Human METTL20 Methylates Lysine Residues Adjacent to the Recognition Loop of the Electron Transfer Flavoprotein in Mitochondria

Background: The electron transfer flavoprotein (ETF) is a mobile electron carrier coupling mitochondrial fatty acid oxidation to respiration. Results: METTL20 is a mitochondrial lysine protein methylase that modifies Lys-199 and -202 of the electron transfer flavoprotein -subunit. Conclusion: METTL20 is a protein methylase in mitochondria that influences protein-protein interactions with ETF. Significance: Mitochondria contain lysine methyltransferase METTL20. In mammalian mitochondria, protein methylation is a relatively uncommon post-transcriptional modification, and the extent of the mitochondrial protein methylome, the modifying methyltransferases, and their substrates have been little studied. As shown here, the -subunit of the electron transfer flavoprotein (ETF) is one such methylated protein. The ETF is a heterodimer of - and -subunits. Lysine residues 199 and 202 of mature ETF are almost completely trimethylated in bovine heart mitochondria, whereas ETF is not methylated. The enzyme responsible for the modifications was identified as methyltransferase-like protein 20 (METTL20). In human 143B cells, the methylation of ETF is less extensive and is diminished further by suppression of METTL20. Tagged METTL20 expressed in HEK293T cells specifically associates with the ETF and promotes the trimethylation of ETF lysine residues 199 and 202. ETF serves as a mobile electron carrier linking dehydrogenases involved in fatty acid oxidation and one-carbon metabolism to the membrane-associated ubiquinone pool. The methylated residues in ETF are immediately adjacent to a protein loop that recognizes and binds to the dehydrogenases. Suppression of trimethylation of ETF in mouse C2C12 cells oxidizing palmitate as an energy source reduced the consumption of oxygen by the cells. These experiments suggest that the oxidation of fatty acids in mitochondria and the passage of electrons via the ETF may be controlled by modulating the protein-protein interactions between the reduced dehydrogenases and the -subunit of the ETF by trimethylation of lysine residues. METTL20 is the first lysine methyltransferase to be found to be associated with mitochondria.