An integrated model for the nucleo-cytoplasmic transport of cytoplasmic poly(A)-binding proteins

Stress-Induced Eukaryotic Translational Regulatory Mechanisms

The eukaryotic protein synthesis process entails intricate stages governed by diverse mechanisms to tightly regulate translation. Translational regulation during stress is pivotal for maintaining cellular homeostasis, ensuring the accurate expression of essential proteins is important for survival. This selective translational control mechanism is integral to cellular adaptation and resilience under adverse conditions. This review manuscript explores various mechanisms involved in selective translational regulation, focusing on mRNA-specific and global regulatory processes. Key aspects of translational control include translation initiation, which is often a rate-limiting step, and involves the formation of the eIF4F complex and recruitment of mRNA to ribosomes. Regulation of translation initiation factors, such as eIF4E, eIF4E2, and eIF2, through phosphorylation and interactions with binding proteins, modulates translation efficiency under stress conditions. This review also highlights the control of translation initiation through factors like the eIF4F complex and the ternary complex and also underscores the importance of eIF2α phosphorylation in stress granule formation and cellular stress responses. Additionally, the impact of amino acid deprivation, mTOR signaling, and ribosome biogenesis on translation regulation and cellular adaptation to stress is also discussed. Understanding the intricate mechanisms of translational regulation during stress provides insights into cellular adaptation mechanisms and potential therapeutic targets for various diseases, offering valuable avenues for addressing conditions associated with dysregulated protein synthesis.

Stress-induced Eukaryotic Translational Regulatory Mechanisms

The eukaryotic protein synthesis process entails intricate stages governed by diverse mechanisms to tightly regulate translation. Translational regulation during stress is pivotal for maintaining cellular homeostasis, ensuring the accurate expression of essential proteins crucial for survival. This selective translational control mechanism is integral to cellular adaptation and resilience under adverse conditions. This review manuscript explores various mechanisms involved in selective translational regulation, focusing on mRNA-specific and global regulatory processes. Key aspects of translational control include translation initiation, which is often a rate-limiting step, and involves the formation of the eIF4F complex and recruitment of mRNA to ribosomes. Regulation of translation initiation factors, such as eIF4E, eIF4E2, and eIF2, through phosphorylation and interactions with binding proteins, modulates translation efficiency under stress conditions. This review also highlights the control of translation initiation through factors like the eIF4F complex and the ternary complex and also underscores the importance of eIF2α phosphorylation in stress granule formation and cellular stress responses. Additionally, the impact of amino acid deprivation, mTOR signaling, and ribosome biogenesis on translation regulation and cellular adaptation to stress is also discussed. Understanding the intricate mechanisms of translational regulation during stress provides insights into cellular adaptation mechanisms and potential therapeutic targets for various diseases, offering valuable avenues for addressing conditions associated with dysregulated protein synthesis.

Translational arrest and mRNA decay are independent activities of alphaherpesvirus virion host shutoff proteins

The herpes simplex virus 1 (HSV1) virion host shutoff (vhs) protein is an endoribonuclease that regulates the translational environment of the infected cell, by inducing the degradation of host mRNA via cellular exonuclease activity. To further understand the relationship between translational shutoff and mRNA decay, we have used ectopic expression to compare HSV1 vhs (vhsH) to its homologues from four other alphaherpesviruses - varicella zoster virus (vhsV), bovine herpesvirus 1 (vhsB), equine herpesvirus 1 (vhsE) and Marek's disease virus (vhsM). Only vhsH, vhsB and vhsE induced degradation of a reporter luciferase mRNA, with poly(A)+ in  situ hybridization indicating a global depletion of cytoplasmic poly(A)+ RNA and a concomitant increase in nuclear poly(A)+ RNA and the polyA tail binding protein PABPC1 in cells expressing these variants. By contrast, vhsV and vhsM failed to induce reporter mRNA decay and poly(A)+ depletion, but rather, induced cytoplasmic G3BP1 and poly(A)+ mRNA- containing granules and phosphorylation of the stress response proteins eIF2α and protein kinase R. Intriguingly, regardless of their apparent endoribonuclease activity, all vhs homologues induced an equivalent general blockade to translation as measured by single-cell puromycin incorporation. Taken together, these data suggest that the activities of translational arrest and mRNA decay induced by vhs are separable and we propose that they represent sequential steps of the vhs host interaction pathway.

