C2ORF29/CNOT11 and CNOT10 form a new module of the CCR4-NOT complex

An RNA-Ligation-Based RACE-PAT Assay to Monitor Poly(A) Tail Length of mRNAs of Interest

In eukaryotes, a non-templated poly-adenosine (poly(A)) tail is added co-transcriptionally to almost every messenger RNA (mRNA). The length of this poly(A) tail changes during the lifetime of mRNAs and has been shown in many circumstances to be an important factor controlling transcript fates. Yet, the measure of the length of this homogenous nucleotide sequence is technically challenging, making it difficult to assess its dynamic variation. In this chapter, we describe an RNA-ligation-based RACE-PAT (Rapid Amplification of cDNA End-Poly(A) Tail) assay to monitor the poly(A) tail length of mRNAs. In the first step, an RNA oligonucleotide is ligated to mRNA 3' ends providing an anchoring site to prime cDNA synthesis, avoiding the bias introduced by oligo(dT)-derived primers. Afterward, reverse transcription is performed with an anchor primer with a unique 5' extension. The choice of the oligonucleotide 3' end at this step allows further flexibility to amplify modified tails, for example, by uridylation. Next, short DNA fragments encompassing the poly(A) tails are amplified by Polymerase Chain Reaction (PCR) using as forward primer, a transcript-specific primer hybridizing close to the transcript polyadenylation signal, and as reverse primer, an oligonucleotide corresponding to the 5' extension of the primer used for cDNA synthesis, ensuring that only cDNAs are amplified. The resulting DNA fragments are then visualized after size fractionation by electrophoresis. This method does not provide exact nucleotide count and composition but has the advantage of allowing the processing of many samples in parallel at a low cost.

Roles of the CCR4-Not complex in translation and dynamics of co-translation events

The Ccr4-Not complex is a global regulator of mRNA metabolism in eukaryotic cells that is most well-known to repress gene expression. Delivery of the complex to mRNAs through a multitude of distinct mechanisms accelerates their decay, yet Ccr4-Not also plays an important role in co-translational processes, such as co-translational association of proteins and delivery of translating mRNAs to organelles. The recent structure of Not5 interacting with the translated ribosome has brought to light that embedded information within the codon sequence can be monitored by recruitment of the Ccr4-Not complex to elongating ribosomes. Thereby, the Ccr4-Not complex is empowered with regulatory decisions determining the fate of proteins being synthesized and their encoding mRNAs. This review will focus on the roles of the complex in translation and dynamics of co-translation events. This article is categorized under: Translation > Mechanisms Translation > Regulation.

Structure and function of molecular machines involved in deadenylation-dependent 5'-3' mRNA degradation

In eukaryotic cells, the synthesis, processing, and degradation of mRNA are important processes required for the accurate execution of gene expression programmes. Fully processed cytoplasmic mRNA is characterised by the presence of a 5'cap structure and 3'poly(A) tail. These elements promote translation and prevent non-specific degradation. Degradation via the deadenylation-dependent 5'-3' degradation pathway can be induced by trans-acting factors binding the mRNA, such as RNA-binding proteins recognising sequence elements and the miRNA-induced repression complex. These factors recruit the core mRNA degradation machinery that carries out the following steps: i) shortening of the poly(A) tail by the Ccr4-Not and Pan2-Pan3 poly (A)-specific nucleases (deadenylases); ii) removal of the 5'cap structure by the Dcp1-Dcp2 decapping complex that is recruited by the Lsm1-7-Pat1 complex; and iii) degradation of the mRNA body by the 5'-3' exoribonuclease Xrn1. In this review, the biochemical function of the nucleases and accessory proteins involved in deadenylation-dependent mRNA degradation will be reviewed with a particular focus on structural aspects of the proteins and enzymes involved.

Specific recognition and ubiquitination of translating ribosomes by mammalian CCR4-NOT

Translation affects messenger RNA stability and, in yeast, this is mediated by the Ccr4-Not deadenylation complex. The details of this process in mammals remain unclear. Here, we use cryogenic electron microscopy (cryo-EM) and crosslinking mass spectrometry to show that mammalian CCR4-NOT specifically recognizes ribosomes that are stalled during translation elongation in an in vitro reconstituted system with rabbit and human components. Similar to yeast, mammalian CCR4-NOT inserts a helical bundle of its CNOT3 subunit into the empty E site of the ribosome. Our cryo-EM structure shows that CNOT3 also locks the L1 stalk in an open conformation to inhibit further translation. CCR4-NOT is required for stable association of the nonconstitutive subunit CNOT4, which ubiquitinates the ribosome, likely to signal stalled translation elongation. Overall, our work shows that human CCR4-NOT not only detects but also enforces ribosomal stalling to couple translation and mRNA decay.

Structure and assembly of the NOT10:11 module of the CCR4-NOT complex

NOT1, NOT10, and NOT11 form a conserved module in the CCR4-NOT complex, critical for post-transcriptional regulation in eukaryotes, but how this module contributes to the functions of the CCR4-NOT remains poorly understood. Here, we present cryo-EM structures of human and chicken NOT1:NOT10:NOT11 ternary complexes to sub-3 Å resolution, revealing an evolutionarily conserved, flexible structure. Through biochemical dissection studies, which include the Drosophila orthologs, we show that the module assembly is hierarchical, with NOT11 binding to NOT10, which then organizes it for binding to NOT1. A short proline-rich motif in NOT11 stabilizes the entire module assembly.

Mechanism of ribosome-associated mRNA degradation during tubulin autoregulation

Microtubules play crucial roles in cellular architecture, intracellular transport, and mitosis. The availability of free tubulin subunits affects polymerization dynamics and microtubule function. When cells sense excess free tubulin, they trigger degradation of the encoding mRNAs, which requires recognition of the nascent polypeptide by the tubulin-specific ribosome-binding factor TTC5. How TTC5 initiates the decay of tubulin mRNAs is unknown. Here, our biochemical and structural analysis reveals that TTC5 recruits the poorly studied protein SCAPER to the ribosome. SCAPER, in turn, engages the CCR4-NOT deadenylase complex through its CNOT11 subunit to trigger tubulin mRNA decay. SCAPER mutants that cause intellectual disability and retinitis pigmentosa in humans are impaired in CCR4-NOT recruitment, tubulin mRNA degradation, and microtubule-dependent chromosome segregation. Our findings demonstrate how recognition of a nascent polypeptide on the ribosome is physically linked to mRNA decay factors via a relay of protein-protein interactions, providing a paradigm for specificity in cytoplasmic gene regulation.

Regulation of eukaryotic mRNA deadenylation and degradation by the Ccr4-Not complex

Accurate and precise regulation of gene expression programmes in eukaryotes involves the coordinated control of transcription, mRNA stability and translation. In recent years, significant progress has been made about the role of sequence elements in the 3' untranslated region for the regulation of mRNA degradation, and a model has emerged in which recruitment of the Ccr4-Not complex is the critical step in the regulation of mRNA decay. Recruitment of the Ccr4-Not complex to a target mRNA results in deadenylation mediated by the Caf1 and Ccr4 catalytic subunits of the complex. Following deadenylation, the 5' cap structure is removed, and the mRNA subjected to 5'-3' degradation. Here, the role of the human Ccr4-Not complex in cytoplasmic deadenylation of mRNA is reviewed, with a particular focus on mechanisms of its recruitment to mRNA by sequence motifs in the 3' untranslated region, codon usage, as well as general mechanisms involving the poly(A) tail.

