ribosome is among the most complex and ancient cellular macromolecular assemblies that plays a central role in protein biosynthesis in all living cells. Its function of translation of genetic information encoded in messenger RNA into protein molecules also extends to subcellular compartments in eukaryotic cells such as apicoplasts, chloroplasts, and mitochondria. The origin of mitochondria is primarily attributed to an early endosymbiotic event between an alpha-proteobacterium and a primitive (archaeal) eukaryotic cell. The timeline of mitochondrial acquisition, the nature of the host, and their diversification have been studied in great detail and are continually being revised as more genomic and structural data emerge. Recent advancements in high-resolution cryo-EM structure determination have provided architectural details of mitochondrial ribosomes (mitoribosomes) from various species, revealing unprecedented diversifications among them. These structures provide novel insights into the evolution of mitoribosomal structure and function. Here, we present a brief overview of the existing mitoribosomal structures in the context of the eukaryotic evolution tree showing their diversification from their last common ancestor.

R, de Oliveira R, Dunham C, Whitford P. Ratchet, swivel, tilt and roll: a complete description of subunit rotation in the ribosome

Protein synthesis by the ribosome requires large-scale rearrangements of the 'small' subunit (SSU; ∼1 MDa), including inter- and intra-subunit rotational motions. However, with nearly 2000 structures of ribosomes and ribosomal subunits now publicly available, it is exceedingly difficult to design experiments based on analysis of all known rotation states. To overcome this, we developed an approach where the orientation of each SSU head and body is described in terms of three angular coordinates (rotation, tilt and tilt direction) and a single translation. By considering the entire RCSB PDB database, we describe 1208 fully-assembled ribosome complexes and 334 isolated small subunits, which span >50 species. This reveals aspects of subunit rearrangements that are universal, and others that are organism/domain-specific. For example, we show that tilt-like rearrangements of the SSU body (i.e. 'rolling') are pervasive in both prokaryotic and eukaryotic (cytosolic and mitochondrial) ribosomes. As another example, domain orientations associated with frameshifting in bacteria are similar to those found in eukaryotic ribosomes. Together, this study establishes a common foundation with which structural, simulation, single-molecule and biochemical efforts can more precisely interrogate the dynamics of this prototypical molecular machine.

Mechanisms of ribosome recycling in bacteria and mitochondria: a structural perspective

In all living cells, the ribosome translates the genetic information carried by messenger RNAs (mRNAs) into proteins. The process of ribosome recycling, a key step during protein synthesis that ensures ribosomal subunits remain available for new rounds of translation, has been largely overlooked. Despite being essential to the survival of the cell, several mechanistic aspects of ribosome recycling remain unclear. In eubacteria and mitochondria, recycling of the ribosome into subunits requires the concerted action of the ribosome recycling factor (RRF) and elongation factor G (EF-G). Recently, the conserved protein HflX was identified in bacteria as an alternative factor that recycles the ribosome under stress growth conditions. The homologue of HflX, the GTP-binding protein 6 (GTPBP6), has a dual role in mitochondrial translation by facilitating ribosome recycling and biogenesis. In this review, mechanisms of ribosome recycling in eubacteria and mitochondria are described based on structural studies of ribosome complexes.

Widespread use of unconventional targeting signals in mitochondrial ribosome proteins

Mitochondrial ribosomes are complex molecular machines indispensable for respiration. Their assembly involves the import of several dozens of mitochondrial ribosomal proteins (MRPs), encoded in the nuclear genome, into the mitochondrial matrix. Proteomic and structural data as well as computational predictions indicate that up to 25% of yeast MRPs do not have a conventional N-terminal mitochondrial targeting signal (MTS). We experimentally characterized a set of 15 yeast MRPs in vivo and found that five use internal MTSs. Further analysis of a conserved model MRP, Mrp17/bS6m, revealed the identity of the internal targeting signal. Similar to conventional MTS-containing proteins, the internal sequence mediates binding to TOM complexes. The entire sequence of Mrp17 contains positive charges mediating translocation. The fact that these sequence properties could not be reliably predicted by standard methods shows that mitochondrial protein targeting is more versatile than expected. We hypothesize that structural constraints imposed by ribosome assembly interfaces may have disfavored N-terminal presequences and driven the evolution of internal targeting signals in MRPs.

