Project description:Poly(A) tails protect RNAs from degradation and deadenylation rates determine RNA stability. Deadenylation has mostly been investigated within the cytoplasm and the dynamics of poly(A) tail length after transcription are not well understood. Combining long-read sequencing with metabolic labeling and cell fractionations, we investigate deadenylation dynamics of newly synthesized poly(A) tails in vitro and in vivo. We report evidence for genome-wide synthesis of poly(A) tails longer than 200 nt which are enriched upon splicing inhibition. Metabolic labeling reveals rapid deadenylation of poly(A) tails within minutes after transcription. Fractionation experiments show that initial deadenylation is a nuclear process, and that different classes of transcripts, including long noncoding RNAs, have distinctive nuclear poly(A) tail lengths. Modelling deadenylation dynamics predicts that deadenylation in the nucleus is by an order of magnitude faster than in the cytoplasm. In summary, we suggest nuclear deadenylation as a novel regulatory layer which may determine stability and abundance before mRNAs reach the cytoplasm.
Project description:Poly(A) tails protect RNAs from degradation and their deadenylation rates determine RNA stability. Although poly(A) tails are generated in the nucleus, deadenylation of tails has mostly been investigated within the cytoplasm. Here, we combined long-read sequencing with metabolic labeling, splicing inhibition and cell fractionation experiments to quantify, separately, the genesis and trimming of nuclear and cytoplasmic tails in vitro and in vivo. We present evidence for genome-wide, nuclear synthesis of tails longer than 200 nt, which are rapidly shortened after transcription. Our data suggests that rapid deadenylation is a nuclear process, and that different classes of transcripts and even transcript isoforms have distinct nuclear tail lengths. For example, many long-noncoding RNAs retain long poly(A) tails. Modeling deadenylation dynamics predicts nuclear deadenylation about 10 times faster than cytoplasmic deadenylation. In summary, our data suggests that nuclear deadenylation might be a key mechanism for regulating mRNA stability, abundance, and subcellular localization.
Project description:RNA tails play integral roles in the control of mRNA translation and decay. Guanylation of poly(A) tail was discovered recently, yet the enzymology and function remain obscure. Here we identify TUT3 (PAPD5) and TUT5 (PAPD7) (TUT3/5) as the enzymes responsible for mRNA guanylation. Purified TUT3/5 predominantly incorporate GTPs to generate a mixed poly(A) tail with intermittent non-adenosine residues. A single guanosine is sufficient to impede the deadenylation complex (CCR4-NOT) which trims the tail and exposes guanosine at the 3′ end. Consistently, the depletion of TUT3/5 leads to a decrease in mRNA half-life and abundance in cells. Thus, TUT3/5 produce a mixed tail which shields the mRNA from rapid deadenylation. Our study unveils the role of mixed tailing, and expands the complexity of post-transcriptional gene regulation.
Project description:RNA tails play integral roles in the control of mRNA translation and decay. Guanylation of poly(A) tail was discovered recently, yet the enzymology and function remain obscure. Here we identify TUT3 (PAPD5) and TUT5 (PAPD7) (TUT3/5) as the enzymes responsible for mRNA guanylation. Purified TUT3/5 predominantly incorporate GTPs to generate a mixed poly(A) tail with intermittent non-adenosine residues. A single guanosine is sufficient to impede the deadenylation complex (CCR4-NOT) which trims the tail and exposes guanosine at the 3′ end. Consistently, the depletion of TUT3/5 leads to a decrease in mRNA half-life and abundance in cells. Thus, TUT3/5 produce a mixed tail which shields the mRNA from rapid deadenylation. Our study unveils the role of mixed tailing, and expands the complexity of post-transcriptional gene regulation.
Project description:N6-methyladenosine (m6A) is the most prevalent internal modification found in mammalian messenger and non-coding RNAs. The discoveries of functionally significant demethylases that reverse this methylation as well as the recently revealed m6A distributions in mammalian transcriptomes strongly indicate regulatory functions of this modification. Here we report the identification and characterization of the mammalian nuclear RNA N6-adenosine methyltransferase core (RNMTC) complex. Besides METTL3, a methyltransferase which was the only known component of RNMTC in the past, we discovered that a previously uncharacterized methyltransferase, METTL14, exhibits a N6-adenosine methyltransferase activity higher than METTL3. Together with WTAP, the third component that dramatically affects the cellular m6A level, these three proteins form the core complex that orchestrates m6A deposition on mammalian nuclear RNA. Biochemistry assays, imaging experiments, as well as transcriptome-wide analyses of the binding sites and their effects on m6A methylation support methylation function and reveal new insights of RNMTC. PAR-CLIP and m6A-seq in HeLa cells
Project description:Animal microRNAs (miRNAs) typically regulate gene expression by binding to partially complementary target sites in the 3' untranslated region (UTR) of messenger RNA (mRNA) reducing its translation and stability. They also commonly induce shortening of the mRNA 3' poly(A) tail, which contributes to their mRNA decay promoting function. The relationship between miRNA-mediated deadenylation and translational repression has been less clear. Using transfection of reporter constructs carrying three imperfectly matching let-7 target sites in the 3' UTR into mammalian cells we observe rapid target mRNA deadenylation that precedes measureable translational repression by endogenous let-7 miRNA. Depleting cells of the argonaute co-factors RCK or TNRC6A can impair let-7-mediated repression despite ongoing mRNA deadenylation, indicating that deadenylation alone is not sufficient to effect full repression. Nevertheless, the magnitude of translational repression by let-7 is diminished when the target reporter lacks a poly(A) tail. Employing an antisense strategy to block deadenylation of target mRNA with poly(A) tail also partially impairs translational repression. On the one hand, these experiments confirm that tail removal by deadenylation is not strictly required for translational repression. On the other hand they show directly that deadenylation can augment miRNA-mediated translational repression in mammalian cells beyond stimulating mRNA decay. Taken together with published work, these results suggest a dual role of deadenylation in miRNA function: it contributes to translational repression as well as mRNA decay and is thus critically involved in establishing the quantitatively appropriate physiological response to miRNAs.