RNA binding proteins Smaug and Cup induce CCR4-NOT-dependent deadenylation of the nanos mRNA in a reconstituted system.

Posttranscriptional regulation of the maternal nanos mRNA is essential for the development of the anterior - posterior axis of the Drosophila embryo. The nanos RNA is regulated by the protein Smaug, which binds to Smaug recognition elements (SREs) in the nanos 3'-UTR and nucleates the assembly of a larger repressor complex including the eIF4E-T paralog Cup and five additional proteins. The Smaug-dependent complex represses translation of nanos and induces its deadenylation by the CCR4-NOT deadenylase. Here we report an in vitro reconstitution of the Drosophila CCR4-NOT complex and Smaug-dependent deadenylation. We find that Smaug by itself is sufficient to cause deadenylation by the Drosophila or human CCR4-NOT complexes in an SRE-dependent manner. CCR4-NOT subunits NOT10 and NOT11 are dispensable, but the NOT module, consisting of NOT2, NOT3 and the C-terminal part of NOT1, is required. Smaug interacts with the C-terminal domain of NOT3. Both catalytic subunits of CCR4-NOT contribute to Smaug-dependent deadenylation. Whereas the CCR4-NOT complex itself acts distributively, Smaug induces a processive behavior. The cytoplasmic poly(A) binding protein (PABPC) has a minor inhibitory effect on Smaug-dependent deadenylation. Among the additional constituents of the Smaug-dependent repressor complex, Cup also facilitates CCR4-NOT-dependent deadenylation, both independently and in cooperation with Smaug.

Reconstitution of 3' end processing of mammalian pre-mRNA reveals a central role of RBBP6.

The 3' ends of almost all eukaryotic mRNAs are generated in an essential two-step processing reaction: endonucleolytic cleavage of an extended precursor followed by the addition of a poly(A) tail. By reconstituting the reaction from overproduced and purified proteins, we provide a minimal list of 14 polypeptides that are essential and two that are stimulatory for RNA processing. In a reaction depending on the polyadenylation signal AAUAAA, the reconstituted system cleaves pre-mRNA at a single preferred site corresponding to the one used in vivo. Among the proteins, cleavage factor I stimulates cleavage but is not essential, consistent with its prominent role in alternative polyadenylation. RBBP6 is required, with structural data showing it to contact and presumably activate the endonuclease CPSF73 through its DWNN domain. The C-terminal domain of RNA polymerase II is dispensable. ATP, but not its hydrolysis, supports RNA cleavage by binding to the hClp1 subunit of cleavage factor II with submicromolar affinity.

Precise Temporal Regulation of Post-transcriptional Repressors Is Required for an Orderly Drosophila Maternal-to-Zygotic Transition.

In animal embryos, the maternal-to-zygotic transition (MZT) hands developmental control from maternal to zygotic gene products. We show that the maternal proteome represents more than half of the protein-coding capacity of Drosophila melanogaster's genome, and that 2% of this proteome is rapidly degraded during the MZT. Cleared proteins include the post-transcriptional repressors Cup, Trailer hitch (TRAL), Maternal expression at 31B (ME31B), and Smaug (SMG). Although the ubiquitin-proteasome system is necessary for clearance of these repressors, distinct E3 ligase complexes target them: the C-terminal to Lis1 Homology (CTLH) complex targets Cup, TRAL, and ME31B for degradation early in the MZT and the Skp/Cullin/F-box-containing (SCF) complex targets SMG at the end of the MZT. Deleting the C-terminal 233 amino acids of SMG abrogates F-box protein interaction and confers immunity to degradation. Persistent SMG downregulates zygotic re-expression of mRNAs whose maternal contribution is degraded by SMG. Thus, clearance of SMG permits an orderly MZT.

Establishment of 5'-3' interactions in mRNA independent of a continuous ribose-phosphate backbone.

Functions of eukaryotic mRNAs are characterized by intramolecular interactions between their ends. We have addressed the question whether 5' and 3' ends meet by diffusion-controlled encounter "through solution" or by a mechanism involving the RNA backbone. For this purpose, we used a translation system derived from Drosophila embryos that displays two types of 5'-3' interactions: Cap-dependent translation initiation is stimulated by the poly(A) tail and inhibited by Smaug recognition elements (SREs) in the 3' UTR. Chimeric RNAs were made consisting of one RNA molecule carrying a luciferase coding sequence and a second molecule containing SREs and a poly(A) tail; the two were connected via a protein linker. The poly(A) tail stimulated translation of such chimeras even when disruption of the RNA backbone was combined with an inversion of the 5'-3' polarity between the open reading frame and poly(A) segment. Stimulation by the poly(A) tail also decreased with increasing RNA length. Both observations suggest that contacts between the poly(A) tail and the 5' end are established through solution, independently of the RNA backbone. In the same chimeric constructs, SRE-dependent inhibition of translation was also insensitive to disruption of the RNA backbone. Thus, tracking of the backbone is not involved in the repression of cap-dependent initiation. However, SRE-dependent repression was insensitive to mRNA length, suggesting that the contact between the SREs in the 3' UTR and the 5' end of the RNA might be established in a manner that differs from the contact between the poly(A) tail and the cap.

