YBEY is an essential biogenesis factor for mitochondrial ribosomes

Ribosome biogenesis is a key cellular process involving numerous protein factors, some of which are deeply conserved. This is namely the case of the bacterial endoribonuclease YbeY, whose loss is associated with ribosomal abnormalities, profound changes in gene expression, altered metabolism and death. Surprisingly, YbeY is also widespread among eukaryotes. Here we provide an in-depth characterisation of the human YBEY homologue localising in mitochondria. We show that YBEY loss results in a severe mitochondrial phenotype and complete inability of mitochondria to translate proteins and, consequently, perform oxidative phosphorylation. Through interactions with the important disease protein p32 and other factors, YBEY functions to recruit the mitoribosomal protein uS11m to the nascent small subunit in order to complete the assembly of initiation-competent ribosomal particles. This likely conserved mechanism shows striking similarities with final stages of the cytosolic ribosome biogenesis and may provide an important late checkpoint before the mitoribosome engages in translation.