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In contrast to the bacterial translation machinery, mitoribosomes and mitochondrial translation factors are highly divergent in terms of composition and architecture. There is increasing evidence that the biogenesis of mitoribosomes is an intricate pathway, involving many assembly factors. To better understand this process, we investigated native assembly intermediates of the mitoribosomal large subunit from the human parasite Trypanosoma brucei using cryo-electron microscopy. We identify 28 assembly factors, 6 of which are homologous to bacterial and eukaryotic ribosome assembly factors. They interact with the partially folded rRNA by specifically recognizing functionally important regions such as the peptidyltransferase center. The architectural and compositional comparison of the assembly intermediates indicates a stepwise modular assembly process, during which the rRNA folds toward its mature state. During the process, several conserved GTPases and a helicase form highly intertwined interaction networks that stabilize distinct assembly intermediates. The presented structures provide general insights into mitoribosomal maturation. © 2020 Elsevier Inc. The structures of two assembly intermediates of the Trypanosoma brucei mitoribosomal large subunit in combination with biochemical analysis provide insights into the stepwise mitoribosomal biogenesis process that involves numerous assembly factors functioning as enzymes or scaffold components. © 2020 Elsevier Inc. ISSN:1097-2765 ISSN:1097-4164

Mitochondrial ribosomes (mitoribosomes) almost exclusively synthesize essential components of the oxidative phosphorylation machinery. Dysfunction of mitochondrial protein biosynthesis leads to human diseases and plays an important role in the altered metabolism of cancer cells. Recent developments in cryo-electron microscopy enabled the structural characterization of complete yeast and mammalian mitoribosomes at near-atomic resolution. Despite originating from ancestral bacterial ribosomes, mitoribosomes have diverged in their composition and architecture. Mitoribosomal proteins are larger and more numerous, forming an extended network around the ribosomal RNA, which is expanded in yeast and highly reduced in mammals. Novel protein elements at the entrance or exit of the mRNA channel imply a different mechanism of mRNA recruitment. The polypeptide tunnel is optimized for the synthesis of hydrophobic proteins and their co-translational membrane insertion.

Structure of the largest, most complex ribosome Ribosomes are two-subunit ribonucleoprotein assemblies that catalyze the translation of messenger RNA into protein. Ribosomal RNAs (rRNAs) play key structural and functional roles. Ramrath et al. report the high-resolution structure of mitochondrial ribosomes from the unicellular parasite Trypanosoma brucei that contain the smallest known rRNAs. The trypanosomal mitoribosome is the most complex ribosomal assembly characterized, with two rRNAs and 126 proteins. The increased protein subunits have substituted for rRNA as an architectural scaffold. The structure also reveals the minimal core needed for ribosome function. Science , this issue p. eaau7735

Nature, 505 (7484), ISSN:0028-0836, ISSN:1476-4687

Nature, 515 (7526), ISSN:0028-0836, ISSN:1476-4687