The long non-coding RNA SAMMSON is essential for uveal melanoma cell survival

Long non-coding RNAs (lncRNAs) can exhibit cell-type and cancer-type specific expression profiles, making them highly attractive as therapeutic targets. Pan-cancer RNA sequencing data revealed broad expression of the SAMMSON lncRNA in uveal melanoma (UM), the most common primary intraocular malignancy in adults. Currently, there are no effective treatments for UM patients with metastatic disease, resulting in a median survival time of 6-12 months. We aimed to investigate the therapeutic potential of SAMMSON inhibition in UM. Antisense oligonucleotide (ASO)-mediated SAMMSON inhibition impaired the growth and viability of a genetically diverse panel of uveal melanoma cell lines. These effects were accompanied by an induction of apoptosis and were recapitulated in two uveal melanoma patient derived xenograft (PDX) models through subcutaneous ASO delivery. SAMMSON pulldown revealed several candidate interaction partners, including various proteins involved in mitochondrial translation. Consequently, inhibition of SAMMSON impaired global, mitochondrial and cytosolic protein translation levels and mitochondrial function in uveal melanoma cells. The present study demonstrates that SAMMSON expression is essential for uveal melanoma cell survival. ASO-mediated silencing of SAMMSON may provide an effective treatment strategy to treat primary and metastatic uveal melanoma patients.

Maintaining mitochondrial ribosome function: The role of ribosome rescue and recycling factors

The universally conserved process of protein biosynthesis is crucial for maintaining cellular homoeostasis and in eukaryotes, mitochondrial translation is essential for aerobic energy production. Mitochondrial ribosomes (mitoribosomes) are highly specialized to synthesize 13 core subunits of the oxidative phosphorylation (OXPHOS) complexes. Although the mitochondrial translation machinery traces its origin from a bacterial ancestor, it has acquired substantial differences within this endosymbiotic environment. The cycle of mitoribosome function proceeds through the conserved canonical steps of initiation, elongation, termination and mitoribosome recycling. However, when mitoribosomes operate in the context of limited translation factors or on aberrant mRNAs, they can become stalled and activation of rescue mechanisms is required. This review summarizes recent advances in the understanding of protein biosynthesis in mitochondria, focusing especially on the mechanistic and physiological details of translation termination, and mitoribosome recycling and rescue.

Mitochondrial Proteostasis Requires Genes Encoded in a Neurodevelopmental Syndrome Locus

Eukaryotic cells maintain proteostasis through mechanisms that require cytoplasmic and mitochondrial translation. Genetic defects affecting cytoplasmic translation perturb synapse development, neurotransmission, and are causative of neurodevelopmental disorders, such as Fragile X syndrome. In contrast, there is little indication that mitochondrial proteostasis, either in the form of mitochondrial protein translation and/or degradation, is required for synapse development and function. Here we focus on two genes deleted in a recurrent copy number variation causing neurodevelopmental disorders, the 22q11.2 microdeletion syndrome. We demonstrate that SLC25A1 and MRPL40, two genes present in the microdeleted segment and whose products localize to mitochondria, interact and are necessary for mitochondrial ribosomal integrity and proteostasis. Our Drosophila studies show that mitochondrial ribosome function is necessary for synapse neurodevelopment, function, and behavior. We propose that mitochondrial proteostasis perturbations, either by genetic or environmental factors, are a pathogenic mechanism for neurodevelopmental disorders.SIGNIFICANCE STATEMENT The balance between cytoplasmic protein synthesis and degradation, or cytoplasmic proteostasis, is required for normal synapse function and neurodevelopment. Cytoplasmic and mitochondrial ribosomes are necessary for two compartmentalized, yet interdependent, forms of proteostasis. Proteostasis dependent on cytoplasmic ribosomes is a well-established target of genetic defects that cause neurodevelopmental disorders, such as autism. Here we show that the mitochondrial ribosome is a neurodevelopmentally regulated organelle whose function is required for synapse development and function. We propose that defective mitochondrial proteostasis is a mechanism with the potential to contribute to neurodevelopmental disease.