Regulation of stress granule formation in human oligodendrocytes

Oligodendrocyte (OL) injury and subsequent loss is a pathologic hallmark of multiple sclerosis (MS). Stress granules (SGs) are membrane-less organelles containing mRNAs stalled in translation and considered as participants of the cellular response to stress. Here we show SGs in OLs in active and inactive areas of MS lesions as well as in normal-appearing white matter. In cultures of primary human adult brain derived OLs, metabolic stress conditions induce transient SG formation in these cells. Combining pro-inflammatory cytokines, which alone do not induce SG formation, with metabolic stress results in persistence of SGs. Unlike sodium arsenite, metabolic stress induced SG formation is not blocked by the integrated stress response inhibitor. Glycolytic inhibition also induces persistent SGs indicating the dependence of SG formation and disassembly on the energetic glycolytic properties of human OLs. We conclude that SG persistence in OLs in MS reflects their response to a combination of metabolic stress and pro-inflammatory conditions.

The DUSP domain of pseudophosphatase MK-STYX interacts with G3BP1 to decrease stress granules

Mitogen activated protein kinase phosphoserine/threonine/tyrosine-binding protein (MK-STYX) is a dual specificity (DUSP) member of the protein tyrosine phosphatase family. It is a pseudophosphatase, which lacks the essential amino acids histidine and cysteine in the catalytic active signature motif (HCX5R). We previously reported that MK-STYX interacts with G3BP1 [Ras-GAP (GTPase-activating protein) SH3 (Src homology 3) domain-binding-1] and reduces stress granules, stalled mRNA. To determine how MK-STYX reduces stress granules, truncated domains, CH2 (cell division cycle 25 phosphatase homology 2) and DUSP, of MK-STYX were used. Wild-type MK-STYX and the DUSP domain significantly decreased stressed granules that were induced by sodium arsenite, in which G3BP1 (a stress granule nucleator) was used as the marker. In addition, HEK/293 and HeLa cells co-expressing G3BP1-GFP and mCherry-MK-STYX, mCherry-MK-STYX-CH2, mCherry-MK-STYX-DUSP or mCherry showed that stress granules were significantly decreased in the presence of wild-type MK-STYX and the DUSP domain of MK-STYX. Further characterization of these dynamics in HeLa cells showed that the CH2 domain increased the number of stress granules within a cell, relative to wild-type and DUSP domain of MK-STYX. To further analyze the interaction of G3BP1 and the domains of MK-STYX, coimmunoprecipitation experiments were performed. Cells co-expressing G3BP1-GFP and mCherry, mCherry-MK-STYX, mCherry-MK-STYX-CH2, or mCherry-MK-STYX-DUSP demonstrated that the DUSP domain of MK-STYX interacts with both G3BP1-GFP and endogenous G3BP1, whereas the CH2 domain of MK-STYX did not coimmunoprecipitate with G3BP1. In addition, G3BP1 tyrosine phosphorylation, which is required for stress granule formation, was decreased in the presence of wild-type MK-STYX or the DUSP domain but increased in the presence of CH2. These data highlight a model for how MK-STYX decreases G3BP1-induced stress granules. The DUSP domain of MK-STYX interacts with G3BP1 and negatively alters its tyrosine phosphorylation- decreasing stress granule formation.

