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Mitochondrial translation occurs on the mitochondrial ribosome, also known as the mitoribosome. The assembly of mitoribosomes is a highly coordinated process. During mitoribosome biogenesis, various assembly factors transiently associate with the nascent ribosome, facilitating the accurate and efficient construction of the mitoribosome. However, the specific factors involved in the assembly process, the precise mechanisms, and the cellular compartments involved in this vital process are not yet fully understood. In this study, we discovered a crucial role for GTP-binding protein 8 (GTPBP8) in the assembly of the mitoribosomal large subunit (mt-LSU) and mitochondrial translation. GTPBP8 is identified as a novel GTPase located in the matrix and peripherally bound to the inner mitochondrial membrane. Importantly, GTPBP8 is specifically associated with the mt-LSU during its assembly. Depletion of GTPBP8 leads to an abnormal accumulation of mt-LSU, indicating that GTPBP8 is critical for proper mt-LSU assembly. Furthermore, the absence of GTPBP8 results in reduced levels of fully assembled 55S monosomes. This impaired assembly leads to compromised mitochondrial translation and, consequently, impaired mitochondrial function. The identification of GTPBP8 as an important player in these processes provides new insights into the molecular mechanisms underlying mitochondrial protein synthesis and its regulation. [ABSTRACT FROM AUTHOR]

Mitoribosomes consist of ribosomal RNA and protein components, coordinated assembly of which is critical for function. We used mitoribosomes from Trypanosoma brucei with reduced RNA and increased protein mass to provide insights into the biogenesis of the mitoribosomal large subunit. Structural characterization of a stable assembly intermediate revealed 22 assembly factors, some of which have orthologues/counterparts/homologues in mammalian genomes. These assembly factors form a protein network that spans a distance of 180 Å, shielding the ribosomal RNA surface. The central protuberance and L7/L12 stalk are not assembled entirely and require removal of assembly factors and remodeling of the mitoribosomal proteins to become functional. The conserved proteins GTPBP7 and mt‐EngA are bound together at the subunit interface in proximity to the peptidyl transferase center. A mitochondrial acyl‐carrier protein plays a role in docking the L1 stalk, which needs to be repositioned during maturation. Additional enzymatically deactivated factors scaffold the assembly while the exit tunnel is blocked. Together, this extensive network of accessory factors stabilizes the immature sites and connects the functionally important regions of the mitoribosomal large subunit. SYNOPSIS: The formation of the mitoribosome is an intricate process involving multiple protein factors that work in coordination. Here, structural characterisation of a T. brucei mitoribosomal large subunit assembly intermediate reveals the specific roles of an extensive assembly factor network. Cryo‐EM of a mitoribosomal large subunit assembly intermediate reveals 22 associated assembly factors.The 180 Å‐spanning network shields the ribosomal RNA surface and connects functionally important regions.The central protuberance and the L7/L12 stalk are pre‐assembled in a non‐functional formConserved proteins GTPBP7 and mt‐EngA bind together at the subunit interfaceA mitochondrial acyl‐carrier protein plays a role in docking the L1 stalk.Assembly factors such as MRM and RPUSD4 represent catalytically‐inactivated enzymes serving as structural mediators. [ABSTRACT FROM AUTHOR]

