Project description:mRNAs are regulated by nucleotide modifications that influence their cellular fate. Two of the most abundant modified nucleotides are N6-methyladenosine (m6A), found within mRNAs, and N6,2′-O-dimethyladenosine (m6Am), which is found at the first transcribed nucleotide. Distinguishing these modifications in mapping studies has been difficult. Here, we identify and biochemically characterize PCIF1, the methyltransferase that generates m6Am. We find that PCIF1 binds and is dependent on the m7G cap. By depleting PCIF1, we generated transcriptome-wide maps that distinguish m6Am and m6A. We find that m6A and m6Am misannotations arise from mRNA isoforms with alternative transcription start sites (TSSs). These isoforms contain m6Am that maps to “internal” sites, increasing the likelihood of misannotation. We find that depleting PCIF1 does not substantially affect mRNA translation but is associated with reduced stability of a subset of m6Am-annotated mRNAs. The discovery of PCIF1 and our accurate mapping technique will facilitate future studies to characterize m6Am’s function.
Project description:The 5' end of eukaryotic mRNAs is protected by the m7G-cap structure. The transcription start site nucleotide is ribose methylated (Nm) in many eukaryotes, while an adenosine at this position is further methylated at the N6 position (m6A) by the mammalian Phosphorylated CTD-interacting Factor 1 (PCIF1) to generate m6Am. Here we show that while loss of cap-specific m6Am in mice does not affect viability or fertility, the Pcif1 mutants display reduced body-weight. Transcriptome analyses of mutant mouse tissues support a role for the cap-specific m6Am modification in stabilizing transcripts. In contrast, the Drosophila Pcif1 is catalytically-dead, but like its mammalian counterpart it retains the ability to associate with the Ser5-phosphorylated CTD of RNA pol II. Finally, we show that the Trypanosoma Pcif1 is an m6Am methylase that contributes to the N6,N6,2?-O-trimethyladenosine (m62Am) in the hypermethylated cap4 structure of Trypanosomatids. Thus, PCIF1 has evolved to function in catalytic and non-catalytic roles.
Project description:To identify m6Am genes that are responsible for HIV inhibition, MeRIP-seq was performed in control and PCIF1 KO T cells infected with HIV.
Project description:To identify m6Am genes that are responsible for HIV inhibition, m6Am-exo-seq was performed in control, PCIF1 KO T cells, and cells infected with HIV.
Project description:Purpose: To identify the in vivo RNA targets of PCIF1, we performed meRIP experiments for PCIF1 KD and control cells using an anti-m6A antibody. We envisioned that the cap-specific PCIF1 should selectively alter the m6Am modification at the 5-terminal region of transcripts. Indeed, we observed a reduction of modification peaks at the 5’-end but not the 3’-UTR regions of mRNA upon PCIF1 KD.
Project description:N6-methylation of 2’-O-methyladenosine (Am) in RNA occurs in eukaryotic cells to generate N6,2’-O-dimethyladenosine (m6Am). Identification of the methyltransferase responsible for m6Am catalysis has accelerated studies on the function of m6Am in RNA processing. While m6Am is generally found in the first transcribed nucleotide of mRNAs, the modification is also found internally within U2 snRNA. However, the writer required for catalyzing internal m6Am formation had remained elusive. By sequencing transcriptome-wide RNA methylation at single-base-resolution, we identified human METTL4 as the writer that directly methylates Am at U2 snRNA position 30 into m6Am. We found that METTL4 localizes to the nucleus and its conserved methyltransferase catalytic site is required for U2 snRNA methylation. By sequencing human cells with overexpressed Mettl4, we determined METTL4’s in vivo target RNA motif specificity. In the absence of Mettl4 in human cells, U2 snRNA lacks m6Am thereby affecting a subset of splicing events that exhibit specific features such as overall 3’ splice-site weakness with certain motif positions more affected than others. This study establishes that METTL4 methylation of U2 snRNA regulates splicing of specific pre-mRNA transcripts.
Project description:Internal bases in mRNA can be subjected to modifications that influence the fate of mRNA in cells. One of the most prevalent modified bases is found at the 5′ end of mRNA, at the first encoded nucleotide adjacent to the 7-methylguanosine cap. Here we show that this nucleotide, N6,2′-O-dimethyladenosine (m6Am), is a reversible modification that influences cellular mRNA fate. Using a transcriptome-wide map of m6Am we find that m6Am-initiated transcripts are markedly more stable than mRNAs that begin with other nucleotides. We show that the enhanced stability of m6Am-initiated transcripts is due to resistance to the mRNA-decapping enzyme DCP2. Moreover, we find that m6Am is selectively demethylated by fat mass and obesity-associated protein (FTO). FTO preferentially demethylates m6Am rather than N6-methyladenosine (m6A), and reduces the stability of m6Am mRNAs. Together, these findings show that the methylation status of m6Am in the 5′ cap is a dynamic and reversible epitranscriptomic modification that determines mRNA stability.
Project description:We previously identified PCIF1 (Phosphorylated CTD Interacting Factor 1) as a novel phosphorylated C-terminal domain (CTD) of RNA polymerase II. We also recently identified PCIF1 as a new cap-specific adenine N6 methyltransferase (CAPAM) responsible for creating N6, 2’-O-dimethyladenosine (m6Am) at the 5’-end of mRNAs. However, it remains unclear how PCIF1 regulates gene expression. To identify genes whose expression levels are affected by PCIF1 knockdown in human cultured cells, we performed gene expression profiling by microarray analysis.