Herpes Simplex Virus 1 Expressing GFP-Tagged Virion Host Shutoff (vhs) Protein Uncouples the Activities of RNA Degradation and Differential Nuclear Retention of the Virus Transcriptome

Virion host shutoff (vhs) protein is an endoribonuclease encoded by herpes simplex virus 1 (HSV1). vhs causes several changes to the infected cell environment that favor the translation of late (L) virus proteins: cellular mRNAs are degraded, immediate early (IE) and early (E) viral transcripts are sequestered in the nucleus with polyA binding protein (PABPC1), and dsRNA is degraded to help dampen the PKR-dependent stress response. To further our understanding of the cell biology of vhs, we constructed a virus expressing vhs tagged at its C terminus with GFP. When first expressed, vhs-GFP localized to juxtanuclear clusters, and later it colocalized and interacted with its binding partner VP16, and was packaged into virions. Despite vhs-GFP maintaining activity when expressed in isolation, it failed to degrade mRNA or relocalise PABPC1 during infection, while viral transcript levels were similar to those seen for a vhs knockout virus. PKR phosphorylation was also enhanced in vhs-GFP infected cells, which is in line with a failure to degrade dsRNA. Nonetheless, mRNA FISH revealed that as in Wt but not Dvhs infection, IE and E, but not L transcripts were retained in the nucleus of vhs-GFP infected cells at late times. These results revealed that the vhs-induced nuclear retention of IE and E transcripts was dependent on vhs expression but not on its endoribonuclease activity, uncoupling these two functions of vhs. IMPORTANCE Like many viruses, herpes simplex virus 1 (HSV1) expresses an endoribonuclease, the virion host shutoff (vhs) protein, which regulates the RNA environment of the infected cell and facilitates the classical cascade of virus protein translation. It does this by causing the degradation of some mRNA molecules and the nuclear retention of others. Here, we describe a virus expressing vhs tagged at its C terminus with a green fluorescent protein (GFP) and show that the vhs-GFP fusion protein retains the physical properties of native vhs but does not induce the degradation of mRNA. Nonetheless, vhs-GFP maintains the ability to trap the early virus transcriptome in the nucleus to favor late protein translation, proving for the first time that mRNA degradation is not a prerequisite for vhs effects on the nuclear transcriptome. This virus, therefore, has uncoupled the nuclear retention and degradation activities of vhs, providing a new understanding of vhs during infection.

Localization and Functional Roles of Components of the Translation Apparatus in the Eukaryotic Cell Nucleus

Components of the translation apparatus, including ribosomal proteins, have been found in cell nuclei in various organisms. Components of the translation apparatus are involved in various nuclear processes, particularly those associated with genome integrity control and the nuclear stages of gene expression, such as transcription, mRNA processing, and mRNA export. Components of the translation apparatus control intranuclear trafficking; the nuclear import and export of RNA and proteins; and regulate the activity, stability, and functional recruitment of nuclear proteins. The nuclear translocation of these components is often involved in the cell response to stimulation and stress, in addition to playing critical roles in oncogenesis and viral infection. Many components of the translation apparatus are moonlighting proteins, involved in integral cell stress response and coupling of gene expression subprocesses. Thus, this phenomenon represents a significant interest for both basic and applied molecular biology. Here, we provide an overview of the current data regarding the molecular functions of translation factors and ribosomal proteins in the cell nucleus.

A novel long non-coding RNA RP11-286H15.1 represses hepatocellular carcinoma progression by promoting ubiquitination of PABPC4

Hepatocellular carcinoma (HCC) is a malignancy found at high frequency around the world. Unfortunately, the scarcity of effective early diagnostic methods invariably results in poor outcomes. Long noncoding RNAs (lncRNAs) are known to regulate the progression of hepatocellular carcinoma (HCC). A novel lncRNA RP11-286H15.1(OTTHUMG00000186042) has been identified and associated with HCC; however, the potential role of RP11-286H15.1 in HCC remains undefined. The transcript abundance of RP11-286H15.1 in 80 pairs of HCC samples and cell lines was evaluated by qRT-PCR analysis. The functional role of RP11-286H15.1 in HCC was tested in vivo and in vitro. The mechanisms underlying the role of RP11-286H15.1 in HCC were explored by RNA pulldown, transcriptome sequencing, and RNA immunoprecipitation (RIP), ubiquitination and fluorescence in situ hybridization (FISH) assays as well as Western blot analysis. The qRT-PCR and FISH assays revealed that RP11-286H15.1 was significantly decreased in HCC, and implied a shorter survival time. RP11-286H15.1 overexpression inhibited HCC cell proliferation and metastasis in vitro and in vivo, whereas RP11-286H15.1 knockdown produced the opposite results. Furthermore, we confirmed that RP11-286H15.1 (620-750 nucleotides) binds to poly(A) binding protein 4 (PABPC4) and promotes its ubiquitination, thus, reducing the stability of TRIM37 and CDC27 mRNAs. Our study demonstrates that a novel lncRNA, RP11-286H15.1, represses HCC progression by promoting PABPC4 ubiquitination. These findings highlight potential therapeutic targets for HCC.