Reconstitution of mammalian cleavage factor II involved in 3' processing of mRNA precursors.

Cleavage factor II (CF II) is a poorly characterized component of the multiprotein complex catalyzing 3' cleavage and polyadenylation of mammalian mRNA precursors. We have reconstituted CF II as a heterodimer of hPcf11 and hClp1. The heterodimer is active in partially reconstituted cleavage reactions, whereas hClp1 by itself is not. Pcf11 moderately stimulates the RNA 5' kinase activity of hClp1; the kinase activity is dispensable for RNA cleavage. CF II binds RNA with nanomolar affinity. Binding is mediated mostly by the two zinc fingers in the C-terminal region of hPcf11. RNA is bound without pronounced sequence-specificity, but extended G-rich sequences appear to be preferred. We discuss the possibility that CF II contributes to the recognition of cleavage/polyadenylation substrates through interaction with G-rich far-downstream sequence elements.

Translational repression of the Drosophila nanos mRNA involves the RNA helicase Belle and RNA coating by Me31B and Trailer hitch.

Translational repression of maternal mRNAs is an essential regulatory mechanism during early embryonic development. Repression of the Drosophila nanos mRNA, required for the formation of the anterior-posterior body axis, depends on the protein Smaug binding to two Smaug recognition elements (SREs) in the nanos 3' UTR. In a comprehensive mass spectrometric analysis of the SRE-dependent repressor complex, we identified Smaug, Cup, Me31B, Trailer hitch, eIF4E, and PABPC, in agreement with earlier data. As a novel component, the RNA-dependent ATPase Belle (DDX3) was found, and its involvement in deadenylation and repression of nanos was confirmed in vivo. Smaug, Cup, and Belle bound stoichiometrically to the SREs, independently of RNA length. Binding of Me31B and Tral was also SRE-dependent, but their amounts were proportional to the length of the RNA and equimolar to each other. We suggest that "coating" of the RNA by a Me31B•Tral complex may be at the core of repression.

Deciphering the mRNP Code: RNA-Bound Determinants of Post-Transcriptional Gene Regulation.

Eukaryotic cells determine the final protein output of their genetic program not only by controlling transcription but also by regulating the localization, translation and turnover rates of their mRNAs. Ultimately, the fate of any given mRNA is determined by the ensemble of all associated RNA-binding proteins (RBPs), non-coding RNAs and metabolites collectively known as the messenger ribonucleoprotein particle (mRNP). Although many mRNA-associated factors have been identified over the past years, little is known about the composition of individual mRNPs and the cooperation of their constituents. In this review we discuss recent progress that has been made on how this 'mRNP code' is established on individual transcripts and how it is interpreted during gene expression in eukaryotic cells.

The nuclear poly(A) binding protein of mammals, but not of fission yeast, participates in mRNA polyadenylation.

The nuclear poly(A) binding protein (PABPN1) has been suggested, on the basis of biochemical evidence, to play a role in mRNA polyadenylation by strongly increasing the processivity of poly(A) polymerase. While experiments in metazoans have tended to support such a role, the results were not unequivocal, and genetic data show that the S. pombe ortholog of PABPN1, Pab2, is not involved in mRNA polyadenylation. The specific model in which PABPN1 increases the rate of poly(A) tail elongation has never been examined in vivo. Here, we have used 4-thiouridine pulse-labeling to examine the lengths of newly synthesized poly(A) tails in human cells. Knockdown of PABPN1 strongly reduced the synthesis of full-length tails of ∼250 nucleotides, as predicted from biochemical data. We have also purified S. pombe Pab2 and the S. pombe poly(A) polymerase, Pla1, and examined their in vitro activities. Whereas PABPN1 strongly increases the activity of its cognate poly(A) polymerase in vitro, Pab2 was unable to stimulate Pla1 to any significant extent. Thus, in vitro and in vivo data are consistent in supporting a role of PABPN1 but not S. pombe Pab2 in the polyadenylation of mRNA precursors.