A distinct assembly pathway of the human 39S late pre-mitoribosome

Assembly of the mitoribosome is largely enigmatic and involves numerous assembly factors. Little is known about their function and the architectural transitions of the pre-ribosomal intermediates. Here, we solve cryo-EM structures of the human 39S large subunit pre-ribosomes, representing five distinct late states. Besides the MALSU1 complex used as bait for affinity purification, we identify several assembly factors, including the DDX28 helicase, MRM3, GTPBP10 and the NSUN4-mTERF4 complex, all of which keep the 16S rRNA in immature conformations. The late transitions mainly involve rRNA domains IV and V, which form the central protuberance, the intersubunit side and the peptidyltransferase center of the 39S subunit. Unexpectedly, we find deacylated tRNA in the ribosomal E-site, suggesting a role in 39S assembly. Taken together, our study provides an architectural inventory of the distinct late assembly phase of the human 39S mitoribosome.

Mitochondrial Protein Translation: Emerging Roles and Clinical Significance in Disease

Mitochondria are one of the most important organelles in cells. Mitochondria are semi-autonomous organelles with their own genetic system, and can independently replicate, transcribe, and translate mitochondrial DNA. Translation initiation, elongation, termination, and recycling of the ribosome are four stages in the process of mitochondrial protein translation. In this process, mitochondrial protein translation factors and translation activators, mitochondrial RNA, and other regulatory factors regulate mitochondrial protein translation. Mitochondrial protein translation abnormalities are associated with a variety of diseases, including cancer, cardiovascular diseases, and nervous system diseases. Mutation or deletion of various mitochondrial protein translation factors and translation activators leads to abnormal mitochondrial protein translation. Mitochondrial tRNAs and mitochondrial ribosomal proteins are essential players during translation and mutations in genes encoding them represent a large fraction of mitochondrial diseases. Moreover, there is crosstalk between mitochondrial protein translation and cytoplasmic translation, and the imbalance between mitochondrial protein translation and cytoplasmic translation can affect some physiological and pathological processes. This review summarizes the regulation of mitochondrial protein translation factors, mitochondrial ribosomal proteins, mitochondrial tRNAs, and mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) in the mitochondrial protein translation process and its relationship with diseases. The regulation of mitochondrial protein translation and cytoplasmic translation in multiple diseases is also summarized.

Structural basis of GTPase-mediated mitochondrial ribosome biogenesis and recycling

Ribosome biogenesis requires auxiliary factors to promote folding and assembly of ribosomal proteins and RNA. Particularly, maturation of the peptidyl transferase center (PTC) is mediated by conserved GTPases, but the molecular basis is poorly understood. Here, we define the mechanism of GTPase-driven maturation of the human mitochondrial large ribosomal subunit (mtLSU) using endogenous complex purification, in vitro reconstitution and cryo-EM. Structures of transient native mtLSU assembly intermediates that accumulate in GTPBP6-deficient cells reveal how the biogenesis factors GTPBP5, MTERF4 and NSUN4 facilitate PTC folding. Addition of recombinant GTPBP6 reconstitutes late mtLSU biogenesis in vitro and shows that GTPBP6 triggers a molecular switch and progression to a near-mature PTC state. Additionally, cryo-EM analysis of GTPBP6-treated mature mitochondrial ribosomes reveals the structural basis for the dual-role of GTPBP6 in ribosome biogenesis and recycling. Together, these results provide a framework for understanding step-wise PTC folding as a critical conserved quality control checkpoint.

Distinct mechanisms of the human mitoribosome recycling and antibiotic resistance

Ribosomes are recycled for a new round of translation initiation by dissociation of ribosomal subunits, messenger RNA and transfer RNA from their translational post-termination complex. Here we present cryo-EM structures of the human 55S mitochondrial ribosome (mitoribosome) and the mitoribosomal large 39S subunit in complex with mitoribosome recycling factor (RRF_mt) and a recycling-specific homolog of elongation factor G (EF-G2_mt). These structures clarify an unusual role of a mitochondria-specific segment of RRF_mt, identify the structural distinctions that confer functional specificity to EF-G2_mt, and show that the deacylated tRNA remains with the dissociated 39S subunit, suggesting a distinct sequence of events in mitoribosome recycling. Furthermore, biochemical and structural analyses reveal that the molecular mechanism of antibiotic fusidic acid resistance for EF-G2_mt is markedly different from that of mitochondrial elongation factor EF-G1_mt, suggesting that the two human EF-G_mts have evolved diversely to negate the effect of a bacterial antibiotic.

