Balancing of mitochondrial translation through METTL8-mediated m³C modification of mitochondrial tRNAs

Mitochondria contain a specific translation machinery for the synthesis of mitochondria-encoded respiratory chain components. Mitochondrial tRNAs (mt-tRNAs) are also generated from the mitochondrial DNA and, similar to their cytoplasmic counterparts, are post-transcriptionally modified. Here, we find that the RNA methyltransferase METTL8 is a mitochondrial protein that facilitates 3-methyl-cytidine (m³C) methylation at position C₃₂ of the mt-tRNA^Ser(UCN) and mt-tRNA^Thr. METTL8 knockout cells show a reduction in respiratory chain activity, whereas overexpression increases activity. In pancreatic cancer, METTL8 levels are high, which correlates with lower patient survival and an enhanced respiratory chain activity. Mitochondrial ribosome profiling uncovered mitoribosome stalling on mt-tRNA^Ser(UCN)- and mt-tRNA^Thr-dependent codons. Further analysis of the respiratory chain complexes using mass spectrometry revealed reduced incorporation of the mitochondrially encoded proteins ND6 and ND1 into complex I. The well-balanced translation of mt-tRNA^Ser(UCN)- and mt-tRNA^Thr-dependent codons through METTL8-mediated m³C₃₂ methylation might, therefore, facilitate the optimal composition and function of the mitochondrial respiratory chain.