mRNA circularization by METTL3-eIF3h enhances translation

N6-methyladenosine (m6A) modification of mRNA catalyzed by METTL3 is enriched at a subset of stop codons. METTL3 can promote translation but the mechanism and widespread relevance remain unknown. Here we show that METTL3 enhances translation only when tethered to reporter mRNA at sites close to the stop codon supporting a mRNA looping mechanism for ribosome recycling and translational control. Electron microscopy revealed the topology of individual polyribosomes with single METTL3 foci found in close proximity to 5’ cap-binding proteins. We identify a direct physical and functional interaction between METTL3 and the eukaryotic translation initiation factor 3 subunit h (eIF3h). METTL3 promotes translation of a large subset of oncogenic mRNAs, including BRD4 that are also m6A-modified in human primary lung tumors. The METTL3-eIF3h interaction is required for enhanced translation, formation of densely packed polyribosomes, and oncogenic transformation. METTL3 depletion inhibits tumorigenicity and sensitizes lung cancer cells to BRD4 inhibition. These findings uncover a mRNA looping mechanism of translation control and identify METTL3-eIF3h as a potential cancer therapeutic target. METTL3 expression was depleted in HeLa cells using lentivirus mediated shRNA knockdown. The METTL3 knockdown cells and the control cells were subjected to sucrose gradient ultracentifuge for the polysome profiling. The RNA samples isolated from the polysome fractions and subpolysome fractions were used for high throughput sequencing for the analysis of translation efficiency. For the analysis of mRNA stability, METTL3 knockdown cells and the control cells were treated with ActD for different time points then the RNA samples were isolated for high throughput sequencing and the subsequent calculation of mRNA halflife.