{ { DISPLAYTITLE:N6-Methyladenosine}}
N6-Methyladenosine (m6A) was originally identified and partially characterised in the 1970s, and is an abundant modification in mRNA and DNA. It is found within some viruses, and most eukaryotes including mammals, insects, plants and yeast. It is also found in tRNA, rRNA, and small nuclear RNA (snRNA) as well as several long non-coding RNA, such as Xist.
The methylation of adenosine is directed by a large m6A methyltransferase complex containing METTL3, which is the subunit that binds S-adenosyl-L-methionine (SAM). In vitro, this methyltransferase complex preferentially methylates RNA oligonucleotides containing GGACU and a similar preference was identified in vivo in mapped m6A sites in Rous sarcoma virus genomic RNA and in bovine prolactin mRNA. More recent studies have characterized other key components of the m6A methyltransferase complex in mammals, including METTL14, Wilms tumor 1 associated protein (WTAP), VIRMA and METTL5. Following a 2010 speculation of m6A in mRNA being dynamic and reversible, the discovery of the first m6A demethylase, fat mass and obesity-associated protein (FTO) in 2011 confirmed this hypothesis and revitalized the interests in the study of m6A. A second m6A demethylase alkB homolog 5 (ALKBH5) was later discovered as well.
The biological functions of m6A are mediated through a group of RNA binding proteins that specifically recognize the methylated adenosine on RNA. These binding proteins are named m6A readers. The YT521-B homology (YTH) domain family of proteins (YTHDF1, YTHDF2, YTHDF3 and YTHDC1) have been characterized as direct m6A readers and have a conserved m6A-binding pocket. Insulin-like growth factor-2 mRNA-binding proteins 1, 2, and 3 (IGF2BP1–3) are reported as a novel class of m6A readers. IGF2BPs use K homology (KH) domains to selectively recognize m6A-containing RNAs and promote their translation and stability. These m6A readers, together with m6A methyltransferases (writers) and demethylases (erasers), establish a complex mechanism of m6A regulation in which writers and erasers determine the distributions of m6A on RNA, whereas readers mediate m6A-dependent functions. m6A has also been shown to mediate a structural switch termed m6A switch.
The specificity of m6A installation on mRNA is controlled by exon architecture and exon junction complexes. Exon junction complexes suppress m6A methylation near exon-exon junctions by packaging nearby RNA and protecting it from methylation by the m6A methyltransferase complex. m6A regions in long internal and terminal exons, away from exon-exon junctions and exon junction complexes, escape suppression and can be methylated by the methyltransferase complex.
Species distribution
Yeast
In budding yeast (Saccharomyces cerevisiae), the homologue of METTL3, IME4 is induced in diploid cells in response to nitrogen and fermentable carbon source starvation and is required for mRNA methylation and the initiation of correct meiosis and sporulation. mRNAs of IME1 and IME2, key early regulators of meiosis, are known to be targets for methylation, as are transcripts of IME4 itself.
Plants
In plants, the majority of the m6A is found within 150 nucleotides before the start of the poly(A) tail.
Mutations of MTA, the Arabidopsis thaliana homologue of METTL3, results in embryo arrest at the globular stage. A >90% reduction of m6A levels in mature plants leads to dramatically altered growth patterns and floral homeotic abnormalities.
Mammals
Mapping of m6A in human and mouse RNA has identified over 18,000 m6A sites in the transcripts of more than 7,000 human genes with a consensus sequence of [G/A/U][G>A]m6AC[U>A/C] consistent with the previously identified motif. The localization of individual m6A sites in many mRNAs is highly similar between human and mouse, and transcriptome-wide analysis reveals that m6A is found in regions of high evolutionary conservation. m6A is found within long internal exons and is preferentially enriched within 3' UTRs and around stop codons. m6A within 3' UTRs is also associated with the presence of microRNA binding sites; roughly 2/3 of the mRNAs which contain an m6A site within their 3' UTR also have at least one microRNA binding site. By integrating all m6A sequencing data, a novel database called RMBase has identified and provided ~200,000 sites in the human and mouse genomes corresponding to N6-Methyladenosines (m6A) in RNA.