ABSTRACT
N-methyladenosine (mA), a ubiquitous RNA modification, is installed by METTL3-METTL14 complex. The structure of the heterodimeric complex between the methyltransferase domains (MTDs) of METTL3 and METTL14 has been previously determined. However, the MTDs alone possess no enzymatic activity. Here we present the solution structure for the zinc finger domain (ZFD) of METTL3, the inclusion of which fulfills the methyltransferase activity of METTL3-METTL14. We show that the ZFD specifically binds to an RNA containing 5'-GGACU-3' consensus sequence, but does not to one without. The ZFD thus serves as the target recognition domain, a structural feature previously shown for DNA methyltransferases, and cooperates with the MTDs of METTL3-METTL14 for catalysis. However, the interaction between the ZFD and the specific RNA is extremely weak, with the binding affinity at several hundred micromolar under physiological conditions. The ZFD contains two CCCH-type zinc fingers connected by an anti-parallel β-sheet. Mutational analysis and NMR titrations have mapped the functional interface to a contiguous surface. As a division of labor, the RNA-binding interface comprises basic residues from zinc finger 1 and hydrophobic residues from β-sheet and zinc finger 2. Further we show that the linker between the ZFD and MTD of METTL3 is flexible but partially folded, which may permit the cooperation between the two domains during catalysis. Together, the structural characterization of METTL3 ZFD paves the way to elucidate the atomic details of the entire process of RNA mA modification.
ABSTRACT
BACKGROUND: At present, the transplantation of bone marrow-derived endothelial progenitor cells (BM-EPCs) or bone marrow-derived hepatocyte stem cells (BDHSCs) is common in the treatment of liver fibrosis, but the combined treatment for liver fibrosis is rarely reported. Combined transplantation of BM-EPCs possessing the function of angiogenesis and BDHSCs possessing the function of hepatocyte regeneration might play a dual anti-fibrosis role. OBJECTIVE: To evaluate the reversal effect on liver fibrosis by the combined transplantation of BM-EPCs and BDHSCs in rats. METHODS: The liver fibrosis rat models were induced with CCl4 subcutaneous injections for 6 weeks. BM-EPCs of rats with liver fibrosis were obtained by culture induction in vitro.BDHSCs of rats with liver fibrosis were obtained by magnetic bead cell sorting.BM-EPCs and/or BDHSCs were transplanted into liver fibrosis rats via the tail vein and branch of the portal vein,and then the effects of BDHSCs transplantatiron on liver fibrosis and liver function were observed. RESULTS AND CONCLUSION: (1) Masson staining results showed transplantations of BDHSCs and BM-EPCs, alone or both, could suppress the formation of collagen fibers. However, the staging scores of liver fibrosis showed that only the combined transplantation of BM-EPCs and BDHSCs could significantly improve liver fibrosis,which was significantly different from the model group(1.75±0.25 vs. 3.00±0.19, P < 0.05). (2) The liver biochemical assay in the blood showed that the levels of all five parameters of alanine aminotransferase, aspartate aminotransferase, total bilirubin, prothrombin time, and activated partial thromboplastin time in the BM-EPCs/BDHSCs group were significantly improved to be equivalent to normal levels, compared with those in the model group (P < 0.05). To conclude, it is an effective treatment for liver fibrosis by the co-transplantation of BM-EPCs and BDHSCs.