Ribosomes are the molecular machines that translate messenger RNAs into proteins. They consist of two subunits. The small one decodes messenger RNA and the large one carries out the polymerization of amino acids to form the corresponding protein. The team of Philippe Giegé in collaboration with the teams of Yaser Hashem (IECB, INSERM, Bordeaux) and Hakim Mireau (IJPB, INRA, Versailles) has determined the specific composition and architecture of Arabidopsis mitochondrial ribosomes. This study is published in the journal Nature Plants.
Mitochondria represent the energy center of eukaryotic cells. For their metabolism as well as for their gene expression, mitochondria combine bacterial-like traits with traits that have evolved in eukaryotes. Translation is the least well-known step of mitochondrial gene expression. In plants, pentatricopeptide repeat (PPR) proteins are involved in all steps of gene expression but their function in mitochondrial translation remains unclear.
Using a biochemical approach, researchers have characterized the mitochondrial ribosome (mitoribosome) of the model plant Arabidopsis and identified its protein composition. 19 plant-specific mitoribosome proteins have been found, among which 10 are PPR proteins. Mutant analysis of genes encoding these PPR proteins, in particular using ribosome profiling, revealed their role in translation. Finally, a cryo-electron microscopy analysis revealed the unique three-dimensional architecture of these mitoribosomes. They are characterized by a very large small subunit, in particular with a new elongated domain never observed to date in other ribosomes.
Structural comparison between Arabidopsis mitoribosome and animal mitoribosome as well as with Arabidopsis cytosolic ribosome, which highlights the originality of this mitoribosome architecture. In particular, it is characterized by the presence of additional domains (circled in red). “SSU” represents small subunits and “LSU” large ribosomal subunits.
This work contributes to understand the evolutionary diversity of translation systems. It illustrates remarkably how evolution has played with mitoribosomes to optimize protein synthesis in mitochondria.