Functional Divergence of bL36m Protein in Yeast and Human Mitochondrial Ribosomes

L36 is a structural protein of the large ribosomal subunit of bacterial, mitochondrial, and chloroplast ribosomes. L36 stabilizes the peptidyl transferase center and the L7/L12 stalk, which is a binding site for the elongation factors during the translation cycle. According to the cryoelectron microscopy data, L36 incorporates into the large ribosomal subunit in both bacteria and mitochondria at the final assembly step. Bacterial L36 is not an essential protein, since deletion of its gene in bacteria did not impair the colony growth or reduce the mRNA translation levels. Deletion of the RTC6 gene coding for the mitochondrial homologue of L36 (bL36m) in Saccharomyces cerevisiae, impeded yeast growth on the media with non-fermentable carbon sources. Our findings indicate that the mitochondrial dysfunction associated with the absence of bL36m was caused by a decreased activity of cytochrome c oxidase complex that resulted from the selective disruption of synthesis of its subunits encoded in the mitochondrial genome. Furthermore, selective inhibition of mitochondrial protein synthesis did not induce critical structural abnormalities of mitochondrial ribosomes or reduce their ability to bind mRNA. Furthermore, we demonstrated that in contrast to S. cerevisiae, the absence of bL36m protein in human cells had no substantial impact on the synthesis of mitochondrially encoded proteins or mitochondrial ribosome assembly. However, the observed reduction in the mitochondrial respiration in the bL36m-deficient cells may be indicative of disturbances in the respiratory chain organization not associated with the mitochondrial translation.