METTL3 and METTL14 (methyltransferase-like) form a methyltransferase complex that methylates adenosine residues at the N6 position of some RNAs, forming N6-methyladenosine (m6A). METTL3, also known as MT-A70, is the catalytic subunit of this complex, and accepts S-adenosyl methionine, a donor for methylation []. m6A, which is present at internal sites of some mRNAs, affects different aspects of mRNA metabolism such as half-life, splicing, and translation [, , , , ].Methyltransferases (EC [intenz:2.1.1.-]) constitute an important class of enzymes present in every life form. They transfer a methyl group most frequently from S-adenosyl L-methionine (SAM or AdoMet) to a nucleophilic acceptor such as oxygen leading to S-adenosyl-L-homocysteine (AdoHcy) and a methylated molecule [, , ]. All these enzymes have in common a conserved region of about 130 amino acid residues that allow them to bind SAM []. The substrates that are methylated by these enzymes cover virtually every kind of biomolecules ranging from small molecules, to lipids, proteins and nucleic acids [, , ]. Methyltransferase are therefore involved in many essential cellular processes including biosynthesis, signal transduction, protein repair, chromatin regulation and gene silencing [, , ]. More than 230 families of methyltransferases have been described so far, of which more than 220 use SAM as the methyl donor.
