Interaction between a poly(A)-specific ribonuclease and the 5' cap influences mRNA deadenylation rates in vitro

We have used an in vitro system that reproduces in vivo aspects of mRNA turnover to elucidate mechanisms of deadenylation. DAN, the major enzyme responsible for poly(A) tail shortening in vitro, specifically interacts with the 5' cap structure of RNA substrates, and this interaction is greatly stimulated by a poly(A) tail. Several observations suggest that cap-DAN interactions are functionally important for the networking between regulated mRNA stability and translation. First, uncapped RNA substrates are inefficiently deadenylated. Second, a stem-loop structure in the 5' UTR dramatically reduces deadenylation by interfering with cap-DAN interactions. Third, the addition of cap binding protein eIF4E inhibits deadenylation in vitro. These data provide insights into the early steps of substrate recognition that target an mRNA for degradation.     

A cell-free mRNA stability assay reveals conservation of the enzymes and mechanisms of mRNA decay between mosquito and mammalian cell lines

The rate of mRNA turnover is an important determinant of levels of gene expression. Although this process has been studied extensively in mammalian cells and yeast, relatively little is known about the mRNA decay pathways in insects. Our analysis found that the vast majority of components of the mRNA decay machinery are conserved between humans and mosquitoes. Moreover, the half-lives of Aedes albopictus mRNAs are within a similar range to those of mammalian mRNAs. In order to investigate mechanistic aspects of mRNA decay in mosquitoes, we developed an in vitro system using cytoplasmic S100 extracts from A. albopictus C6/36 cells. Using this decay assay, we show here that all the pathways of mRNA turnover that have been observed in mammalian cells (deadenylation, decapping, 3′-to-5′ exonucleolytic decay and 5′-to-3′ exonucleolytic decay) are active in C6/36 extracts. Finally, we present compelling evidence that the major deadenylase in C6/36 extracts is likely to be a homolog of the human poly(A) specific ribonuclease, PARN. Our results suggest a high level of conservation in the factors and pathways of mRNA decay between mosquitoes and humans.     

The poly(A)-binding protein and an mRNA stability protein jointly regulate an endoribonuclease activity

We previously identified a sequence-specific erythroid cell-enriched endoribonuclease (ErEN) activity involved in the turnover of the stable alpha-globin mRNA. We now demonstrate that ErEN activity is regulated by the poly(A) tail. The unadenylated alpha-globin 3' untranslated region (3'UTR) was an efficient substrate for ErEN cleavage, while the polyadenylated 3'UTR was inefficiently cleaved in an in vitro decay assay. The influence of the poly(A) tail was mediated through the poly(A)-binding protein (PABP) bound to the poly(A) tail, which can inhibit ErEN activity. ErEN cleavage of an adenylated alpha-globin 3'UTR was accentuated upon depletion of PABP from the cytosolic extract, while addition of recombinant PABP reestablished the inhibition of endoribonuclease cleavage. PABP inhibited ErEN activity indirectly through an interaction with the alphaCP mRNA stability protein. Sequestration of alphaCP resulted in an increase of ErEN cleavage activity, regardless of the polyadenylation state of the RNA. Using electrophoretic mobility shift assays, PABP was shown to enhance the binding efficiency of alphaCP to the alpha-globin 3'UTR, which in turn protected the ErEN target sequence. Conversely, the binding of PABP to the poly(A) tail was also augmented by alphaCP, implying that a stable higher-order structural network is involved in stabilization of the alpha-globin mRNA. Upon deadenylation, the interaction of PABP with alphaCP would be disrupted, rendering the alpha-globin 3'UTR more susceptible to endoribonuclease cleavage. The data demonstrated a specific role for PABP in protecting the body of an mRNA in addition to demonstrating PABP's well-characterized effect of stabilizing the poly(A) tail.     