Oligoadenylation of 3' decay intermediates promotes cytoplasmic mRNA degradation in Drosophila cells.

Post-transcriptional 3' end addition of nucleotides is important in a variety of RNA decay pathways. We have examined the 3' end addition of nucleotides during the decay of the Hsp70 mRNA and a corresponding reporter RNA in Drosophila S2 cells by conventional sequencing of cDNAs obtained after mRNA circularization and by deep sequencing of dedicated libraries enriched for 3' decay intermediates along the length of the mRNA. Approximately 5%-10% of 3' decay intermediates carried nonencoded oligo(A) tails with a mean length of 2-3 nucleotides. RNAi experiments showed that the oligoadenylated RNA fragments were intermediates of exosomal decay and the noncanonical poly(A) polymerase Trf4-1 was mainly responsible for A addition. A hot spot of A addition corresponded to an intermediate of 3' decay that accumulated upon inhibition of decapping, and knockdown of Trf4-1 increased the abundance of this intermediate, suggesting that oligoadenylation facilitates 3' decay. Oligoadenylated 3' decay intermediates were found in the cytoplasmic fraction in association with ribosomes, and fluorescence microscopy revealed a cytoplasmic localization of Trf4-1. Thus, oligoadenylation enhances exosomal mRNA degradation in the cytoplasm.

Mitochondrial poly(A) polymerase is involved in tRNA repair.

Transcription of the mitochondrial genome results in polycistronic precursors, which are processed mainly by the release of tRNAs interspersed between rRNAs and mRNAs. In many metazoan mitochondrial genomes some tRNA genes overlap with downstream genes; in the case of human mitochondria the genes for tRNA(Tyr) and tRNA(Cys) overlap by one nucleotide. It has previously been shown that processing of the common precursor releases an incomplete tRNA(Tyr) lacking the 3'-adenosine. The 3'-terminal adenosine has to be added before addition of the CCA end and subsequent aminoacylation. We show that the mitochondrial poly(A) polymerase (mtPAP) is responsible for this A addition. In vitro, a tRNA(Tyr) lacking the discriminator is a substrate for mtPAP. In vivo, an altered mtPAP protein level affected tRNA(Tyr) maturation, as shown by sequencing the 3' ends of mitochondrial tRNAs. Complete repair could be reconstituted in vitro with three enzymes: mtPAP frequently added more than one A to the 3' end of the truncated tRNA, and either the mitochondrial deadenylase PDE12 or the endonuclease RNase Z trimmed the oligo(A) tail to a single A before CCA addition. An enzyme machinery that evolved primarily for other purposes thus allows to tolerate the frequent evolutionary occurrence of gene overlaps.

Reconstitution of CPSF active in polyadenylation: recognition of the polyadenylation signal by WDR33.

Cleavage and polyadenylation specificity factor (CPSF) is the central component of the 3' processing machinery for polyadenylated mRNAs in metazoans: CPSF recognizes the polyadenylation signal AAUAAA, providing sequence specificity in both pre-mRNA cleavage and polyadenylation, and catalyzes pre-mRNA cleavage. Here we show that of the seven polypeptides that have been proposed to constitute CPSF, only four (CPSF160, CPSF30, hFip1, and WDR33) are necessary and sufficient to reconstitute a CPSF subcomplex active in AAUAAA-dependent polyadenylation, whereas CPSF100, CPSF73, and symplekin are dispensable. WDR33 is required for binding of reconstituted CPSF to AAUAAA-containing RNA and can be specifically UV cross-linked to such RNAs, as can CPSF30. Transcriptome-wide identification of WDR33 targets by photoactivatable ribonucleoside-enhanced cross-linking and immunoprecipitation (PAR-CLIP) showed that WDR33 binds in and very close to the AAUAAA signal in vivo with high specificity. Thus, our data indicate that the large CPSF subunit participating in recognition of the polyadenylation signal is WDR33 and not CPSF160, as suggested by previous studies.

Biophysical and biochemical analysis of hnRNP K: arginine methylation, reversible aggregation and combinatorial binding to nucleic acids.

