Cryo-EM maps: Mitoribosomal small subunit maturation involves formation of initiation-like complexes

Final cryo-EM maps for the complexes described in the article Mitoribosomal small subunit maturation involves formation of initiation-like complexes Abstract: Mitochondrial ribosomes (mitoribosomes) play a central role in synthesizing mitochondrial inner membrane proteins responsible for oxidative phosphorylation. Although mitoribosomes from different organisms exhibit considerable structural variations, recent insights into mitoribosome assembly suggest that mitoribosome maturation follows common principles and involves a number of conserved assembly factors. To investigate the steps involved in the assembly of the mitoribosomal small subunit (mt-SSU) we determined the cryo-electron microscopy structures of middle and late assembly intermediates of the Trypanosoma brucei mitochondrial small subunit (mt-SSU) at 3.6 and 3.7 Å resolution, respectively. We identified five novel assembly factors that together with the mitochondrial initiation factor 2 specifically interact with functionally important regions of the rRNA including the decoding center, thereby preventing premature mRNA or large subunit binding. Structural comparison of assembly intermediates with mature mt-SSU combined with RNAi experiments suggests a new, non-canonical role of mitochondrial initiation factor 2 and a stepwise assembly process, where modular exchange of ribosomal proteins and assembly factors together with mt-IF-2 ensure proper 9S rRNA folding and protein maturation during the final steps of assembly.