High-resolution cryo-EM structures and biochemical analyses of the human mitoribosome, in complex with mitochondria-specific factors mediating mitoribosome recycling, RRFmt and EF-G2mt, offer insight into mechanisms of mitoribosome recycling and resistance to antibiotic fusidic acid.

Mitochondrial ribosomal small subunit proteins (MRPS) MRPS6 and MRPS23 show dysregulation in breast cancer affecting tumorigenic cellular processes

Human Mitoribosomal Small Subunit unit (MRPS) family of genes appears to have role in cancer. Gene expression analysis of select MRPS genes (n = 9) in 15 cancer cell lines showed altered expression in cancer cells. Protein levels of MRPS6, MRPS23 showed significant overexpression in breast cancer cells and tissues. Interestingly, their overexpression did not correlate with mitochondrial ribosome translated COX2 protein levels in breast cancer. Subcellular fractionation analysis showed a distinct presence of MRPS23 in the nuclear fraction. GST/MRP6 and GST/MRPS23 pulldown assays identified 32 novel protein-protein interactions (PPIs) and MRPS23-RIPK3 interaction was validated. Co-expression module identification tool (CEMi) analysis of breast cancer gene expression and MRPS6 and MRPS23 interactions revealed hub interactions in gene expression modules having functional roles in cancer-associated cellular processes. Based on PPI network analysis a novel interaction MRPS23-p53 was validated. Knockdown of MRPS6 and MRPS23 decreased proliferation, expression of select mesenchymal markers, oncogenes, and increased expression of tumor suppressor genes. Taken together present study has revealed that MRPS6 and MRPS23 genes have pro-tumorigenic functions in breast cancer.

Structural basis of translation termination, rescue, and recycling in mammalian mitochondria

The mitochondrial translation system originates from a bacterial ancestor but has substantially diverged in the course of evolution. Here, we use single-particle cryo-electron microscopy (cryo-EM) as a screening tool to identify mitochondrial translation termination mechanisms and to describe them in molecular detail. We show how mitochondrial release factor 1a releases the nascent chain from the ribosome when it encounters the canonical stop codons UAA and UAG. Furthermore, we define how the peptidyl-tRNA hydrolase ICT1 acts as a rescue factor on mitoribosomes that have stalled on truncated messages to recover them for protein synthesis. Finally, we present structural models detailing the process of mitochondrial ribosome recycling to explain how a dedicated elongation factor, mitochondrial EFG2 (mtEFG2), has specialized for cooperation with the mitochondrial ribosome recycling factor to dissociate the mitoribosomal subunits at the end of the translation process.

The Mitochondrial Ribosome: A World of Opportunities for Mitochondrial Dysfunction Toward Parkinson's Disease

Significance: Mitochondrial ribosomes (mitoribosomes) are organelles that translate mitochondrial messenger RNA in the matrix and, in mammals, have evolved to translate 13 polypeptides of the pathway that performs oxidative phosphorylation (OXPHOS). Although a number of devastating diseases result from defects in this mitochondrial translation apparatus, most are associated with genetic mutations and little is known about allelopathic defects caused by antibiotics, toxins, or nonproteinogenic amino acids. Recent Advances: The levels of mitochondrial ribosomal subunits 12S and 16S ribosomal RNA (rRNA) in cells/tissues from patients carrying mutations in these genes have been associated with alterations in mitochondrial translation efficiency and with impaired OXPHOS activities, as well as with the severity of clinical phenotypes. In recent decades, important studies revealed a prominent role of mitochondrial dysfunction in Parkinson's disease (PD); however, the involvement of mitoribosomes remains largely unknown. Critical Issues: Considering that mitoribosomal structure and function can determine the efficiency of OXPHOS and that an impaired mitochondrial respiratory chain is a common finding in PD, we argue that the mitoribosome may be key to disease onset and progression. With this review, we comprehensively integrate the available knowledge on the composition, assembly, and role of the mitoribosome in mitochondrial efficiency, reflecting on its possible involvement in the etiopathogenesis of this epidemic disease as an appealing research avenue. Future Directions: If a direct correlation between mitoribosome failure and PD pathology is demonstrated, these mitochondrial organelles will provide valuable early clinical markers and potentially attractive targets for the development of innovative PD-directed therapeutic agents.

