Biogenesis of cytosolic ribosomes requires the essential iron-sulphur protein Rli1p and mitochondria

Mitochondria perform a central function in the biogenesis of cellular iron - sulphur (Fe/S) proteins. It is unknown to date why this biosynthetic pathway is indispensable for life, the more so as no essential mitochondrial Fe/S proteins are known. Here, we show that the soluble ATP-binding cassette (ABC) protein Rli1p carries N-terminal Fe/S clusters that require the mitochondrial and cytosolic Fe/ S protein biogenesis machineries for assembly. Mutations in critical cysteine residues of Rli1p abolish association with Fe/ S clusters and lead to loss of cell viability. Hence, the essential character of Fe/ S clusters in Rli1p explains the indispensable character of mitochondria in eukaryotes. We further report that Rli1p is associated with ribosomes and with Hcr1p, a protein involved in rRNA processing and translation initiation. Depletion of Rli1p causes a nuclear export defect of the small and large ribosomal subunits and subsequently a translational arrest. Thus, ribosome biogenesis and function are intimately linked to the crucial role of mitochondria in the maturation of the essential Fe/ S protein Rli1p.