Xenopus cold-inducible RNA-binding protein 2 interacts with ElrA,the Xenopus homolog of HuR,and inhibits deadenylation of specific mRNAs

Xenopus cold-inducible RNA-binding protein 2 (xCIRP2) is a major cytoplasmic RNA-binding protein in oocytes. In this study, we identify another RNA-binding protein ElrA, the Xenopus homolog of HuR, as an interacting protein of xCIRP2 by yeast two-hybrid screening. As ElrA stabilizes the RNA body in the in vitro mRNA stability system, we examine the role of xCIRP2 in the stabilization of mRNA and find that xCIRP2 inhibits deadenylation of AU-rich element-containing mRNA. These results suggest that xCIRP2 and ElrA may be involved in the regulation of mRNA stability at different steps. By immunoprecipitation with anti-xCIRP2 antibody, we find that xCIRP2 interacts with several mRNAs including mRNA encoding the centrosomal kinase Nek2B in oocytes. xCIRP2 also inhibits deadenylation of the mRNA substrate containing the 3'-untranslated region of Nek2B mRNA in the in vitro system. Our results suggest that xCIRP2 associates with specific mRNAs and can regulate the length of poly(A) tail in Xenopus oocytes.     

Ccr4p is the catalytic subunit of a Ccr4p/Pop2p/Notp mRNA deadenylase complex in Saccharomyces cerevisiae

The major pathways of mRNA turnover in eukaryotic cells are initiated by shortening of the poly(A) tail. Recent work has identified Ccr4p and Pop2p as components of the major cytoplasmic deadenylase in yeast. We now demonstrate that CCR4 encodes the catalytic subunit of the deadenylase and that Pop2p is dispensable for catalysis. In addition, we demonstrate that at least some of the Ccr4p/Pop2p-associated Not proteins are cytoplasmic, and lesions in some of the NOT genes can lead to defects in mRNA deadenylation rates. The Ccr4p deadenylase is inhibited in vitro by addition of the poly(A) binding protein (Pab1p), suggesting that dissociation of Pab1p from the poly(A) tail may be rate limiting for deadenylation in vivo. In addition, the rapid deadenylation of the COX17 mRNA, which is controlled by a member of the Pumilio family of deadenylation activators Puf3p, requires an active Ccr4p/Pop2p/Not deadenylase. These results define the Ccr4p/Pop2p/Not complex as the cytoplasmic deadenylase in yeast and identify positive and negative regulators of this enzyme complex.     

RNA degradation by the exosome is promoted by a nuclear polyadenylation complex

The exosome complex of 3'-5' exonucleases participates in RNA maturation and quality control and can rapidly degrade RNA-protein complexes in vivo. However, the purified exosome showed weak in vitro activity, indicating that rapid RNA degradation requires activating cofactors. This work identifies a nuclear polyadenylation complex containing a known exosome cofactor, the RNA helicase Mtr4p; a poly(A) polymerase, Trf4p; and a zinc knuckle protein, Air2p. In vitro, the Trf4p/Air2p/Mtr4p polyadenylation complex (TRAMP) showed distributive RNA polyadenylation activity. The presence of the exosome suppressed poly(A) tail addition, while TRAMP stimulated exosome degradation through structured RNA substrates. In vivo analyses showed that TRAMP is required for polyadenylation and degradation of rRNA and snoRNA precursors that are characterized exosome substrates. Poly(A) tails stimulate RNA degradation in bacteria, suggesting that this is their ancestral function. We speculate that this function was maintained in eukaryotic nuclei, while cytoplasmic mRNA poly(A) tails acquired different roles in translation.     

Analysis of the products of mRNA decapping and 3'-to-5' decay by denaturing gel electrophoresis

The majority of mRNA turnover is mediated either by mRNA decapping/5'-to-3' decay or exosome-mediated 3'-to-5' exonucleolytic decay. Current assays to assess mRNA decapping in vitro using cap-labeled RNA substrates rely on one-dimensional thin layer chromatography. This approach does not, however, resolve free phosphate from 7meGDP, the product of Dcp1p-mediated mRNA decapping. This can result in misinterpretation of the levels of mRNA decapping due to the generation of free phosphate following the action of the unrelated scavenger decapping activity on the products of exosome-mediated decay. In this report, we describe a simple denaturing acrylamide gel-based assay that faithfully resolves all of the possible products that can be generated from cap-labeled RNA substrates by turnover enzymes present in cell extracts. This approach allows a one-step assay to quantitatively assess the contributions of the exosome and DCP-1-type decapping on turnover of an RNA substrate in vitro. We have applied this assay to recalculate the effect of competition of cap-binding proteins on decapping in yeast. In addition, we have used the assay to confirm observations made on regulated mRNA decapping in mammalian extracts that contain much higher levels of exosome activity than yeast extracts.     

