Abstract
Mitochondrial ribosomes are functionally specialized for the synthesis of several essential inner membrane proteins of the respiratory chain. Although remarkable progress has been made toward understanding the structure of mitoribosomes, the pathways and factors that facilitate their biogenesis remain largely unknown. The long unstructured domains of unassembled ribosomal proteins are highly prone to misfolding and often require dedicated chaperones to prevent aggregation. To date, chaperones that ensure safe delivery to the assembling ribosome have not been identified in the mitochondrion. In this study, a respiratory synthetic lethality screen revealed a role for an evolutionarily conserved mitochondrial matrix protein called Mam33 in Saccharomyces cerevisiae mitoribosome biogenesis. We found that the absence of Mam33 results in misassembled, aggregated ribosomes and a respiratory lethal phenotype in combination with other ribosome-assembly mutants. Using sucrose gradient sedimentation, native affinity purifications, in vitro binding assays, and SILAC-based quantitative proteomics, we found that Mam33 does not associate with the mature mitoribosome, but directly binds a subset of unassembled large subunit proteins. Based on these data, we propose that Mam33 binds specific mitoribosomal proteins to ensure proper assembly.
Highlights
Mitochondrial ribosomes are functionally specialized for the synthesis of several essential inner membrane proteins of the respiratory chain
We recently identified an evolutionarily conserved protein called Mam33 that promotes the translation of Cox1 in yeast mitochondria [33]
We screened for genes that functionally compensate for Mam33 during respiration and identified mutants with Cox1-specific translation defects and misassembled, aggregated mitochondrial ribosomes
Summary
Henry‡§1 From the ‡Department of Molecular Biology, Rowan University School of Osteopathic Medicine, Stratford, New Jersey 08084 and the ‡§Graduate School of Biomedical Sciences, Rowan University, Stratford, New Jersey 08084
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