Project description:The exon junction complex is deposited at 24nt upstream of exon-exon junctions, but not at every junction. The core complex is comprosed of 4 proteins, eIF4A3, Magoh, Y14 and MLN51. Here we performed immunoprecipitation of Y14 with subsequent iCLIP of eIF4A3 in HeLa cells to identify the crosslink sites of the exon junction complex, in particular eIF4A3.
Project description:Analysis of miRNA-targeted cellular NMD substrates in HeLa cell. The hypothesis tested in the present study was that endogenous NMD substrates containing long 3' untranslated region may targeted for miRNA. Results provide important information expanding the roles of miRISC in the posttranscriptional regulation of gene expression: a new cross-talk between miRNA-mediated gene silencing and NMD. ABSTRACT: Imperfect base-pairing between microRNA (miRNA) and the 3â-untranslated region (3âUTR) of target mRNA triggers translational repression of the target mRNA. Here, we provide evidence that human Argonaute 2 (Ago2) targets cap-binding protein (CBP)80/20- and exon junction complex (EJC)-bound mRNAs and inhibits nonsense-mediated mRNA decay (NMD), which is tightly restricted to CBP80/20-bound mRNAs. Furthermore, microarray analyses reveal that a subset of cellular transcripts, which are expected to be targeted for NMD, is stabilized by miRNA-mediated gene silencing. The regulation of NMD by miRNAs will shed light on a new post-transcriptional regulation mechanism of gene expression in mammalian cells Total RNA obtained from HeLa cells with downregulation of Ago2 or Ago2/UPF1 by siRNA. The up- or down-regulated transcripts were compared to control siRNA treated HeLa cell RNA extract. Significant transcripts were confirmed by replication.
Project description:Nonsense-mediated mRNA decay (NMD) is essential for removing premature termination codon (PTC)-containing transcripts from cells. Studying the NMD pathway in model organisms can help to elucidate the NMD mechanism of humans and improves our understanding of how this biologically important process has evolved. Protozoa are among the earliest branching eukaryotes. Their NMD mechanism is poorly understood and may be primordial. We demonstrate that highly conserved Upf proteins (Upf1a, Upf2, and Upf3) are involved in the NMD pathway of the ciliate, Tetrahymena thermophila. We further show that a novel protozoa-specific nuclease, Smg6L, is responsible for destroying many NMD-targeted transcripts. The transcriptome-wide identification and characterization of NMD-targeted transcripts in vegetative Tetrahymena cells showed that many have exon–exon junctions downstream of the termination codon. However, Tetrahymena homologs of exon junction complex (EJC) core components do not form a complex and are dispensable for NMD, suggesting that NMD is EJC independent in this early branching eukaryote.
Project description:We identify a brain-specific microRNA—miR-128—that represses Nonsense Mediated mRNA Decay (NMD) and thereby controls batteries of transcripts in neural cells. miR-128 represses NMD by targeting the RNA helicase UPF1 and the exon-junction complex core component MLN51. We employed exon arrays for this analysis, as this platform detects expression levels of individual exons and thus allows detection of not only differentially expressed transcripts (DETs), but also alternative isoform transcripts (AITs). The latter is particularly relevant to our study because alternative RNA processing events (e.g., RNA splicing, alternative promoter usage, and alternative polyadenylation-site usage) often place a translation termination codon in a premature context and thereby trigger NMD.
Project description:N6-methyladenosine (m6A), a widespread destabilizing mark on mRNA, is non-uniformly distributed across the transcriptome, yet the basis for its selective deposition is unknown. Here, we uncover that m6A deposition is not selective. Instead, m6A distribution is exclusion-based: m6A-consensus harboring sites are methylated by default, unless they are within a window of up to ~200 nt from an exon-intron junction. A simple model which we extensively validate, relying exclusively on presence of m6A motifs and exon-intron architecture allows high accuracy recapitulation of experimentally-measured m6A profiles and of the vast majority of m6A hallmarks. Finally, we provide strong evidence that this exclusion is mediated by the exon-junction complex. Our findings establish a mechanism by which the memory of nuclear RNA splicing is covalently etched on an mRNA in the form of m6A, and determines its cytoplasmic stability, with broad implications on the regulation, function, and evolution of the exon-junction complex and m6A.