Four paralogues of RPL12 are differentially associated to ribosome in plant mitochondria

Ribosomal protein L12 is the only component present in four copies in the ribosome. In prokaryotes as well as in yeast and human mitochondria, all copies correspond to the same RPL12. By contrast, we present here evidence that plant mitochondria contain four different RPL12 proteins. Compared to E. coli RPL12, the four mature RPL12 variants show a conserved C-terminal region that contains all the functional domains of prokaryotic RPL12 but three of them present an additional N-terminal extension containing either an acidic or a basic domain and a high level of proline residues. All proteins have a potential mitochondrial N-terminal targeting sequence and were imported in vitro into isolated mitochondria. Using RPL12 antibodies, the four variants were shown to be present in a potato mitochondrial ribosome fraction. Moreover, the four proteins reacted differently to the destabilization of ribosomes. This suggests either a heterogeneous RPL12 composition among each ribosome and/or a heterogeneous population of plant mitochondrial ribosomes.