The Dynamic Poly(A) Tail Acts as a Signal Hub in mRNA Metabolism

In eukaryotes, mRNA metabolism requires a sophisticated signaling system. Recent studies have suggested that polyadenylate tail may play a vital role in such a system. The poly(A) tail used to be regarded as a common modification at the 3' end of mRNA, but it is now known to be more than just that. It appears to act as a platform or hub that can be understood in two ways. On the one hand, polyadenylation and deadenylation machinery constantly regulates its dynamic activity; on the other hand, it exhibits the ability to recruit RNA-binding proteins and then interact with diverse factors to send various signals to regulate mRNA metabolism. In this paper, we outline the main complexes that regulate the dynamic activities of poly(A) tails, explain how these complexes participate polyadenylation/deadenylation process and summarize the diverse signals this hub emit. We are trying to make a point that the poly(A) tail can metaphorically act as a "flagman" who is supervised by polyadenylation and deadenylation and sends out signals to regulate the orderly functioning of mRNA metabolism.

Delineation of the clinical profile of CNOT2 haploinsufficiency and overview of the IDNADFS phenotype

CNOT2 haploinsufficiency underlies a rare neurodevelopmental disorder named Intellectual Developmental disorder with NAsal speech, Dysmorphic Facies, and variable Skeletal anomalies (IDNADFS, OMIM 618608). The condition clinically overlaps with chromosome 12q15 deletion syndrome, suggesting a major contribution of CNOT2 haploinsufficiency to the latter. CNOT2 is a member of the CCR4-NOT complex, which is a master regulator of multiple cellular processes, including gene expression, RNA deadenylation, and protein ubiquitination. To date, less than 20 pathogenic 12q15 microdeletions encompassing CNOT2, together with a single truncating variant of the gene, and two large intragenic deletions have been reported. Due to the small number of affected subjects described so far, the clinical profile of IDNADFS has not been fully delineated. Here we report five unrelated individuals, three of which carrying de novo intragenic CNOT2 variants, one presenting with a multiexon intragenic deletion, and an additional case of 12q15 microdeletion syndrome. Finally, we assess the features of IDNADFS by reviewing published and present affected individuals and reevaluate the clinical phenotype of this neurodevelopmental disorder.

The human CNOT1-CNOT10-CNOT11 complex forms a structural platform for protein-protein interactions

The evolutionary conserved CCR4-NOT complex functions in the cytoplasm as the main mRNA deadenylase in both constitutive mRNA turnover and regulated mRNA decay pathways. The versatility of this complex is underpinned by its modular multi-subunit organization, with distinct structural modules actuating different functions. The structure and function of all modules are known, except for that of the N-terminal module. Using different structural approaches, we obtained high-resolution data revealing the architecture of the human N-terminal module composed of CNOT1, CNOT10, and CNOT11. The structure shows how two helical domains of CNOT1 sandwich CNOT10 and CNOT11, leaving the most conserved domain of CNOT11 protruding into solvent as an antenna. We discovered that GGNBP2, a protein identified as a tumor suppressor and spermatogenic factor, is a conserved interacting partner of the CNOT11 antenna domain. Structural and biochemical analyses thus pinpoint the N-terminal CNOT1-CNOT10-CNOT11 module as a conserved protein-protein interaction platform.

Molecular Insights into mRNA Polyadenylation and Deadenylation

Poly(A) tails are present on almost all eukaryotic mRNAs, and play critical roles in mRNA stability, nuclear export, and translation efficiency. The biosynthesis and shortening of a poly(A) tail are regulated by large multiprotein complexes. However, the molecular mechanisms of these protein machineries still remain unclear. Recent studies regarding the structural and biochemical characteristics of those protein complexes have shed light on the potential mechanisms of polyadenylation and deadenylation. This review summarizes the recent structural studies on pre-mRNA 3′-end processing complexes that initiate the polyadenylation and discusses the similarities and differences between yeast and human machineries. Specifically, we highlight recent biochemical efforts in the reconstitution of the active human canonical pre-mRNA 3′-end processing systems, as well as the roles of RBBP6/Mpe1 in activating the entire machinery. We also describe how poly(A) tails are removed by the PAN2-PAN3 and CCR4-NOT deadenylation complexes and discuss the emerging role of the cytoplasmic poly(A)-binding protein (PABPC) in promoting deadenylation. Together, these recent discoveries show that the dynamic features of these machineries play important roles in regulating polyadenylation and deadenylation.

RNF219 regulates CCR4-NOT function in mRNA translation and deadenylation

Post-transcriptional regulatory mechanisms play a role in many biological contexts through the control of mRNA degradation, translation and localization. Here, we show that the RING finger protein RNF219 co-purifies with the CCR4-NOT complex, the major mRNA deadenylase in eukaryotes, which mediates translational repression in both a deadenylase activity-dependent and -independent manner. Strikingly, RNF219 both inhibits the deadenylase activity of CCR4-NOT and enhances its capacity to repress translation of a target mRNA. We propose that the interaction of RNF219 with the CCR4-NOT complex directs the translational repressive activity of CCR4-NOT to a deadenylation-independent mechanism.

RNF219 attenuates global mRNA decay through inhibition of CCR4-NOT complex-mediated deadenylation

The CCR4-NOT complex acts as a central player in the control of mRNA turnover and mediates accelerated mRNA degradation upon HDAC inhibition. Here, we explored acetylation-induced changes in the composition of the CCR4-NOT complex by purification of the endogenously tagged scaffold subunit NOT1 and identified RNF219 as an acetylation-regulated cofactor. We demonstrate that RNF219 is an active RING-type E3 ligase which stably associates with CCR4-NOT via NOT9 through a short linear motif (SLiM) embedded within the C-terminal low-complexity region of RNF219. By using a reconstituted six-subunit human CCR4-NOT complex, we demonstrate that RNF219 inhibits deadenylation through the direct interaction of the α-helical SLiM with the NOT9 module. Transcriptome-wide mRNA half-life measurements reveal that RNF219 attenuates global mRNA turnover in cells, with differential requirement of its RING domain. Our results establish RNF219 as an inhibitor of CCR4-NOT-mediated deadenylation, whose loss upon HDAC inhibition contributes to accelerated mRNA turnover.

Crystal structure and functional properties of the human CCR4-CAF1 deadenylase complex

The CCR4 and CAF1 deadenylases physically interact to form the CCR4-CAF1 complex and function as the catalytic core of the larger CCR4-NOT complex. Together, they are responsible for the eventual removal of the 3′-poly(A) tail from essentially all cellular mRNAs and consequently play a central role in the posttranscriptional regulation of gene expression. The individual properties of CCR4 and CAF1, however, and their respective contributions in different organisms and cellular environments are incompletely understood. Here, we determined the crystal structure of a human CCR4-CAF1 complex and characterized its enzymatic and substrate recognition properties. The structure reveals specific molecular details affecting RNA binding and hydrolysis, and confirms the CCR4 nuclease domain to be tethered flexibly with a considerable distance between both enzyme active sites. CCR4 and CAF1 sense nucleotide identity on both sides of the 3′-terminal phosphate, efficiently differentiating between single and consecutive non-A residues. In comparison to CCR4, CAF1 emerges as a surprisingly tunable enzyme, highly sensitive to pH, magnesium and zinc ions, and possibly allowing distinct reaction geometries. Our results support a picture of CAF1 as a primordial deadenylase, which gets assisted by CCR4 for better efficiency and by the assembled NOT proteins for selective mRNA targeting and regulation.