Expression and affinity purification of recombinant mammalian Mitochondrial Ribosomal Small Subunit (MRPS) proteins and protein-protein interaction analysis indicate putative role in tumorigenic cellular processes

MRPS group of proteins are structural constituents of the small subunit of mitoribosomes involved in translation. Recent studies indicate role in tumorigenic process, however, unlike cytosolic ribosomal proteins, knowledge on the role of MRPS proteins in alternate cellular processes is very limited. Mapping protein-protein interactions (PPIs) onto known cellular processes can be a valuable tool to identify novel protein functions. In this study, to identify PPIs of MRPS proteins, we have constructed thirty-one GST/MRPS fusion clones. GST/MRPS fusion proteins were confirmed by MALDI-TOF analysis. GST pull-downs were performed using eight GST/MRPS proteins (MRPS9, MRPS10, MRPS11, MRPS18B, MRPS31, MRPS33, MRPS38, MRPS39), GST alone as pull-down control, and HEK293 cell lysate as the source for anchor proteins followed by nLC/MS/MS analysis and probable PPIs of eight MRPS proteins were identified. Three PPIs from GST pull-downs and interaction between six MRPS proteins and p53 previously reported in PPI database were validated. The PPI network analysis revealed putative role in cellular processes with implications for tumorigenesis. Gene expression screening of a cancer cell line panel indicated overexpression of MRPS10 and MRPS31 in breast cancer. Co-expression module identification tool analysis of breast cancer gene expression and MRPS10 and MRPS31 PPIs revealed putative role for PPI with ACADSB in fatty acid oxidation process regulated by brain-derived neurotrophic factor (BDNF) signaling pathway.

Dual function of GTPBP6 in biogenesis and recycling of human mitochondrial ribosomes

Translation and ribosome biogenesis in mitochondria require auxiliary factors that ensure rapid and accurate synthesis of mitochondrial proteins. Defects in translation are associated with oxidative phosphorylation deficiency and cause severe human diseases, but the exact roles of mitochondrial translation-associated factors are not known. Here we identify the functions of GTPBP6, a homolog of the bacterial ribosome-recycling factor HflX, in human mitochondria. Similarly to HflX, GTPBP6 facilitates the dissociation of ribosomes in vitro and in vivo. In contrast to HflX, GTPBP6 is also required for the assembly of mitochondrial ribosomes. GTPBP6 ablation leads to accumulation of late assembly intermediate(s) of the large ribosomal subunit containing ribosome biogenesis factors MTERF4, NSUN4, MALSU1 and the GTPases GTPBP5, GTPBP7 and GTPBP10. Our data show that GTPBP6 has a dual function acting in ribosome recycling and biogenesis. These findings contribute to our understanding of large ribosomal subunit assembly as well as ribosome recycling pathway in mitochondria.

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.

Recent Advances in Archaeal Translation Initiation

Translation initiation (TI) allows accurate selection of the initiation codon on a messenger RNA (mRNA) and defines the reading frame. In all domains of life, translation initiation generally occurs within a macromolecular complex made up of the small ribosomal subunit, the mRNA, a specialized methionylated initiator tRNA, and translation initiation factors (IFs). Once the start codon is selected at the P site of the ribosome and the large subunit is associated, the IFs are released and a ribosome competent for elongation is formed. However, even if the general principles are the same in the three domains of life, the molecular mechanisms are different in bacteria, eukaryotes, and archaea and may also vary depending on the mRNA. Because TI mechanisms have evolved lately, their studies bring important information about the evolutionary relationships between extant organisms. In this context, recent structural data on ribosomal complexes and genome-wide studies are particularly valuable. This review focuses on archaeal translation initiation highlighting its relationships with either the eukaryotic or the bacterial world. Eukaryotic features of the archaeal small ribosomal subunit are presented. Ribosome evolution and TI mechanisms diversity in archaeal branches are discussed. Next, the use of leaderless mRNAs and that of leadered mRNAs having Shine-Dalgarno sequences is analyzed. Finally, the current knowledge on TI mechanisms of SD-leadered and leaderless mRNAs is detailed.