Rotavirus Infection Alters Splicing of the Stress-Related Transcription Factor XBP1

XBP1 is a stress-regulated transcription factor also involved in mammalian host defenses and innate immune response. Our investigation of XBP1 RNA splicing during rotavirus infection revealed that an additional XBP1 RNA (XBP1es) that corresponded to exon skipping in the XBP1 pre-RNA is induced depending on the rotavirus strain used. We show that the translation product of XBP1es (XBP1es) has trans-activation properties similar to those of XBP1 on ER stress response element (ERSE) containing promoters. Using monoreassortant between ES⁺ ("skipping") and ES^- ("nonskipping") strains of rotavirus, we show that gene 7 encoding the viral translation enhancer NSP3 is involved in this phenomenon and that exon skipping parallels the nuclear relocalization of cytoplasmic PABP. We further show, using recombinant rotaviruses carrying chimeric gene 7, that the ES⁺ phenotype is linked to the eIF4G-binding domain of NSP3. Because the XBP1 transcription factor is involved in stress and immunological responses, our results suggest an alternative way to activate XBP1 upon viral infection or nuclear localization of PABP.IMPORTANCE Rotavirus is one of the most important pathogens causing severe gastroenteritis in young children worldwide. Here we show that infection with several rotavirus strains induces an alternative splicing of the RNA encoding the stressed-induced transcription factor XBP1. The genetic determinant of XBP1 splicing is the viral RNA translation enhancer NSP3. Since XBP1 is involved in cellular stress and immune responses and since the XBP1 protein made from the alternatively spliced RNA is an active transcription factor, our observations raise the question of whether alternative splicing is a cellular response to rotavirus infection.

Multiple Posttranscriptional Strategies To Regulate the Herpes Simplex Virus 1 vhs Endoribonuclease

The herpes simplex virus 1 (HSV-1) virion host shutoff (vhs) protein is an endoribonuclease that binds to the cellular translation initiation machinery and degrades associated mRNAs, resulting in the shutoff of host protein synthesis. Hence, its unrestrained activity is considered lethal, and it has been proposed that vhs is regulated by two other virus proteins, VP22 and VP16. We have found that during infection, translation of vhs requires VP22 but not the VP22-VP16 complex. Moreover, in the absence of VP22, vhs is not overactive against cellular or viral transcripts. In transfected cells, vhs was also poorly translated, correlating with the aberrant localization of its mRNA. Counterintuitively, vhs mRNA was predominantly nuclear in cells where vhs protein was detected. Likewise, transcripts from cotransfected plasmids were also retained in the same nuclei where vhs mRNA was located, while poly(A) binding protein (PABP) was relocalized to the nucleus in a vhs-dependent manner, implying a general block to mRNA export. Coexpression of VP16 and VP22 rescued the cytoplasmic localization of vhs mRNA but failed to rescue vhs translation. We identified a 230-nucleotide sequence in the 5' region of vhs that blocked its translation and, when transferred to a heterologous green fluorescent protein transcript, reduced translation without altering mRNA levels or localization. We propose that expression of vhs is tightly regulated by a combination of inherent untranslatability and autoinduced nuclear retention of its mRNA that results in a negative feedback loop, with nuclear retention but not translation of vhs mRNA being the target of rescue by the vhs-VP16-VP22 complex.IMPORTANCE A myriad of gene expression strategies has been discovered through studies carried out on viruses. This report concerns the regulation of the HSV-1 vhs endoribonuclease, a virus factor that is important for counteracting host antiviral responses by degrading their mRNAs but that must be regulated during infection to ensure that it does not act against and inhibit the virus itself. We show that regulation of vhs involves multifaceted posttranscriptional cellular and viral processes, including aberrant mRNA localization and a novel, autoregulated negative feedback loop to target its own and coexpressed mRNAs for nuclear retention, an activity that is relieved by coexpression of two other virus proteins, VP22 and VP16. These studies reveal the interplay of strategies by which multiple virus-encoded factors coordinate gene expression at the time that they are needed. These findings are broadly relevant to both virus and cellular gene expression.

