Structural insight into the human mitochondrial tRNA purine N1-methyltransferase and ribonuclease P complexes

Mitochondrial tRNAs are transcribed as long polycistronic transcripts of precursor tRNAs and undergo posttranscriptional modifications such as endonucleolytic processing and methylation required for their correct structure and function. Among them, 5-end processing and purine 9 N1-methylation of mitochondrial tRNA are catalyzed by two proteinaceous complexes with overlapping subunit composition. The Mg2-dependent RNase P complex for 5-end cleavage comprises the methyltransferase domain– containing protein tRNA methyltransferase 10C, mitochondrial RNase P subunit (TRMT10C/MRPP1), short-chain oxidoreductase hydroxysteroid 17-dehydroge-nase 10 (HSD17B10/MRPP2), and metallonuclease KIAA0391/ MRPP3. An MRPP1–MRPP2 subcomplex also catalyzes the formation of 1-methyladenosine/1-methylguanosine at position 9 using S-adenosyl-L-methionine as methyl donor. However, a lack of structural information has precluded insights into how these complexes methylate and process mitochondrial tRNA. Here, we used a combination of X-ray crystallography, interaction and activity assays, and small angle X-ray scattering (SAXS) to gain structural insight into the two tRNA modification complexes and their components. The MRPP1 N terminus is involved in tRNA binding and monomer–monomer self-interaction, whereas the C-terminal SPOUT fold contains key residues for S-adenosyl-L-methionine binding and N1-methylation.SCOPUS: ar.j