Methyltransferase-like 3 (METTL3) is a 70 kDa protein, 580 amino acids in length. It is the principal catalytic agent for most of the N6– methyladenosine (m6A) modifications that occur in eukaryotic cell mRNA. METTL3 typically places methyl adducts at the adenosine in motifs, such as DRACH, RAC, or RRACH. As such, it influences mRNA splicing, stability, expression, and degradation. Recent studies show that it even tags miRNA and lncRNA. This broad spectrum of influence in the most prevalent of RNA modifications affects cellular proliferation and determination, impacting the course of viral infections, cardiovascular disease, neurological disorders, and cancer. However, it cannot methylate by itself alone.1
METTL3 as a Complex
The N-terminal half of METTL3 is important for protein co-factor interactions. Two CCCH-type zinc finger domains (ZnF1 and ZnF2) in its midsection target and bind single-stranded RNA 5′-GGACU-3′ consensus sequences. The C-terminus is dominated by its methyltransferase domain that utilizes S-adenosylmethionine as the principal methyl donor. Still, with all these features, METTL3 is almost completely catalytically inactive without the participation of METTL14.2
When METTL3 and METTL14 form a heterodimer, they form the catalytically active m6A-METTL Complex (MAC). METTL14 plays a key structural role that stabilizes METTL3’s interaction with its substrate RNA, allowing efficient adenosine methylation.2
MAC associates with WTAP, VIRMA, ZC3H13, RBM15, and HAKAI to form the m6A-METTL-associated complex (MACOM). The N-terminal domain of METTL3 binds WTAP to potentiate m6A deposition and MAC localization to nuclear speckles. VIRMA acts as bridge between MAC-WTAP (the WMM complex) and other components, while further aiding overall complex targeting to m6A methylation sites in the 3′-UTR and stop codon regions of mRNA. RBM15’s RNA binding motifs direct the MACOM to U-rich RNA regions. ZC3H13 promotes interaction between RBM15 and WTAP, while modulating the nuclear localization of MACOM. HAKAI helps target m6A methylation sites, located in the 5′-UTR and start codon regions of mRNA.2
Upregulated METTL3 expression in several cancers has been found to, by itself, increase the translational efficiency of key drivers, such as EGFR and TAZ. When its overexpression outstrips the supply of its usual partners, its N-terminal region can recruit translation initiation factors, like eIF3h, eIF4e, and CBP800, to m6A-modified 3′-UTR mRNA regions on certain transcripts and facilitate greater expression through ribosomal recycling.3
Implications for Health and Disease
m6A modification via METTL3 is important for normal cell cycle regulation, apoptosis, autophagy, and differentiation. Its general effect is to modulate these functions, dependent upon context. In particular, it drives proper bone and neural development. Aberrant upregulation of activity in adulthood has been implicated in hypertension, atherosclerosis, cardiac hypertrophy, and heart failure. Conversely, downregulation in the central nervous system is now associated with neurodegenerative disorders, such as Alzheimer’s Disease.2
Nonetheless, upregulation of METTL3 activity has its most profound effects in cancer. While methylation via METTL3 acts as a tumor suppressor in very few instances, its dysregulation overwhelmingly acts as a tumor promoter in many cancers. METTL3 and its co-factor proteins enhance the proliferative MYC, WNT/b-catenin, BCL-2, and PI3K/AKT/mTOR pathways, while diminishing the potency of the p53 pathway. Acting upon yet other gene transcripts or miRNAs, its complexes promote resistance to targeted therapy, chemotherapy, immunotherapy, and radiotherapy.4
Managing dysregulated METTL3-mediated m6A modification via direct catalytic inhibitors, inactive SAM analogues, suppressive miRNA upregulation, and CRISPR-directed strategies is now a significant focus of cancer research.4
Discovery of METTL3 modulators will hopefully lead to new therapeutic cancer treatments. BellBrook Labs offers discovery tools to advance the search for METTL3 inhibitors. Our AptaFluor SAH Assay provides an HTS-ready assay kit to measure METTL3/METTL14 activity and screen and profile for inhibitors. The assay relies on an SAH-sensing RNA aptamer that provides the ultra-sensitivity required to measure activity at physiological KM concentrations.
- Zeng, C. et al. (2020) Roles of METTL3 in cancer: mechanisms and therapeutic targeting. Journal of Hematology & Oncology, 13:117. Review. https://doi.org/10.1186/s13045-020-00951-w.
- Fiorentino, F. et al. (2023) METTL3 from Target Validation to the First Small-Molecule Inhibitors: A Medicinal Chemistry Journey. J. Med. Chem. 66(3), 1654–1677. Review. https://doi.org/10.1021/acs.jmedchem.2c01601.
- Liu, S. et al. (2020) METTL3 plays multiple functions in biological processes. Am J Cancer Res. 10(6), 1631–1646. Review. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7339281/.
- Huang, W. et al. (2021) N6-methyladenosine methyltransferases: functions, regulation, and clinical potential. J Hematol Oncol. 14(117). Review. https://doi.org/10.1186/s13045-021-01129-8.