The functional characterization of phosphorylation of tristetraprolin at C-terminal NOT1-binding domain

Background

Tristetraprolin (TTP) family proteins contain conserved tandem CCCH zinc-finger binding to AU-rich elements and C-terminal NOT1-binding domain. TTP is phosphorylated extensively in cells, and its mRNA destabilization activity is regulated by protein phosphorylation.

Methods

We generated an antibody against phospho-Serine316 located at the C-terminal NOT1-binding site and examined TTP phosphorylation in LPS-stimulated RAW264.7 cells. Knockout of TTP was created in RAW264.7 cells using CRISPR/Cas9 gene editing to explore TTP functions.

Results

We demonstrated that Ser316 was phosphorylated by p90 ribosomal S6 kinase 1 (RSK1) and p38-activated protein kinase (MK2) and dephosphorylated by Protein Phosphatase 2A (PP2A). A phosphorylation-mimic mutant of S316D resulted in dissociation with the CCR4-NOT deadenylase complex through weakening interaction with CNOT1. Furthermore, Ser316 and serines 52 and 178 were independently contributed to the CCR4-NOT complex recruitment in the immunoprecipitation assay using phosphor-mimic mutants. In RAW264.7 macrophages, TTP was induced, and Ser316 was phosphorylated through RSK1 and MK2 by LPS stimulation. Knockout of TTP resulted in TNFα mRNA increased due to mRNA stabilization. Overexpression of non-phosphorylated S316A TTP mutant can restore TTP activity and lead to TNFα mRNA decreased. GST pull-down and RNA pull-down analyses demonstrated that endogenous TTP with Ser316 phosphorylation decreased the interaction with CNOT1.

Conclusions

Our results suggest that the TTP-mediated mRNA stability is modulated by Ser316 phosphorylation via regulating the TTP interaction with the CCR4-NOT deadenylase complex.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12950-021-00288-2.

A conserved motif in human BTG1 and BTG2 proteins mediates interaction with the poly(A) binding protein PABPC1 to stimulate mRNA deadenylation

Antiproliferative BTG/Tob proteins interact directly with the CAF1 deadenylase subunit of the CCR4-NOT complex. This binding requires the presence of two conserved motifs, boxA and boxB, characteristic of the BTG/Tob APRO domain. Consistently, these proteins were shown to stimulate mRNA deadenylation and decay in several instances. Two members of the family, BTG1 and BTG2, were reported further to associate with the protein arginine methyltransferase PRMT1 through a motif, boxC, conserved only in this subset of proteins. We recently demonstrated that BTG1 and BTG2 also contact the first RRM domain of the cytoplasmic poly(A) binding protein PABPC1. To decipher the mode of interaction of BTG1 and BTG2 with partners, we performed nuclear magnetic resonance experiments as well as mutational and biochemical analyses. Our data demonstrate that, in the context of an APRO domain, the boxC motif is necessary and sufficient to allow interaction with PABPC1 but, unexpectedly, that it is not required for BTG2 association with PRMT1. We show further that the presence of a boxC motif in an APRO domain endows it with the ability to stimulate deadenylation in cellulo and in vitro. Overall, our results identify the molecular interface allowing BTG1 and BTG2 to activate deadenylation, a process recently shown to be necessary for maintaining T-cell quiescence.

Genome-Wide Study of NOT2_3_5 Protein Subfamily in Cotton and Their Necessity in Resistance to Verticillium wilt

The Negative on TATA-less (NOT) 2_3_5 domain proteins play key roles in mRNA metabolism and transcription regulation, but few comprehensive studies have focused on this protein family in plants. In our study, a total of 30 NOT2_3_5 genes were identified in four cotton genomes: Gossypium. arboretum, G. raimondii, G. hirsutum and G. barbadense. Phylogenetic analysis showed that all the NOT2_3_5 domain proteins were divided into two classes. The NOT2_3_5 genes were expanded frequently, and segmental duplication had significant effects in their expansion process. The cis-regulatory elements analysis of NOT2_3_5 promoter regions indicated that NOT2_3_5 domain proteins might participate in plant growth and development processes and responds to exogenous stimuli. Expression patterns demonstrated that all of the GhNOT2_3_5 genes were expressed in the majority of tissues and fiber development stages, and that these genes were induced by multiple stresses. Quantitative real-time PCR showed that GbNOT2_3_5 genes were up-regulated in response to verticillium wilt and the silencing of GbNOT2_3_5-3/8 and GbNOT2_3_5-4/9 led to more susceptibility to verticillium wilt than controls. Identification and analysis of the NOT2_3_5 protein family will be beneficial for further research on their biological functions.

RNA decay machinery safeguards immune cell development and immunological responses

mRNA decay systems control mRNA abundance by counterbalancing transcription. Several recent studies show that mRNA decay pathways are crucial to conventional T and B cell development in vertebrates, in addition to suppressing autoimmunity and excessive inflammatory responses. Selective mRNA degradation triggered by the CCR4-NOT deadenylase complex appears to be required in lymphocyte development, cell quiescence, V(D)J (variable-diversity-joining) recombination, and prevention of inappropriate apoptosis in mice. Moreover, a recent study suggests that mRNA decay may be involved in preventing human hyperinflammatory disease. These findings imply that mRNA decay pathways in humans and mice do not simply maintain mRNA homeostatic turnover but can also precisely regulate immune development and immunological responses by selectively targeting mRNAs.

The Regulatory Properties of the Ccr4-Not Complex

The mammalian Ccr4–Not complex, carbon catabolite repression 4 (Ccr4)-negative on TATA-less (Not), is a large, highly conserved, multifunctional assembly of proteins that acts at different cellular levels to regulate gene expression. In the nucleus, it is involved in the regulation of the cell cycle, chromatin modification, activation and inhibition of transcription initiation, control of transcription elongation, RNA export, nuclear RNA surveillance, and DNA damage repair. In the cytoplasm, the Ccr4–Not complex plays a central role in mRNA decay and affects protein quality control. Most of our original knowledge of the Ccr4–Not complex is derived, primarily, from studies in yeast. More recent studies have shown that the mammalian complex has a comparable structure and similar properties. In this review, we summarize the evidence for the multiple roles of both the yeast and mammalian Ccr4–Not complexes, highlighting their similarities.

