Interconnected assembly factors regulate the biogenesis of mitoribosomal large subunit in trypanosomes

Mitoribosomes synthesize essential proteins encoded in the mitochondrial genome. They consist of the ribosomal RNA and protein components, coordinated assembly of which is critical for function. Divergent mitoribosomes with reduced RNA and increased protein mass were described in , a mammalian parasite causing African Trypanosomiases. Here, we report that the large subunit of mitoribosome requires at least 15 biogenesis factors, which we characterized in complex with an assembly intermediate using cryo-EM. These assembly factors form a protein network that spans over 180 Å and connects between functional regions, allowing to coordinate their maturation. The conserved proteins mt-RbgA and mt-EngA are stably bound together at the subunit interface and mitochondrial acyl-carrier proteins play a structural role in constraining the L1 stalk. The characterized assembly intermediate forms an extensive network connecting between functionally important regions of the mitoribosome.