Mitochondria are eukaryotic organelles of bacterial origin where respiration takes place to produce cellular chemical energy. These reactions are catalyzed by the respiratory chain complexes located in the inner mitochondrial membrane. Notably, key components of the respiratory chain complexes are encoded on the mitochondrial chromosome and their expression relies on a dedicated mitochondrial translation machinery. Defects in the mitochondrial gene expression machinery lead to a variety of diseases in humans mostly affecting tissues with high energy demand such as the nervous system, the heart, or the muscles. The mitochondrial translation system has substantially diverged from its bacterial ancestor, including alterations in the mitoribosomal architecture, multiple changes to the set of translation factors and striking reductions in otherwise conserved tRNA elements. Although a number of structures of mitochondrial ribosomes from different species have been determined, our mechanistic understanding of the mitochondrial translation cycle remains largely unexplored. Here, we present two cryo‐EM reconstructions of human mitochondrial elongation factor G1 bound to the mammalian mitochondrial ribosome at two different steps of the tRNA translocation reaction during translation elongation. Our structures explain the mechanism of tRNA and mRNA translocation on the mitoribosome, the regulation of mtEFG1 activity by the ribosomal GTPase‐associated center, and the basis of decreased susceptibility of mtEFG1 to the commonly used antibiotic fusidic acid. Synopsis: Mitochondrial translation relies on both conserved and mitochondria‐specific features. Cryo‐EM structures provide insights into tRNA translocation during the elongation stage of mitochondrial translation, which is catalyzed by mtEFG1 on the mitoribosome. tRNA‐mRNA translocation is based on conserved large‐scale motions of the small ribosomal subunit head, and interaction of mtEFG1 with the tRNA‐mRNA module.Closure of the ribosomal GTPase‐associated center facilitates translocation of tRNAs by elongation factor G.Increased stability of mtEFG1 switch‐1 rationalizes decreased susceptibility of mitochondrial translation to the antibiotic fusidic acid.Mitochondria‐specific L1 stalk element compensates for loss of flexible L1 stalk rRNA base. [ABSTRACT FROM AUTHOR]

In Saccharomyces cerevisiae, the mitoribosomal RNA of the minor subunit, 15S rRNA, is transcribed as a bicistronic transcript along with tRNAW. 5′ and 3′ sequences flanking the mature transcript must be removed by cleavage at the respective junctions before incorporating it into the mitoribosome. An in vivo dose–response triphasic system was created to elucidate the role of Ccm1p in the processing of 15S rRNA: Ccm1p supply ("On"), deprivation ("Off"), and resupply ("Back on"). After 72 h under "Off" status, the cells started to exhibit a complete mutant phenotype as assessed by their lack of growth in glycerol medium, while keeping their mitochondrial DNA integrity (ρ+). Full functionality of mitochondria was reacquired upon "Back on." 15S rRNA levels and phenotype followed the Ccm1p intramitochondrial concentrations throughout the "On–Off–Back on" course. Under "Off" status, cells gradually accumulated unprocessed 5′ and 3′ junctions, which reached significant levels at 72–96 h, probably due to a saturation of the mitochondrial degradosome (mtEXO). The Ccm1p/mtEXO mutant (Δccm1/Δdss1) showed a copious accumulation of 15S rRNA primary transcript forms, which were cleaved upon Ccm1p resupply. The gene that codes for the RNA component of RNase P was conserved in wild-type and mutant strains. Our results indicate that Ccm1p is crucial in processing the 15S rRNA primary transcript and does not stabilize the already mature 15S rRNA. Consequently, failure of this function in Δccm1 cells results, as it happens to any other unprocessed primary transcripts, in total degradation of 15S rRNA by mtEXO, whose mechanism of action is discussed. [ABSTRACT FROM AUTHOR]

Oxidative phosphorylation (OXPHOS) is the mechanism whereby ATP, the major energy source for the cell, is produced by harnessing cellular respiration in the mitochondrion. This is facilitated by five multi-subunit complexes housed within the inner mitochondrial membrane. These complexes, with the exception of complex II, are of a dual genetic origin, requiring expression from nuclear and mitochondrial genes. Mitochondrially encoded mRNA is translated on the mitochondrial ribosome (mitoribosome) and the recent release of the near atomic resolution structure of the mammalian mitoribosome has highlighted its peculiar features. However, whereas some aspects of mitochondrial translation are understood, much is to be learnt about the presentation of mitochondrial mRNA to the mitoribosome, the biogenesis of the machinery, the exact role of the membrane, the constitution of the translocon/insertion machinery and the regulation of translation in the mitochondrion. This review addresses our current knowledge of mammalian mitochondrial gene expression, highlights key questions and indicates how defects in this process can result in profound mitochondrial disease. [ABSTRACT FROM AUTHOR]