Cytoplasmic Relocalization and Colocalization with Viroplasms of Host Cell Proteins, and Their Role in Rotavirus Infection

Rotavirus replicates in the cytoplasm of infected cells in unique virus-induced cytoplasmic inclusion bodies called viroplasms (VMs), which are nucleated by two essential viral nonstructural proteins, NSP2 and NSP5. However, the precise composition of the VM, the intracellular localization of host proteins during virus infection, and their association with VMs or role in rotavirus growth remained largely unexplored. Mass spectrometry analyses revealed the presence of several host heterogeneous nuclear ribonucleoproteins (hnRNPs), AU-rich element-binding proteins (ARE-BPs), and cytoplasmic proteins from uninfected MA104 cell extracts in the pulldown (PD) complexes of the purified viroplasmic proteins NSP2 and NSP5. Immunoblot analyses of PD complexes from RNase-treated and untreated cell extracts, analyses of coimmunoprecipitation complexes using RNase-treated infected cell lysates, and direct binding assays using purified recombinant proteins further demonstrated that the interactions of the majority of the hnRNPs and ARE-BPs with viroplasmic proteins are RNA independent. Time course immunoblot analysis of the nuclear and cytoplasmic fractions from rotavirus-infected and mock-infected cells and immunofluorescence confocal microscopy analyses of virus-infected cells revealed a surprising sequestration of the majority of the relocalized host proteins in viroplasms. Analyses of ectopic overexpression and small interfering RNA (siRNA)-mediated downregulation of expression revealed that host proteins either promote or inhibit viral protein expression and progeny virus production in virus-infected cells. This study demonstrates that rotavirus induces the cytoplasmic relocalization and sequestration of a large number of nuclear and cytoplasmic proteins in viroplasms, subverting essential cellular processes in both compartments to promote rapid virus growth, and reveals that the composition of rotavirus viroplasms is much more complex than is currently understood.IMPORTANCE Rotavirus replicates exclusively in the cytoplasm. Knowledge on the relocalization of nuclear proteins to the cytoplasm or the role(s) of host proteins in rotavirus infection is very limited. In this study, it is demonstrated that rotavirus infection induces the cytoplasmic relocalization of a large number of nuclear RNA-binding proteins (hnRNPs and AU-rich element-binding proteins). Except for a few, most nuclear hnRNPs and ARE-BPs, nuclear transport proteins, and some cytoplasmic proteins directly interact with the viroplasmic proteins NSP2 and NSP5 in an RNA-independent manner and become sequestered in the viroplasms of infected cells. The host proteins differentially affected viral gene expression and virus growth. This study demonstrates that rotavirus induces the relocalization and sequestration of a large number of host proteins in viroplasms, affecting host processes in both compartments and generating conditions conducive for virus growth in the cytoplasm of infected cells.

Poly(A)-binding proteins and mRNA localization: who rules the roost?

RNA-binding proteins are often multifunctional, interact with a variety of protein partners and display complex localizations within cells. Mammalian cytoplasmic poly(A)-binding proteins (PABPs) are multifunctional RNA-binding proteins that regulate multiple aspects of mRNA translation and stability. Although predominantly diffusely cytoplasmic at steady state, they shuttle through the nucleus and can be localized to a variety of cytoplasmic foci, including those associated with mRNA storage and localized translation. Intriguingly, PABP sub-cellular distribution can alter dramatically in response to cellular stress or viral infection, becoming predominantly nuclear and/or being enriched in induced cytoplasmic foci. However, relatively little is known about the mechanisms that govern this distribution/relocalization and in many cases PABP functions within specific sites remain unclear. Here we discuss the emerging evidence with respect to these questions in mammals.

