2020同步年報

Life Science 047 D NA methylation is the key mechanism of epigenetic regulation involved in various cellular processes, including gene expression, tumorigenesis and genomic im- printing. In mammals, DNA methylation is achieved by the enzymatic activity of DNA methyltransferases (DNMTs) on transferring the methyl group from the cofactor, S-adenos- ylmethionine, to the C5 position of cytosine. Mammalian DNMTs consist of four members—DNMT1, DNMT3A, DNMT3B and DNMT3L. DNMT3A and DNMT3B are so-called de novo methyltransferases, as they establish new DNA methylation patterns during embryogenesis and genomic imprinting during germline development. The activities of DNMT3A and DNMT3B are stimulated by interacting with the inac- tive DNMT member, DNMT3L. 1 Although DNMT3B shares a similar domain arrangement and high sequence identity in the methyltransferase do- main (~85%) with DNMT3A, these two enzymes have distinct physiological or pathophysiological roles and enzymatic properties. DNMT3A methylates imprinted Fig. 1 : (a) Crystal structures of a DNMT3B–3L complex bound without and (b) with DNA. The two SAH molecules are represented with a stick model; catalytic loops are shown in purple and TRD loops are displayed in yellow. (c) Overlay of the catalytic loop and (d) the TRD loop of apo (white) and DNA-bound (colored) complexes. (e) TRD loops of DNMT3B interact at the major groove of a DNA duplex in the DNMT3B-3L-DNA (CpGpG) struc- ture. (f) The cytosine (C5) in CpG sites is flipped from the DNA helix by the catalytic loop. The enlarged panel (right) reveals the hydrogen-bonded network surrounding the cytosine. (g) Methyltransferase activities of DNMT3B–3L and the C651A and V657G mutants. (h) A water channel located near the C5 atom of the cytosine of the CpG sites was identified. Cytosines of the CpG sites are shown as magenta sticks, whereas water molecules are shown as pink spheres. (i) Proposed working mechanism for methylation by DNMT3B. [Reproduced from Ref. 7] Human DNMT3B Structure: Insights into Flanking Sequence Preference and Processive Methylation Unlike that of DNA methyltransferase 3A (DNMT3A), methylation of DNA by DNMT3B occurs in a non-cooperative processive manner, allowing the enzyme to methylate repeated CpG sites along a DNA strand. A combination of structural and biochemical analyses of the DNMT3B catalytic do- main shows that DNMT3B adopts a more stable target recognition domain (TRD) loop for proces- sive methylation and a flanking sequence preference different from those of DNMT3A.