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).[18]In vitro, this methyltransferase complex preferentially methylates RNA oligonucleotides containing GGACU[19] and a similar preference was identified in vivo in mapped m6A sites in Rous sarcoma virus genomic RNA[20] and in bovine prolactin mRNA.[21] More recent studies have characterized other key components of the m6A methyltransferase complex in mammals, including METTL14,[22][23] Wilms tumor 1 associated protein (WTAP),[22][24]VIRMA[25] and METTL5.[26] Following a 2010 speculation of m6A in mRNA being dynamic and reversible,[27] the discovery of the first m6A demethylase, fat mass and obesity-associated protein (FTO) in 2011[28] 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.[29]
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.[17][30][31][32][33] Insulin-like growth factor-2 mRNA-binding proteins 1, 2, and 3 (IGF2BP1–3) are reported as a novel class of m6A readers.[34] IGF2BPs use K homology (KH) domains to selectively recognize m6A-containing RNAs and promote their translation and stability.[34] 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.[35]
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. [36]
^ abBeemon K, Keith J (June 1977). "Localization of N6-methyladenosine in the Rous sarcoma virus genome". Journal of Molecular Biology. 113 (1): 165–179. doi:10.1016/0022-2836(77)90047-X. PMID196091.
^Wei CM, Gershowitz A, Moss B (January 1976). "5'-Terminal and internal methylated nucleotide sequences in HeLa cell mRNA". Biochemistry. 15 (2): 397–401. doi:10.1021/bi00647a024. PMID174715.
^Levis R, Penman S (April 1978). "5'-terminal structures of poly(A)+ cytoplasmic messenger RNA and of poly(A)+ and poly(A)- heterogeneous nuclear RNA of cells of the dipteran Drosophila melanogaster". Journal of Molecular Biology. 120 (4): 487–515. doi:10.1016/0022-2836(78)90350-9. PMID418182.
^Kennedy TD, Lane BG (June 1979). "Wheat embryo ribonucleates. XIII. Methyl-substituted nucleoside constituents and 5'-terminal dinucleotide sequences in bulk poly(AR)-rich RNA from imbibing wheat embryos". Canadian Journal of Biochemistry. 57 (6): 927–931. doi:10.1139/o79-112. PMID476526.
^Xu C, Wang X, Liu K, Roundtree IA, Tempel W, Li Y, et al. (November 2014). "Structural basis for selective binding of m6A RNA by the YTHDC1 YTH domain". Nature Chemical Biology. 10 (11): 927–929. doi:10.1038/nchembio.1654. PMID25242552.