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

Mitochondria contain a specific translation machinery for the synthesis of respiratory chain components encoded on the mitochondrial genome. Mitochondrial tRNAs (mt-tRNAs) are also generated from the mitochondrial genome and, similar to their cytoplasmic counterparts, are modified at various positions. Here, we find that the RNA methyltransferase METTL8, is a mitochondrial protein that facilitates m3C methylation at position C32 of mt-tRNASer(UCN) and mt-tRNAThr. METTL8 knock out cells show reduced and over expressing cells enhanced respiratory chain activity. In pancreatic cancer, METTL8 levels are high, which correlates with patient survival. Indeed, METTL8 up regulation stimulates respiratory chain activity in these cells. Ribosome occupancy analysis using ribosome profiling revealed ribosome stalling on mt-tRNASer(UCN) and mt-tRNAThr codons and mass spectrometry analysis of native ribosomal subcomplexes unraveled reduced respiratory chain incorporation of the mitochondria encoded proteins ND6 and ND1. A well-balanced translation of mt-tRNASer(UCN) and mt-tRNAThr codons through METTL8-mediated C32 methylation might therefore provide optimal respiratory chain compositions and function. Overall design: 4 mitoribosome profiling datasets of HEK293 cells. One control dataset, two biological replicates containing METTL8 knockouts, and one with a inducible METTL8 overexpression