Analysis of CNOT Family Gene Expression, Clinicopathological Features, and Prognosis Value in Hepatocellular Carcinoma

The carbon catabolite repressor 4-negative on TATA (CCR4-NOT) complex, abbreviated CNOT, has deadenylation and 3'-5' exonuclease activity, mediates deadenylation in the degradation of RNA, initiates the exonuclease degradation pathway, and participates in tumor gene regulation. CNOT proteins comprise a family of global transcriptional regulators that are evolutionarily conserved in eukaryotic cells. Several subunit types of the CNOT complex have been discovered; however, little is known about the role of different subunits in tumorigenesis and development. We observed overexpression of CNOT1-11 in liver cancer and correlations with clinicopathological characteristics. The expression of some CNOTs subunits was associated with histological grades, lymph node metastasis, and tumor stages in patients with hepatocellular carcinoma (HCC). Our data suggested that some CNOTs can be used as predictors of poor prognosis in HCC patients. At the same time, we conducted an analysis of CNOTs mutations in HCC patients. Moreover, we selected CNOT6, CNOT10, and CNOT11 for protein interaction network analysis and Gene Ontology enrichment analysis to investigate their related biological processes and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Finally, the results of western blot and quantitative reverse transcription-PCR (qRT-PCR) experiments were consistent with the database findings. Results of this study suggest that CNOT6, CNOT10, and CNOT11, acting as regulators of transcription, may play an important role in the development of HCC and may serve as biological markers in the diagnosis and prognosis of HCC.

Intra-species differences in population size shape life history and genome evolution

The evolutionary forces shaping life history divergence within species are largely unknown. Turquoise killifish display differences in lifespan among wild populations, representing an ideal natural experiment in evolution and diversification of life history. By combining genome sequencing and population genetics, we investigate the evolutionary forces shaping lifespan among wild turquoise killifish populations. We generate an improved reference genome assembly and identify genes under positive and purifying selection, as well as those evolving neutrally. Short-lived populations from the outer margin of the species range have small population size and accumulate deleterious mutations in genes significantly enriched in the WNT signaling pathway, neurodegeneration, cancer and the mTOR pathway. We propose that limited population size due to habitat fragmentation and repeated population bottlenecks, by increasing the genome-wide mutation load, exacerbates the effects of mutation accumulation and cumulatively contribute to the short adult lifespan.

A Quantitative Genetic Interaction Map of HIV Infection

We have developed a platform for quantitative genetic interaction mapping using viral infectivity as a functional readout and constructed a viral host-dependency epistasis map (vE-MAP) of 356 human genes linked to HIV function, comprising >63,000 pairwise genetic perturbations. The vE-MAP provides an expansive view of the genetic dependencies underlying HIV infection and can be used to identify drug targets and study viral mutations. We found that the RNA deadenylase complex, CNOT, is a central player in the vE-MAP and show that knockout of CNOT1, 10, and 11 suppressed HIV infection in primary T cells by upregulating innate immunity pathways. This phenotype was rescued by deletion of IRF7, a transcription factor regulating interferon-stimulated genes, revealing a previously unrecognized host signaling pathway involved in HIV infection. The vE-MAP represents a generic platform that can be used to study the global effects of how different pathogens hijack and rewire the host during infection.

HELZ directly interacts with CCR4-NOT and causes decay of bound mRNAs

The putative UPF1-like SF1 helicase HELZ directly interacts with the CCR4–NOT deadenylase complex to induce translational repression and 5′-to-3′ decay of bound mRNAs.

Eukaryotic superfamily (SF) 1 helicases have been implicated in various aspects of RNA metabolism, including transcription, processing, translation, and degradation. Nevertheless, until now, most human SF1 helicases remain poorly understood. Here, we have functionally and biochemically characterized the role of a putative SF1 helicase termed “helicase with zinc-finger,” or HELZ. We discovered that HELZ associates with various mRNA decay factors, including components of the carbon catabolite repressor 4-negative on TATA box (CCR4–NOT) deadenylase complex in human and Drosophila melanogaster cells. The interaction between HELZ and the CCR4–NOT complex is direct and mediated by extended low-complexity regions in the C-terminal part of the protein. We further reveal that HELZ requires the deadenylase complex to mediate translational repression and decapping-dependent mRNA decay. Finally, transcriptome-wide analysis of Helz-null cells suggests that HELZ has a role in the regulation of the expression of genes associated with the development of the nervous system.

Identification of Arabidopsis CCR4-NOT Complexes with Pumilio RNA-Binding Proteins, APUM5 and APUM2

CCR4/CAF1 are widely conserved deadenylases in eukaryotes. They form a large complex that includes NOT1 as a scaffold protein and various NOT proteins that are core components of multiple levels of gene expression control. The CCR4-NOT complex also contains several RNA-binding proteins as accessory proteins, which are required for target recognition by CCR4/CAF1 deadenylases. AtCCR4a/b, orthologs of human CCR4 in Arabidopsis, have various physiological effects. AtCCR4 isoforms are likely to have specific target mRNAs related to each physiological effect; however, AtCCR4 does not have RNA-binding capability. Therefore, identifying factors that interact with AtCCR4a/b is indispensable to understand its function as a regulator of gene expression, as well as the target mRNA recognition mechanism. Here, we identified putative components of the AtCCR4-NOT complex using co-immunoprecipitation in combination with mass spectrometry using FLAG-tagged AtCCR4b and subsequent verification with a yeast two-hybrid assay. Interestingly, four of 11 AtCAF1 isoforms interacted with both AtCCR4b and AtNOT1, whereas two isoforms interacted only with AtNOT1 in yeast two-hybrid assays. These results imply that Arabidopsis has multiple CCR4-NOT complexes with various combinations of deadenylases. We also revealed that the RNA-binding protein Arabidopsis Pumilio 5 and 2 interacted with AtCCR4a/b in the cytoplasm with a few foci.

Reconstitution of recombinant human CCR4-NOT reveals molecular insights into regulated deadenylation

CCR4-NOT is a conserved multiprotein complex which regulates eukaryotic gene expression principally via shortening of poly(A) tails of messenger RNA or deadenylation. Here, we reconstitute a complete, recombinant human CCR4-NOT complex. Our reconstitution strategy permits strict compositional control to test mechanistic hypotheses with purified component variants. CCR4-NOT is more active and selective for poly(A) than the isolated exonucleases, CCR4a and CAF1, which have distinct deadenylation profiles in vitro. The exonucleases require at least two out of three conserved non-enzymatic modules (CAF40, NOT10:NOT11 or NOT) for full activity in CCR4-NOT. CAF40 and the NOT10:NOT11 module both bind RNA directly and stimulate deadenylation in a partially redundant manner. Linear motifs from different RNA-binding factors that recruit CCR4-NOT to specific mRNAs via protein-protein interactions with CAF40 can inhibit bulk deadenylation. We reveal an additional layer of regulatory complexity to the human deadenylation machinery, which may prime it either for general or target-specific degradation.

The CCR4-NOT complex shortens poly(A) tails of messenger RNAs. By biochemical reconstitution of the entire human CCR4-NOT complex, the authors show the stimulatory roles of non-enzymatic subunits and the importance of the interaction between CAF40 and RNA binding proteins in targeted deadenylation.

