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Abstract
Mitochondria are the powerhouse of eukaryotic cells. They possess their own gene expression machineries where highly divergent and specialized ribosomes, named hereafter mitoribosomes, translate the few essential messenger RNAs still encoded by mitochondrial genomes. Here, we present a biochemical and structural characterization of the mitoribosome in the model green alga Chlamydomonas reinhardtii, as well as a functional study of some of its specific components. Single particle cryo-electron microscopy resolves how the Chlamydomonas mitoribosome is assembled from 13 rRNA fragments encoded by separate non-contiguous gene pieces. Additional proteins, mainly OPR, PPR and mTERF helical repeat proteins, are found in Chlamydomonas mitoribosome, revealing the structure of an OPR protein in complex with its RNA binding partner. Targeted amiRNA silencing indicates that these ribosomal proteins are required for mitoribosome integrity. Finally, we use cryo-electron tomography to show that Chlamydomonas mitoribosomes are attached to the inner mitochondrial membrane via two contact points mediated by Chlamydomonas-specific proteins. Our study expands our understanding of mitoribosome diversity and the various strategies these specialized molecular machines adopt for membrane tethering.
Highlights
Mitochondria are the powerhouse of eukaryotic cells
Our study provides an example of a mitoribosome composed of numerous ribosomal RNAs (rRNAs) fragments, revealing a strikingly divergent blueprint for building this conserved molecular machine
To analyze the C. reinhardtii mitoribosome, mitochondria were purified and used for mitoribosome isolation following a procedure based on sucrose density gradient separation
Summary
Mitochondria are the powerhouse of eukaryotic cells. They possess their own gene expression machineries where highly divergent and specialized ribosomes, named hereafter mitoribosomes, translate the few essential messenger RNAs still encoded by mitochondrial genomes. Our structure reveals how the reduced and fragmented rRNAs are organized and stabilized in the mitoribosome via numerous Chlamydomonas-specific r-proteins. The Chlamydomonas mitoribosome contains 47 r-proteins in the LSU and 36 in the SSU.
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