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.

与细菌翻译机制相反，线粒体和线粒体翻译因子在组成和结构上差异很大。越来越多的证据表明，线粒体的生物发生是一个复杂的途径，涉及许多组装因子。为了更好地了解这一过程，我们研究了来自人寄生虫布鲁氏锥虫的线粒体大亚基的天然装配中间体使用低温电子显微镜。我们确定了28个装配因子，其中6个与细菌和真核生物核糖体装配因子同源。它们通过特异性识别功能上重要的区域（如肽基转移酶中心）与部分折叠的rRNA相互作用。组装中间体的结构和组成比较表明了逐步的模块化组装过程，在此过程中，rRNA向其成熟状态折叠。在此过程中，几个保守的GTPases和解旋酶形成高度交织的相互作用网络，可稳定不同的组装中间体。提出的结构提供了对线粒体成熟的一般见解。