The Ccr4-Not Complex: Architecture and Structural Insights

The Ccr4-Not complex is an essential multi-subunit protein complex that plays a fundamental role in eukaryotic mRNA metabolism and has a multitude of different roles that impact eukaryotic gene expression . It has a conserved core of three Not proteins, the Ccr4 protein, and two Ccr4 associated factors, Caf1 and Caf40. A fourth Not protein, Not4, is conserved, but is only a stable subunit of the complex in yeast. Certain subunits have been duplicated during evolution, with functional divergence, such as Not3 in yeast, and Ccr4 or Caf1 in human. However the complex includes only one homolog for each protein. In addition, species-specific subunits are part of the complex, such as Caf130 in yeast or Not10 and Not11 in human. Two conserved catalytic functions are associated with the complex, deadenylation and ubiquitination . The complex adopts an L-shaped structure, in which different modules are bound to a large Not1 scaffold protein. In this chapter we will summarize our current knowledge of the architecture of the complex and of the structure of its constituents.

1-Hydroxy-xanthine derivatives inhibit the human Caf1 nuclease and Caf1-containing nuclease complexes via Mg2+-dependent binding

In eukaryotic cells, cytoplasmic mRNA is characterised by a 3′ poly(A) tail. The shortening and removal of poly(A) tails (deadenylation) by the Ccr4‐Not nuclease complex leads to reduced translational efficiency and RNA degradation. Using recombinant human Caf1 (CNOT7) enzyme as a screening tool, we recently described the discovery and synthesis of a series of substituted 1‐hydroxy‐3,7‐dihydro‐1H‐purine‐2,6‐diones (1‐hydroxy‐xanthines) as inhibitors of the Caf1 catalytic subunit of the Ccr4‐Not complex. Here, we used a chemiluminescence‐based AMP detection assay to show that active 1‐hydroxy‐xanthines inhibit both isolated Caf1 enzyme and human Caf1‐containing complexes that also contain the second nuclease subunit Ccr4 (CNOT6L) to a similar extent, indicating that the active site of the Caf1 nuclease subunit does not undergo substantial conformational change when bound to other Ccr4‐Not subunits. Using differential scanning fluorimetry, we also show that binding of active 1‐hydroxy‐xanthines requires the presence of Mg²⁺ ions, which are present in the active site of Caf1.

A conserved CAF40-binding motif in metazoan NOT4 mediates association with the CCR4-NOT complex

The multisubunit CCR4-NOT mRNA deadenylase complex plays important roles in the posttranscriptional regulation of gene expression. The NOT4 E3 ubiquitin ligase is a stable component of the CCR4-NOT complex in yeast but does not copurify with the human or Drosophila melanogaster complex. Here we show that the C-terminal regions of human and D. melanogaster NOT4 contain a conserved sequence motif that directly binds the CAF40 subunit of the CCR4-NOT complex (CAF40-binding motif [CBM]). In addition, nonconserved sequences flanking the CBM also contact other subunits of the complex. Crystal structures of the CBM-CAF40 complex reveal a mutually exclusive binding surface for NOT4 and Roquin or Bag of marbles mRNA regulatory proteins. Furthermore, CAF40 depletion or structure-guided mutagenesis to disrupt the NOT4-CAF40 interaction impairs the ability of NOT4 to elicit decay of tethered reporter mRNAs in cells. Together with additional sequence analyses, our results reveal the molecular basis for the association of metazoan NOT4 with the CCR4-NOT complex and show that it deviates substantially from yeast. They mark the NOT4 ubiquitin ligase as an ancient but nonconstitutive cofactor of the CCR4-NOT deadenylase with potential recruitment and/or effector functions.

The central region of CNOT1 and CNOT9 stimulates deadenylation by the Ccr4-Not nuclease module

Regulated degradation of cytoplasmic mRNA is important for the accurate execution of gene expression programmes in eukaryotic cells. A key step in this process is the shortening and removal of the mRNA poly(A) tail, which can be achieved by the recruitment of the multi-subunit Ccr4-Not nuclease complex via sequence-specific RNA-binding proteins or the microRNA machinery. The Ccr4-Not complex contains several modules that are attached to its large subunit CNOT1. Modules include the nuclease module, which associates with the MIF4G domain of CNOT1 and contains the catalytic subunits Caf1 and Ccr4, as well as the module containing the non-catalytic CNOT9 subunit, which binds to the DUF3819 domain of CNOT1. To understand the contributions of the individual modules to the activity of the complex, we have started to reconstitute sub-complexes of the human Ccr4-Not complex containing one or several functional modules. Here, we report the reconstitution of a pentameric complex including a BTG2-Caf1-Ccr4 nuclease module, CNOT9 and the central region of CNOT1 encompassing the MIF4G and DUF3819 domains. By comparing the biochemical activities of the pentameric complex and the nuclease module, we conclude that the CNOT1-CNOT9 components stimulate deadenylation by the nuclease module. In addition, we show that a pentameric complex containing the melanoma-associated CNOT9 P131L variant is able to support deadenylation similar to a complex containing the wild-type CNOT9 protein.

Structural and biochemical analysis of a NOT1 MIF4G-like domain of the CCR4-NOT complex

The CCR4-NOT complex plays a central role in the regulation of gene expression and degradation of messenger RNAs. The multisubunit complex assembles on the NOT1 protein, which acts as a 'scaffold' and is highly conserved in eukaryotes. NOT1 consists of a series of helical domains that serve as docking sites for other CCR4-NOT subunits. We describe a crystal structure of a connector domain of NOT1 from the thermophilic fungus Chaetomium thermophilum (Ct). Comparative structural analysis indicates that this domain adopts a MIF4G-like fold and we have termed it the MIF4G-C domain. Solution scattering studies indicate that the human MIF4G-C domain likely adopts a very similar fold to the Ct MIF4G-C. MIF4G domains have been described to mediate interactions with DEAD-box helicases such as DDX6. However, comparison of the interfaces of the MIF4G-C with the MIF4G domain of NOT1 that interacts with DDX6 reveals key structural differences that explain why the MIF4G-C does not bind DDX6. We further show that the human MIF4G-C does not interact stably with other subunits of the CCR4-NOT complex. The structural conservation of the MIF4G-C domain suggests that it may have an important but presently undefined role in the CCR4-NOT complex.

Drosophila Bag-of-marbles directly interacts with the CAF40 subunit of the CCR4-NOT complex to elicit repression of mRNA targets

Drosophila melanogaster Bag-of-marbles (Bam) promotes germline stem cell (GSC) differentiation by repressing the expression of mRNAs encoding stem cell maintenance factors. Bam interacts with Benign gonial cell neoplasm (Bgcn) and the CCR4 deadenylase, a catalytic subunit of the CCR4-NOT complex. Bam has been proposed to bind CCR4 and displace it from the CCR4-NOT complex. Here, we investigated the interaction of Bam with the CCR4-NOT complex by using purified recombinant proteins. Unexpectedly, we found that Bam does not interact with CCR4 directly but instead binds to the CAF40 subunit of the complex in a manner mediated by a conserved N-terminal CAF40-binding motif (CBM). The crystal structure of the Bam CBM bound to CAF40 reveals that the CBM peptide adopts an α-helical conformation after binding to the concave surface of the crescent-shaped CAF40 protein. We further show that Bam-mediated mRNA decay and translational repression depend entirely on Bam's interaction with CAF40. Thus, Bam regulates the expression of its mRNA targets by recruiting the CCR4-NOT complex through interaction with CAF40.