Human GTPBP5 (MTG2) fuels mitoribosome large subunit maturation by facilitating 16S rRNA methylation

Biogenesis of mammalian mitochondrial ribosomes (mitoribosomes) involves several conserved small GTPases. Here, we report that the Obg family protein GTPBP5 or MTG2 is a mitochondrial protein whose absence in a TALEN-induced HEK293T knockout (KO) cell line leads to severely decreased levels of the 55S monosome and attenuated mitochondrial protein synthesis. We show that a fraction of GTPBP5 co-sediments with the large mitoribosome subunit (mtLSU), and crosslinks specifically with the 16S rRNA, and several mtLSU proteins and assembly factors. Notably, the latter group includes MTERF4, involved in monosome assembly, and MRM2, the methyltransferase that catalyzes the modification of the 16S mt-rRNA A-loop U1369 residue. The GTPBP5 interaction with MRM2 was also detected using the proximity-dependent biotinylation (BioID) assay. In GTPBP5-KO mitochondria, the mtLSU lacks bL36m, accumulates an excess of the assembly factors MTG1, GTPBP10, MALSU1 and MTERF4, and contains hypomethylated 16S rRNA. We propose that GTPBP5 primarily fuels proper mtLSU maturation by securing efficient methylation of two 16S rRNA residues, and ultimately serves to coordinate subunit joining through the release of late-stage mtLSU assembly factors. In this way, GTPBP5 provides an ultimate quality control checkpoint function during mtLSU assembly that minimizes premature subunit joining to ensure the assembly of the mature 55S monosome.

Structural basis of mitochondrial translation

Translation of mitochondrial messenger RNA (mt-mRNA) is performed by distinct mitoribosomes comprising at least 36 mitochondria-specific proteins. How these mitoribosomal proteins assist in the binding of mt-mRNA and to what extent they are involved in the translocation of transfer RNA (mt-tRNA) is unclear. To visualize the process of translation in human mitochondria, we report ~3.0 Å resolution structure of the human mitoribosome, including the L7/L12 stalk, and eight structures of its functional complexes with mt-mRNA, mt-tRNAs, recycling factor and additional trans factors. The study reveals a transacting protein module LRPPRC-SLIRP that delivers mt-mRNA to the mitoribosomal small subunit through a dedicated platform formed by the mitochondria-specific protein mS39. Mitoribosomal proteins of the large subunit mL40, mL48, and mL64 coordinate translocation of mt-tRNA. The comparison between those structures shows dynamic interactions between the mitoribosome and its ligands, suggesting a sequential mechanism of conformational changes.

Structures of the human mitochondrial ribosome bound to EF-G1 reveal distinct features of mitochondrial translation elongation

The mammalian mitochondrial ribosome (mitoribosome) and its associated translational factors have evolved to accommodate greater participation of proteins in mitochondrial translation. Here we present the 2.68-3.96 Å cryo-EM structures of the human 55S mitoribosome in complex with the human mitochondrial elongation factor G1 (EF-G1mt) in three distinct conformational states, including an intermediate state and a post-translocational state. These structures reveal the role of several mitochondria-specific (mito-specific) mitoribosomal proteins (MRPs) and a mito-specific segment of EF-G1mt in mitochondrial tRNA (tRNAmt) translocation. In particular, the mito-specific C-terminal extension in EF-G1mt is directly involved in translocation of the acceptor arm of the A-site tRNAmt. In addition to the ratchet-like and independent head-swiveling motions exhibited by the small mitoribosomal subunit, we discover significant conformational changes in MRP mL45 at the nascent polypeptide-exit site within the large mitoribosomal subunit that could be critical for tethering of the elongating mitoribosome onto the inner-mitochondrial membrane.

Structural basis for ribosome recycling by RRF and tRNA

The bacterial ribosome is recycled into subunits by two conserved proteins, elongation factor G (EF-G) and the ribosome recycling factor (RRF). The molecular basis for ribosome recycling by RRF and EF-G remains unclear. Here, we report the crystal structure of a posttermination Thermus thermophilus 70S ribosome complexed with EF-G, RRF and two transfer RNAs at a resolution of 3.5 Å. The deacylated tRNA in the peptidyl (P) site moves into a previously unsuspected state of binding (peptidyl/recycling, p/R) that is analogous to that seen during initiation. The terminal end of the p/R-tRNA forms nonfavorable contacts with the 50S subunit while RRF wedges next to central inter-subunit bridges, illuminating the active roles of tRNA and RRF in dissociation of ribosomal subunits. The structure uncovers a missing snapshot of tRNA as it transits between the P and exit (E) sites, providing insights into the mechanisms of ribosome recycling and tRNA translocation.

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.