Abstract Analysis of arginine methylation, which affects specific protein interactions in eukaryotic cells, requires access to methylated protein for biophysical and biochemical studies. Methylation of heterogeneous nuclear ribonucleoprotein K (hnRNP K) upon co-expression with protein arginine methyltransferase 1 in E. coli was monitored by mass spectrometry and found to be identical to the modification of hnRNP K purified from mammalian cells. Recombinant non-methylated and arginine-methylated hnRNP K (MethnRNP K) were used to characterize self-aggregation and nucleic acid binding. Analytical ultracentrifugation and static light scattering experiments revealed that hnRNP K methylation does not impact reversible self-aggregation, which can be prevented by high ionic strength and organic additives. Filter binding assays were used to compare the binding of non-methylated and MethnRNP K to the pyrimidine repeat-containing differentiation control element (DICE) of reticulocyte 15-lipoxygenase mRNA 3' UTR. No affinity differences were detected for both hnRNP K variants. A series of oligonucleotides carrying various numbers of C4 motifs at different positions was used in steady state competition assays with fluorescently-labeled functional differentiation control element (2R). Quantitative evaluation indicated that all hnRNP K homology domains of hnRNP K contribute differentially to RNA binding, with KH1-KH2 acting as a tandem domain and KH3 as an individual binding domain.

Deadenylation of mRNA by the CCR4-NOT complex in Drosophila: molecular and developmental aspects.

Controlled shortening of the poly(A) tail of mRNAs is the first step in eukaryotic mRNA decay and can also be used for translational inactivation of mRNAs. The CCR4-NOT complex is the most important among a small number of deadenylases, enzymes catalyzing poly(A) tail shortening. Rates of poly(A) shortening differ between mRNAs as the CCR4-NOT complex is recruited to specific mRNAs by means of either sequence-specific RNA binding proteins or miRNAs. This review summarizes our current knowledge concerning the subunit composition and deadenylation activity of the Drosophila CCR4-NOT complex and the mechanisms by which the complex is recruited to particular mRNAs. We discuss genetic data implicating the complex in the regulation of specific mRNAs, in particular in the context of development.

Crystal structures of the novel cytosolic 5'-nucleotidase IIIB explain its preference for m7GMP.

5'-nucleotidases catalyze the hydrolytic dephosphorylation of nucleoside monophosphates. As catabolic enzymes they contribute significantly to the regulation of cellular nucleotide levels; misregulation of nucleotide metabolism and nucleotidase deficiencies are associated with a number of diseases. The seven human 5'-nucleotidases differ with respect to substrate specificity and cellular localization. Recently, the novel cytosolic 5'-nucleotidase III-like protein, or cN-IIIB, has been characterized in human and Drosophila. cN-IIIB exhibits a strong substrate preference for the modified nucleotide 7-methylguanosine monophosphate but the structural reason for this preference was unknown. Here, we present crystal structures of cN-IIIB from Drosophila melanogaster bound to the reaction products 7-methylguanosine or cytidine. The structural data reveal that the cytosine- and 7-methylguanine moieties of the products are stacked between two aromatic residues in a coplanar but off-centered position. 7-methylguanosine is specifically bound through π-π interactions and distinguished from unmodified guanosine by additional cation-π coulomb interactions between the aromatic side chains and the positively charged 7-methylguanine. Notably, the base is further stabilized by T-shaped edge-to-face stacking of an additional tryptophan packing perpendicularly against the purine ring and forming, together with the other aromates, an aromatic slot. The structural data in combination with site-directed mutagenesis experiments reveal the molecular basis for the broad substrate specificity of cN-IIIB but also explain the substrate preference for 7-methylguanosine monophosphate. Analyzing the substrate specificities of cN-IIIB and the main pyrimidine 5'-nucleotidase cN-IIIA by mutagenesis studies, we show that cN-IIIA dephosphorylates the purine m7GMP as well, hence redefining its substrate spectrum. Docking calculations with cN-IIIA and m7GMP as well as biochemical data reveal that Asn69 does not generally exclude the turnover of purine substrates thus correcting previous suggestions.

Assaying mRNA deadenylation in vitro.

Deadenylation is the removal of poly(A) tails from mRNA. This chapter presents two methods to assay deadenylation in vitro. The first is a quick and quantitative assay for the degradation of radiolabeled poly(A) that can easily be adapted to be used for many different enzymes. The second method uses an extract from Drosophila embryos to catalyze the deadenylation of an RNA dependent on a specific sequence that also directs deadenylation in vivo.

Assaying mRNA deadenylation in vivo.

Deadenylation is the removal of poly(A) tails from mRNA. Here, we present two methods for assaying deadenylation in vivo. The first is a method for measuring bulk poly(A) tail lengths. When combined with a block in transcription, the method can be used for measuring the rate of bulk poly(A) tail shortening. The second is an RT-PCR method to determine the poly(A) tail lengths of individual RNAs. Again in combination with a block of transcription, the method permits the rate of deadenylation of an individual RNA to be measured.

Smaug destroys a huge treasure.

Smaug, a protein repressing translation and inducing mRNA decay, directly controls an unexpectedly large number of maternal mRNAs driving early Drosophila development.