Characterization of deadenylation in trypanosome extracts and its inhibition by poly(A)-binding protein Pab1p

The stability of mRNAs is an important point in the regulation of gene expression in eukaryotes. The mRNA turnover pathways have been identified in yeast and mammals. However, mRNA turnover pathways in trypanosomes have not been widely studied. Deadenylation is the first step in the major mRNA turnover pathways of yeast and mammals. To better understand mRNA degradation processes in these organisms, we have developed an in vitro mRNA turnover system that is functional for deadenylation. In this system, addition of poly(A) homopolymer activates the deadenylation of poly(A) tails. The trypanosomal deadenylase activity is a 3'-->5' exonuclease specific for adenylate residues, generates 5'-AMP as a product, is magnesium dependent, and is inhibited by neomycin B sulfate. These characteristics suggest similarity with other eukaryotic deadenylases. Furthermore, this activity is cap independent, indicating a potential difference between the trypanosomal activity and PARN, but suggesting similarity to Ccr4p/Pop2p activities. Extracts immunodepleted of Pab1p required the addition of poly(A) competition to activate deadenylation. Trypanosomal Pab1p functions as an inhibitor of the activity under in vitro conditions. Pab1p appears to be one of several mRNA stability proteins in trypanosomal extracts.     

Plant mitochondrial polyadenylated mRNAs are degraded by a 3'- to 5'-exoribonuclease activity, which proceeds unimpeded by stable secondary structures

Recently, we and others have reported that mRNAs may be polyadenylated in plant mitochondria, and that polyadenylation accelerates the degradation rate of mRNAs. To further characterize the molecular mechanisms involved in plant mitochondrial mRNA degradation, we have analyzed the polyadenylation and degradation processes of potato atp9 mRNAs. The overall majority of polyadenylation sites of potato atp9 mRNAs is located at or in the vicinity of their mature 3'-extremities. We show that a 3'- to 5'-exoribonuclease activity is responsible for the preferential degradation of polyadenylated mRNAs as compared with non-polyadenylated mRNAs, and that 20-30 adenosine residues constitute the optimal poly(A) tail size for inducing degradation of RNA substrates in vitro. The addition of as few as seven non-adenosine nucleotides 3' to the poly(A) tail is sufficient to almost completely inhibit the in vitro degradation of the RNA substrate. Interestingly, the exoribonuclease activity proceeds unimpeded by stable secondary structures present in RNA substrates. From these results, we propose that in plant mitochondria, poly(A) tails added at the 3' ends of mRNAs promote an efficient 3'- to 5'- degradation process.     

Translation and identification of the viral mRNA species isolated from subcellular fractions of vesicular stomatitis virus-infected cells.

The cytoplasm of vesicular stomatitis virus (VSV)-infected BHK cells has been separated into a fraction containing the membrane-bound polysomes and the remaining supernatant fraction. Total poly(A)-containing RNA was isolated from each fraction and purified. A 17S class of VSV mRNA was found associated almost exclusively with the membrane-bound polysomes, whereas 14,5S and 12S RNAs were found mostly in the postmembrane cytoplasmic supernatant. Poly(A)-containing VSV RNA synthesized in vitro by purified virus was resolved into the same size classes. The individual RNA fractions isolated from VSV-infected cells or synthesized in vitro were translated in cell-free extracts of wheat germ, and their polypeptide products were compared by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. The corresponding in vivo and in vitro RNA fractions qualitatively direct the synthesis of the same viral polypeptides and therefore appear to contain the same mRNA species. By tryptic peptide analysis of their translation products, the in vivo VSV mRNA species have been identified. The 17S RNA, which is compartmentalized on membrane-bound polysomes, codes for a protein of molecular weight 63,000 (P-63) which is most probably a nonglycosylated form of the viral glycoprotein, G. Of the viral RNA species present in the remaining cytoplasmic supernatant, the 14.5S RNA codes almost exclusively for the N protein, whereas the 12S RNA codes predominantly for both the NS and M proteins of the virion.