Tryptophan-Mediated Interactions between Tristetraprolin and the CNOT9 Subunit Are Required for CCR4-NOT Deadenylase Complex Recruitment

The zinc-finger protein tristetraprolin (TTP) binds to AU-rich elements present in the 3' untranslated regions of transcripts that mainly encode proteins of the inflammatory response. TTP-bound mRNAs are targeted for destruction via recruitment of the eight-subunit deadenylase complex "carbon catabolite repressor protein 4 (CCR4)-negative on TATA-less (NOT)," which catalyzes the removal of mRNA poly-(A) tails, the first obligatory step in mRNA decay. Here we show that a novel interaction between TTP and the CCR4-NOT subunit, CNOT9, is required for recruitment of the deadenylase complex. In addition to CNOT1, CNOT9 is now included in the identified CCR4-NOT subunits shown to interact with TTP. We find that both the N- and C-terminal domains of TTP are involved in an interaction with CNOT9. Through a combination of SPOT peptide array, site-directed mutagenesis, and bio-layer interferometry, we identified several conserved tryptophan residues in TTP that serve as major sites of interaction with two tryptophan-binding pockets of CNOT9, previously found to interact with another modulator GW182. We further demonstrate that these interactions are also required for recruitment of the CCR4-NOT complex and TTP-directed decay of an mRNA containing an AU-rich element in its 3'-untranslated region. Together the results reveal new molecular details for the TTP-CNOT interaction that shape an emerging mechanism whereby TTP targets inflammatory mRNAs for deadenylation and decay.

Alternative splicing of CNOT7 diversifies CCR4-NOT functions

The CCR4-associated factor CAF1, also called CNOT7, is a catalytic subunit of the CCR4–NOT complex, which has been implicated in all aspects of the mRNA life cycle, from mRNA synthesis in the nucleus to degradation in the cytoplasm. In human cells, alternative splicing of the CNOT7 gene yields a second CNOT7 transcript leading to the formation of a shorter protein, CNOT7 variant 2 (CNOT7v2). Biochemical characterization indicates that CNOT7v2 interacts with CCR4–NOT subunits, although it does not bind to BTG proteins. We report that CNOT7v2 displays a distinct expression profile in human tissues, as well as a nuclear sub-cellular localization compared to CNOT7v1. Despite a conserved DEDD nuclease domain, CNOT7v2 is unable to degrade a poly(A) tail in vitro and preferentially associates with the protein arginine methyltransferase PRMT1 to regulate its activity. Using both in vitro and in cellulo systems, we have also demonstrated that CNOT7v2 regulates the inclusion of CD44 variable exons. Altogether, our findings suggest a preferential involvement of CNOT7v2 in nuclear processes, such as arginine methylation and alternative splicing, rather than mRNA turnover. These observations illustrate how the integration of a splicing variant inside CCR4–NOT can diversify its cell- and tissue-specific functions.

Prediction of Protein Complexes in Trypanosoma brucei by Protein Correlation Profiling Mass Spectrometry and Machine Learning

A disproportionate number of predicted proteins from the genome sequence of the protozoan parasite Trypanosoma brucei, an important human and animal pathogen, are hypothetical proteins of unknown function. This paper describes a protein correlation profiling mass spectrometry approach, using two size exclusion and one ion exchange chromatography systems, to derive sets of predicted protein complexes in this organism by hierarchical clustering and machine learning methods. These hypothesis-generating proteomic data are provided in an open access online data visualization environment (http://134.36.66.166:8083/complex_explorer). The data can be searched conveniently via a user friendly, custom graphical interface. We provide examples of both potential new subunits of known protein complexes and of novel trypanosome complexes of suggested function, contributing to improving the functional annotation of the trypanosome proteome. Data are available via ProteomeXchange with identifier PXD005968.

Acetylation-Dependent Control of Global Poly(A) RNA Degradation by CBP/p300 and HDAC1/2

Acetylation of histones and transcription-related factors is known to exert epigenetic and transcriptional control of gene expression. Here we report that histone acetyltransferases (HATs) and histone deacetylases (HDACs) also regulate gene expression at the posttranscriptional level by controlling poly(A) RNA stability. Inhibition of HDAC1 and HDAC2 induces massive and widespread degradation of normally stable poly(A) RNA in mammalian and Drosophila cells. Acetylation-induced RNA decay depends on the HATs p300 and CBP, which acetylate the exoribonuclease CAF1a, a catalytic subunit of the CCR4-CAF1-NOT deadenlyase complex and thereby contribute to accelerating poly(A) RNA degradation. Taking adipocyte differentiation as a model, we observe global stabilization of poly(A) RNA during differentiation, concomitant with loss of CBP/p300 expression. Our study uncovers reversible acetylation as a fundamental switch by which HATs and HDACs control the overall turnover of poly(A) RNA.

The CCR4-NOT complex contributes to repression of Major Histocompatibility Complex class II transcription

The multi-subunit CCR4 (carbon catabolite repressor 4)-NOT (Negative on TATA) complex serves as a central coordinator of all different steps of eukaryotic gene expression. Here we performed a systematic and comparative analysis of cells where the CCR4-NOT subunits CNOT1, CNOT2 or CNOT3 were individually downregulated using doxycycline-inducible shRNAs. Microarray experiments showed that downregulation of either CNOT subunit resulted in elevated expression of major histocompatibility complex class II (MHC II) genes which are found in a gene cluster on chromosome 6. Increased expression of MHC II genes after knock-down or knock-out of either CNOT subunit was seen in a variety of cell systems and also in naïve macrophages from CNOT3 conditional knock-out mice. CNOT2-mediated repression of MHC II genes occurred also in the absence of the master regulator class II transactivator (CIITA) and did not cause detectable changes of the chromatin structure at the chromosomal MHC II locus. CNOT2 downregulation resulted in an increased de novo transcription of mRNAs whereas tethering of CNOT2 to a regulatory region governing MHC II expression resulted in diminished transcription. These results expand the known repertoire of CCR4-NOT members for immune regulation and identify CNOT proteins as a novel group of corepressors restricting class II expression.

Isolation of protein complexes from the model legume Medicago truncatula by tandem affinity purification in hairy root cultures

Tandem affinity purification coupled to mass spectrometry (TAP-MS) is one of the most powerful techniques to isolate protein complexes and elucidate protein interaction networks. Here, we describe the development of a TAP-MS strategy for the model legume Medicago truncatula, which is widely studied for its ability to produce valuable natural products and to engage in endosymbiotic interactions. As biological material, transgenic hairy roots, generated through Agrobacterium rhizogenes-mediated transformation of M. truncatula seedlings, were used. As proof of concept, proteins involved in the cell cycle, transcript processing and jasmonate signalling were chosen as bait proteins, resulting in a list of putative interactors, many of which confirm the interologue concept of protein interactions, and which can contribute to biological information about the functioning of these bait proteins in planta. Subsequently, binary protein-protein interactions among baits and preys, and among preys were confirmed by a systematic yeast two-hybrid screen. Together, by establishing a M. truncatula TAP-MS platform, we extended the molecular toolbox of this model species.

