ChemRAP uncovers specific mRNA translation regulation via RNA 5' phospho-methylation.

5'-end modifications play key roles in determining RNA fates. Phospho-methylation is a noncanonical cap occurring on either 5'-PPP or 5'-P ends. We used ChemRAP, in which affinity purification of cellular proteins with chemically synthesized modified RNAs is coupled to quantitative proteomics, to identify 5'-Pme "readers". We show that 5'-Pme is directly recognized by EPRS, the central subunit of the multisynthetase complex (MSC), through its linker domain, which has previously been involved in key noncanonical EPRS and MSC functions. We further determine that the 5'-Pme writer BCDIN3D regulates the binding of EPRS to specific mRNAs, either at coding regions rich in MSC codons, or around start codons. In the case of LRPPRC (leucine-rich pentatricopeptide repeat containing), a nuclear-encoded mitochondrial protein associated with the French Canadian Leigh syndrome, BCDIN3D deficiency abolishes binding of EPRS around its mRNA start codon, increases its translation but ultimately results in LRPPRC mislocalization. Overall, our results suggest that BCDIN3D may regulate the translation of specific mRNA via RNA-5'-Pme.

BCDIN3D RNA methyltransferase stimulates Aldolase C expression and glycolysis through let-7 microRNA in breast cancer cells.

Type II diabetes (T2D) and specific cancers share many risk factors, however, the molecular mechanisms underlying these connections are often not well-understood. BCDIN3D is an RNA modifying enzyme that methylates specific precursor microRNAs and tRNAHis. In addition to breast cancer, BCDIN3D may also be linked to metabolism, as its gene locus is associated with obesity and T2D. In order to uncover metabolic pathways regulated by BCDIN3D in cancer, we performed an unbiased analysis of the metabolome, transcriptome, and proteome of breast cancer cells depleted for BCDIN3D. Intersection of these analyses showed that BCDIN3D-depleted cells have increased levels of Fructose 1,6 Bisphosphate (F1,6-BP), the last six-carbon glycolytic intermediate accompanied by reduced glycolytic capacity. We further show that elevated F1,6-BP is due to downregulation of Aldolase C (ALDOC), an enzyme that cleaves F1,6-BP mainly in the brain, but whose high expression/amplification is associated with poor prognosis in breast cancer. BCDIN3D regulates ALDOC through a non-canonical mechanism involving the crucial let-7 microRNA family and its target site on the 3'UTR of ALDOC. Overall, our results reveal an important connection between BCDIN3D, let-7 and glycolysis that may be relevant to breast cancer, obesity, and T2D.

Crosstalk between the RNA Methylation and Histone-Binding Activities of MePCE Regulates P-TEFb Activation on Chromatin.

RNAP II switching from the paused to the productive transcription elongation state is a pivotal regulatory step that requires specific phosphorylations catalyzed by the P-TEFb kinase. Nucleosolic P-TEFb activity is inhibited by its interaction with the ribonuclear protein complex built around the 7SK small nuclear RNA (7SK snRNP). MePCE is the RNA methyltransferase that methylates and stabilizes 7SK in the nucleosol. Here, we report that MePCE also binds chromatin through the histone H4 tail to serve as a P-TEFb activator at specific genes important for cellular identity. Notably, this histone binding abolishes MePCE's RNA methyltransferase activity toward 7SK, which explains why MePCE-bound P-TEFb on chromatin may not be associated with the full 7SK snRNP and is competent for RNAP II activation. Overall, our results suggest that crosstalk between the histone-binding and RNA methylation activities of MePCE regulates P-TEFb activation on chromatin in a 7SK- and Brd4-independent manner.

Click Quantitative Mass Spectrometry Identifies PIWIL3 as a Mechanistic Target of RNA Interference Activator Enoxacin in Cancer Cells.

Enoxacin is a small molecule that stimulates RNA interference (RNAi) and acts as a growth inhibitor selectively in cancer but not in untransformed cells. Here, we used alkenox, a clickable enoxacin surrogate, coupled with quantitative mass spectrometry, to identify PIWIL3 as a mechanistic target of enoxacin. PIWIL3 is an Argonaute protein of the PIWI subfamily that is mainly expressed in the germline and that mediates RNAi through piRNAs. Our results suggest that cancer cells re-express PIWIL3 to repress RNAi through miRNAs and thus open a new opportunity for cancer-specific targeting.