Baixing Wu

Readers, writers and erasers of N6-methylated adenosine modification

N6-methyladenosine (m6A) as the most prevalent internal modification in mammalian RNAs has been increasingly realized as an important reversible mark that participates in various biological processes and cancer pathogenesis. In this review, we discuss the catalytic mechanisms of MT-A70 domain family proteins for mediating adenosine N6-methylation, the removal of this RNA mark by members of ALKB homologue domain family proteins, and the recognition of these m6A-modified RNAs by YTH domain family proteins. Our discussions focus on the recent advances in our understandings of the structural and functional properties of N6-methyladenosine methyltransferases, demethylases and reader proteins. Overall, we aim to mechanistically explain the reversible and dynamic nature of this unique RNA internal modification that contributes to the complexity of RNA-mediated gene regulation, and inspire new studies in epitranscriptomics.

A DNA Structure Containing AgI‐Mediated G:G and C:C Base Pairs

Metal-mediated base pairs have been extensively utilized in many research fields, including genetic-code extension, novel therapeutics development, and nanodevice design. Compared to other cations, AgI is more flexible in pairing with natural base pairs. Herein, we present a DNA structure containing two C-AgI -C pairs and the first reported G-AgI -G pair in a short 8mer DNA strand. This structure not only provides detailed insight into these AgI -mediated base-pairing patterns in DNA, but also represents the first nonhelical DNA structure driven by heavy-metal ions, thus further contributing to the structural diversity of DNA. This unique complex structure is highly sequence-dependent, thus implying functional potentials as a new DNA aptamer that can bind and recognize silver ions. These results not only advance our understanding of the interactions between AgI and nucleobases, but also provide a unique structural component for the rational design of new DNA nanodevices.

Flexibility and stabilization of HgII-mediated C:T and T:T base pairs in DNA duplex

Abstract Owing to their great potentials in genetic code extension and the development of nucleic acid-based functional nanodevices, DNA duplexes containing HgII-mediated base pairs have been extensively studied during the past 60 years. However, structural basis underlying these base pairs remains poorly understood. Herein, we present five high-resolution crystal structures including one first-time reported C–HgII–T containing duplex, three T–HgII–T containing duplexes and one native duplex containing T–T pair without HgII. Our structures suggest that both C–T and T–T pairs are flexible in interacting with the HgII ion with various binding modes including N3–HgII–N3, N4–HgII–N3, O2–HgII–N3 and N3–HgII–O4. Our studies also reveal that the overall conformations of the C–HgII–T and T–HgII–T pairs are affected by their neighboring residues via the interactions with the solvent molecules or other metal ions, such as SrII. These results provide detailed insights into the interactions between HgII and nucleobases and the structural basis for the rational design of C–HgII–T or T–HgII–T containing DNA nanodevices in the future.

Structural basis for single-stranded RNA recognition and cleavage by C3PO

Translin and translin-associated factor-x are highly conserved in eukaroytes; they can form heteromeric complexes (known as C3POs) and participate in various nucleic acid metabolism pathways. In humans and Drosophila, C3POs cleave the fragmented siRNA passenger strands and facilitate the activation of RNA-induced silencing complex, the effector complex of RNA interference (RNAi). Here, we report three crystal structures of Nanoarchaeum equitans (Ne) C3PO. The apo-NeC3PO structure adopts an open form and unravels a potential substrates entryway for the first time. The NeC3PO:ssRNA and NeC3PO:ssDNA complexes fold like closed football with the substrates captured at the inner cavities. The NeC3PO:ssRNA structure represents the only catalytic form C3PO complex available to date; with mutagenesis and in vitro cleavage assays, the structure provides critical insights into the substrate binding and the two-cation-assisted catalytic mechanisms that are shared by eukaryotic C3POs. The work presented here further advances our understanding on the RNAi pathway.

Ribonuclease activity of MARF1 controls oocyte RNA homeostasis and genome integrity in mice

Significance Although MARF1 (meiosis regulator and mRNA stability factor 1) is an ancient protein, identification of its function in mammalian female germ cell development and fertility is recent. It is crucial for the progression of oocyte meiosis and defense against the ravages of retrotransposons, which can cause damage to the oocyte’s genome. These processes are dependent upon the ability of MARF1 to act alone both to bind RNA and to function as a ribonuclease during oogenesis. Here we reveal the molecular structure and functional mechanisms that enable MARF1 activity and provide insight into the complex posttranscriptional processes that shape the oocyte transcriptome. Producing normal eggs for fertilization and species propagation requires completion of meiosis and protection of the genome from the ravages of retrotransposons. Mutation of Marf1 (meiosis regulator and mRNA stability factor 1) results in defects in both these key processes in mouse oocytes and thus in infertility. MARF1 was predicted to have ribonuclease activity, but the structural basis for the function of MARF1 and the contribution of its putative ribonuclease domain to the mutant oocyte phenotype was unknown. Therefore, we resolved the crystal structures of key domains of MARF1 and demonstrated by biochemical and mutagenic analyses that the ribonuclease activity of MARF1 controls oocyte meiotic progression and retrotransposon surveillance. The N-terminal NYN domain of MARF1 resembles the nuclease domains of Vpa0982, T4 RNase H, and MCPIP1 and contains four conserved aspartate residues, D178, D215, D246, and D272. The C-terminal LOTUS domain of MARF1 adopts a winged helix-turn-helix fold and binds ssRNA and dsRNA. Purified MARF1 cleaved ssRNAs in vitro, but this cleavage activity was abolished by mutations of conserved aspartates in its NYN domain and truncation of the LOTUS domain. Furthermore, a point mutation in the D272 residue in vivo caused a female-only infertile phenotype in mice, with failure of meiotic resumption and elevation of Line1 and Iap retrotransposon transcripts and DNA double-strand breaks in oocytes. Therefore, the ribonuclease activity of MARF1 controls oocyte meiosis and genome integrity. This activity depends upon conserved aspartic residues in the catalytic NYN domain and the RNA-binding activity of the LOTUS domain.

High-resolution DNA quadruplex structure containing all the A-, G-, C-, T-tetrads

Abstract DNA can form diverse structures, which predefine their physiological functions. Besides duplexes that carry the genetic information, quadruplexes are the most well-studied DNA structures. In addition to their important roles in recombination, replication, transcription and translation, DNA quadruplexes have also been applied as diagnostic aptamers and antidisease therapeutics. Herein we further expand the sequence and structure complexity of DNA quadruplex by presenting a high-resolution crystal structure of DNA1 (5′-AGAGAGATGGGTGCGTT-3′). This is the first quadruplex structure that contains all the internal A-, G-, C-, T-tetrads, A:T:A:T tetrads and bulged nucleotides in one single structure; as revealed by site-specific mutagenesis and biophysical studies, the central ATGGG motif plays important role in the quadruplex formation. Interestingly, our structure also provides great new insights into cation recognition, including the first-time reported Pb2+, by tetrad structures.