Beyond the known functions of the CCR4-NOT complex in gene expression regulatory mechanisms: New structural insights to unravel CCR4-NOT mRNA processing machinery

Large protein assemblies are usually the effectors of major cellular processes. The intricate cell homeostasis network is divided into numerous interconnected pathways, each controlled by a set of protein machines. One of these master regulators is the CCR4-NOT complex, which ultimately controls protein expression levels. This multisubunit complex assembles around a scaffold platform, which enables a wide variety of well-studied functions from mRNA synthesis to transcript decay, as well as other tasks still being identified. Solving the structure of the entire CCR4-NOT complex will help to define the distribution of its functions. The recently published three-dimensional reconstruction of the complex, in combination with the known crystal structures of some of the components, has begun to address this. Methodological improvements in structural biology, especially in cryoelectron microscopy, encourage further structural and protein-protein interaction studies, which will advance our comprehension of the gene expression machinery.

A balanced chromosomal translocation involving chromosomes 3 and 16 in a patient with Mayer-Rokitansky-Kuster-Hauser syndrome reveals new candidate genes at 3p22.3 and 16p13.3

BACKGROUND

Mayer-Rokitansky-Kuster-Hauser (MRKH) syndrome, or the congenital absence of uterus and vagina, is the most severe anomaly of the female reproductive tract. It affects 1 in 5,000 females, and is the second most common cause of primary amenorrhea. The etiology remains unknown in most patients, although four single gene defects and some repetitive copy number variants (CNVs) have been identified. Translocations in MRKH patients are very rare, and reported only in three patients previously without breakpoint mapping. We have identified the fourth MRKH translocation patient and are the first to characterize the breakpoints mapped by molecular methods.

RESULTS

The proband is a 17- year old white female with agenesis of the uterus and vagina who had a peripheral blood karyotype revealing a de novo balanced translocation 46,XX,t(3;16)(p22.3;p13.3)dn. There were no known related anomalies present in the proband or her family. No CNVs were found by chromosomal microarray analysis, and no genes were directly disrupted by the translocation. DNA sequencing of six nearby candidate genes-TRIM71, CNOT10, ZNF200, OR1F1, ZNF205, and ZNF213-did not reveal any mutations. RT-qPCR of proband lymphoblast RNA for 20 genes near the breakpoints of 3p22.3 and 16p13.3 showed significantly altered expression levels for four genes in the proband compared to three white female controls, after correction for multiple comparisons. Reduced expression was seen for CMTM7 and CCR4 on 3p22.3, while increased expression was observed for IL32 and MEFV on 16p13.3.

CONCLUSION

We have mapped the breakpoints of our t(3;16)(p22.3;p13.3) translocation patient using molecular methods to within 13.6 kb at 3p22.3 and within 1.9 kb for 16p13.3 and have suggested 10 nearby genes that become plausible candidate genes for future study.

BTG2 bridges PABPC1 RNA-binding domains and CAF1 deadenylase to control cell proliferation

While BTG2 plays an important role in cellular differentiation and cancer, its precise molecular function remains unclear. BTG2 interacts with CAF1 deadenylase through its APRO domain, a defining feature of BTG/Tob factors. Our previous experiments revealed that expression of BTG2 promoted mRNA poly(A) tail shortening through an undefined mechanism. Here we report that the APRO domain of BTG2 interacts directly with the first RRM domain of the poly(A)-binding protein PABPC1. Moreover, PABPC1 RRM and BTG2 APRO domains are sufficient to stimulate CAF1 deadenylase activity in vitro in the absence of other CCR4–NOT complex subunits. Our results unravel thus the mechanism by which BTG2 stimulates mRNA deadenylation, demonstrating its direct role in poly(A) tail length control. Importantly, we also show that the interaction of BTG2 with the first RRM domain of PABPC1 is required for BTG2 to control cell proliferation.

BTG2 promotes mRNA poly(A) tail shortening and regulates cellular differentiation. Here, Stupfler et al. show that the BTG2 APRO domain interacts with PABPC1 RRM1, allowing the former to recruit and stimulate the poly(A) tail shortening activity of CAF1 deadenylase and to control cell proliferation.

The architecture of the Schizosaccharomyces pombe CCR4-NOT complex

CCR4-NOT is a large protein complex present both in cytoplasm and the nucleus of eukaryotic cells. Although it is involved in a variety of distinct processes related to expression of genetic information such as poly(A) tail shortening, transcription regulation, nuclear export and protein degradation, there is only fragmentary information available on some of its nine subunits. Here we show a comprehensive structural characterization of the native CCR4-NOT complex from Schizosaccharomyces pombe. Our cryo-EM 3D reconstruction of the complex, combined with techniques such as immunomicroscopy, RNA-nanogold labelling, docking of the available high-resolution structures and models of different subunits and domains, allow us to propose its full molecular architecture. We locate all functionally defined domains endowed with deadenylating and ubiquitinating activities, the nucleus-specific RNA-interacting subunit Mmi1, as well as surfaces responsible for protein–protein interactions. This information provides insight into cooperation of the different CCR4-NOT complex functions.

CCR4-NOT is a protein complex involved in a variety of important genetic processes. Here, the authors report the mid-resolution structure of this complex, and model the positions and contacts between the subunits, providing structural support for the previously reported functions of the complex.

Architecture of the ubiquitylation module of the yeast Ccr4-Not complex

The Ccr4-Not complex regulates eukaryotic gene expression at multiple levels, including mRNA turnover, translational repression, and transcription. We have studied the ubiquitylation module of the yeast Ccr4-Not complex and addressed how E3 ligase binds cognate E2 and how it is tethered to the complex. The 2.8-Å resolution crystal structure of the N-terminal RING domain of Not4 in complex with Ubc4 shows the detailed interactions of this E3-E2 complex. The 3.6-Å resolution crystal structure of the C-terminal domain of the yeast Not4 in complex with the C-terminal domain of Not1 reveals how a largely extended region at the C-terminus of Not4 wraps around a HEAT-repeat region of Not1. This C-terminal region of Not4 is only partly conserved in metazoans, rationalizing its weaker Not1-binding properties. The structural and biochemical data show how Not1 can incorporate both the ubiquitylation module and the Not2-Not3/5 module concomitantly in the Ccr4-Not complex.

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The Not1 C-terminal domain tethers the Not4 ubiquitylation module to yeast Ccr4-Not

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A low-complexity region of Not4 wraps around the C-terminal HEAT repeats of Not1

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In metazoans, Not4 lacks residues that confer high affinity binding to Not1 in yeast

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Not1_C can recruit Not4 and Not2-Not5 concomitantly to the Ccr4-Not complex

The Not5 subunit of the ccr4-not complex connects transcription and translation

Recent studies have suggested that a sub-complex of RNA polymerase II composed of Rpb4 and Rpb7 couples the nuclear and cytoplasmic stages of gene expression by associating with newly made mRNAs in the nucleus, and contributing to their translation and degradation in the cytoplasm. Here we show by yeast two hybrid and co-immunoprecipitation experiments, followed by ribosome fractionation and fluorescent microscopy, that a subunit of the Ccr4-Not complex, Not5, is essential in the nucleus for the cytoplasmic functions of Rpb4. Not5 interacts with Rpb4; it is required for the presence of Rpb4 in polysomes, for interaction of Rpb4 with the translation initiation factor eIF3 and for association of Rpb4 with mRNAs. We find that Rpb7 presence in the cytoplasm and polysomes is much less significant than that of Rpb4, and that it does not depend upon Not5. Hence Not5-dependence unlinks the cytoplasmic functions of Rpb4 and Rpb7. We additionally determine with RNA immunoprecipitation and native gel analysis that Not5 is needed in the cytoplasm for the co-translational assembly of RNA polymerase II. This stems from the importance of Not5 for the association of the R2TP Hsp90 co-chaperone with polysomes translating RPB1 mRNA to protect newly synthesized Rpb1 from aggregation. Hence taken together our results show that Not5 interconnects translation and transcription.

Multifunctional roles of the mammalian CCR4-NOT complex in physiological phenomena

The carbon catabolite repression 4 (CCR4)–negative on TATA-less (NOT) complex serves as one of the major deadenylases of eukaryotes. Although it was originally identified and characterized in yeast, recent studies have revealed that the CCR4–NOT complex also exerts important functions in mammals, -including humans. However, there are some differences in the composition and functions of the CCR4–NOT complex between mammals and yeast. It is noteworthy that each subunit of the CCR4–NOT complex has unique, multifunctional roles and is responsible for various physiological phenomena. This heterogeneity and versatility of the CCR4–NOT complex makes an overall understanding of this complex difficult. Here, we describe the functions of each subunit of the mammalian CCR4–NOT complex and discuss the molecular mechanisms by which it regulates homeostasis in mammals. Furthermore, a possible link between the disruption of the CCR4–NOT complex and various diseases will be discussed. Finally, we propose that the analysis of mice with each CCR4–NOT subunit knocked out is an effective strategy for clarifying its complicated functions and networks in mammals.

Novel roles of the CCR4-NOT complex

The CCR4-NOT complex is a multi-subunit protein complex evolutionarily conserved across eukaryotes which regulates several aspects of gene expression. A fascinating model is emerging in which this complex acts as a regulation platform, controlling gene products 'from birth to death' through the coordination of different cellular machineries involved in diverse cellular functions. Recently the CCR4-NOT functions have been extended to the control of the innate immune response through the regulation of interferon signaling. Thus, a more comprehensive picture of how CCR4-NOT allows the rapid adaptation of cells to external stress, from transcription to mRNA and protein decay, is presented and discussed here. Overall, CCR4-NOT permits the efficient and rapid adaptation of cellular gene expression in response to changes in environmental conditions and stimuli.

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.

Insights into the structure and architecture of the CCR4-NOT complex

The CCR4–NOT complex is a highly conserved, multifunctional machinery with a general role in controlling mRNA metabolism. It has been implicated in a number of different aspects of mRNA and protein expression, including mRNA degradation, transcription initiation and elongation, ubiquitination, and protein modification. The core CCR4–NOT complex is evolutionarily conserved and consists of at least three NOT proteins and two catalytic subunits. The L-shaped complex is characterized by two functional modules bound to the CNOT1/Not1 scaffold protein: the deadenylase or nuclease module containing two enzymes required for deadenylation, and the NOT module. In this review, we will summarize the currently available information regarding the three-dimensional structure and assembly of the CCR4–NOT complex, in order to provide insight into its roles in mRNA degradation and other biological processes.

Deadenylation: enzymes, regulation, and functional implications

Lengths of the eukaryotic messenger RNA (mRNA) poly(A) tails are dynamically changed by the opposing effects of poly(A) polymerases and deadenylases. Modulating poly(A) tail length provides a highly regulated means to control almost every stage of mRNA lifecycle including transcription, processing, quality control, transport, translation, silence, and decay. The existence of diverse deadenylases with distinct properties highlights the importance of regulating poly(A) tail length in cellular functions. The deadenylation activity can be modulated by subcellular locations of the deadenylases, cis-acting elements in the target mRNAs, trans-acting RNA-binding proteins, posttranslational modifications of deadenylase and associated factors, as well as transcriptional and posttranscriptional regulation of the deadenylase genes. Among these regulators, the physiological functions of deadenylases are largely dependent on the interactions with the trans-acting RNA-binding proteins, which recruit deadenylases to the target mRNAs. The task of these RNA-binding proteins is to find and mark the target mRNAs based on their sequence features. Regulation of the regulators can switch on or switch off deadenylation and thereby destabilize or stabilize the targeted mRNAs, respectively. The distinct domain compositions and cofactors provide various deadenylases the structural basis for the recruitments by distinct RNA-binding protein subsets to meet dissimilar cellular demands. The diverse deadenylases, the numerous types of regulators, and the reversible posttranslational modifications together make up a complicated network to precisely regulate intracellular mRNA homeostasis. This review will focus on the diverse regulators of various deadenylases and will discuss their functional implications, remaining problems, and future challenges.

Heterogeneity and complexity within the nuclease module of the Ccr4-Not complex

The shortening of the poly(A) tail of cytoplasmic mRNA (deadenylation) is a pivotal step in the regulation of gene expression in eukaryotic cells. Deadenylation impacts on both regulated mRNA decay as well as the rate of mRNA translation. An important enzyme complex involved in poly(A) shortening is the Ccr4-Not deadenylase. In addition to at least six non-catalytic subunits, it contains two distinct subunits with ribonuclease activity: a Caf1 subunit, characterized by a DEDD (Asp-Glu-Asp-Asp) domain, and a Ccr4 component containing an endonuclease-exonuclease-phosphatase (EEP) domain. In vertebrate cells, the complexity of the complex is further increased by the presence of paralogs of the Caf1 subunit (encoded by either CNOT7 or CNOT8) and the occurrence of two Ccr4 paralogs (encoded by CNOT6 or CNOT6L). In plants, there are also multiple Caf1 and Ccr4 paralogs. Thus, the composition of the Ccr4-Not complex is heterogeneous. The potential differences in the intrinsic enzymatic activities of the paralogs will be discussed. In addition, the potential redundancy, cooperation, and/or the extent of unique roles for the deadenylase subunits of the Ccr4-Not complex will be reviewed. Finally, novel approaches to study the catalytic roles of the Caf1 and Ccr4 subunits will be discussed.

Structure and assembly of the NOT module of the human CCR4-NOT complex

The CCR4-NOT deadenylase complex is a master regulator of translation and mRNA stability. Its NOT module orchestrates recruitment of the catalytic subunits to target mRNAs. We report the crystal structure of the human NOT module formed by the CNOT1, CNOT2 and CNOT3 C-terminal (-C) regions. CNOT1-C provides a rigid scaffold consisting of two perpendicular stacks of HEAT-like repeats. CNOT2-C and CNOT3-C heterodimerize through their SH3-like NOT-box domains. The heterodimer is stabilized and tightly anchored to the surface of CNOT1 through an unexpected intertwined arrangement of peptide regions lacking defined secondary structure. These assembly peptides mold onto their respective binding surfaces and form extensive interfaces. Mutagenesis of individual interfaces and perturbation of endogenous protein ratios cause defects in complex assembly and mRNA decay. Our studies provide a structural framework for understanding the recruitment of the CCR4-NOT complex to mRNA targets.