Neutrophil-derived alpha defensins control inflammation by inhibiting macrophage mRNA translation

Neutrophils are the first and most numerous cells to arrive at the site of an inflammatory insult and are among the first to die. We previously reported that alpha defensins, released from apoptotic human neutrophils, augmented the antimicrobial capacity of macrophages while also inhibiting the biosynthesis of proinflammatory cytokines. In vivo, alpha defensin administration protected mice from inflammation, induced by thioglychollate-induced peritonitis or following infection withSalmonella entericaserovar Typhimurium. We have now dissected the antiinflammatory mechanism of action of the most abundant neutrophil alpha defensin, Human Neutrophil Peptide 1 (HNP1). Herein we show that HNP1 enters macrophages and inhibits protein translation without inducing the unfolded-protein response or affecting mRNA stability. In a cell-free in vitro translation system, HNP1 powerfully inhibited both cap-dependent and cap-independent mRNA translation while maintaining mRNA polysomal association. This is, to our knowledge, the first demonstration of a peptide released from one cell type (neutrophils) directly regulating mRNA translation in another (macrophages). By preventing protein translation, HNP1 functions as a "molecular brake" on macrophage-driven inflammation, ensuring both pathogen clearance and the resolution of inflammation with minimal bystander tissue damage.

Continuous and Discontinuous RNA Synthesis in Coronaviruses

Replication of the coronavirus genome requires continuous RNA synthesis, whereas transcription is a discontinuous process unique among RNA viruses. Transcription includes a template switch during the synthesis of subgenomic negative-strand RNAs to add a copy of the leader sequence. Coronavirus transcription is regulated by multiple factors, including the extent of base-pairing between transcription-regulating sequences of positive and negative polarity, viral and cell protein-RNA binding, and high-order RNA-RNA interactions. Coronavirus RNA synthesis is performed by a replication-transcription complex that includes viral and cell proteins that recognize cis-acting RNA elements mainly located in the highly structured 5' and 3' untranslated regions. In addition to many viral nonstructural proteins, the presence of cell nuclear proteins and the viral nucleocapsid protein increases virus amplification efficacy. Coronavirus RNA synthesis is connected with the formation of double-membrane vesicles and convoluted membranes. Coronaviruses encode proofreading machinery, unique in the RNA virus world, to ensure the maintenance of their large genome size.

Poly(A)-binding proteins are required for diverse biological processes in metazoans

PABPs [poly(A)-binding proteins] bind to the poly(A) tail of eukaryotic mRNAs and are conserved in species ranging from yeast to human. The prototypical cytoplasmic member, PABP1, is a multifunctional RNA-binding protein with roles in global and mRNA-specific translation and stability, consistent with a function as a central regulator of mRNA fate in the cytoplasm. More limited insight into the molecular functions of other family members is available. However, the consequences of disrupting PABP function in whole organisms is less clear, particularly in vertebrates, and even more so in mammals. In the present review, we discuss current and emerging knowledge with respect to the functions of PABP family members in whole animal studies which, although incomplete, already underlines their biological importance and highlights the need for further intensive research in this area.

Interrelation between protein synthesis, proteostasis and life span

The production of newly synthesized proteins is a key process of protein homeostasis that initiates the biosynthetic flux of proteins and thereby determines the composition, stability and functionality of the proteome. Protein synthesis is highly regulated on multiple levels to adapt the proteome to environmental and physiological challenges such as aging and proteotoxic conditions. Imbalances of protein folding conditions are sensed by the cell that then trigger a cascade of signaling pathways aiming to restore the protein folding equilibrium. One regulatory node to rebalance proteostasis upon stress is the control of protein synthesis itself. Translation is reduced as an immediate response to perturbations of the protein folding equilibrium that can be observed in the cytosol as well as in the organelles such as the endoplasmatic reticulum and mitochondria. As reduction of protein synthesis is linked to life span increase, the signaling pathways regu-lating protein synthesis might be putative targets for treatments of age-related diseases. Eukaryotic cells have evolved a complex system for protein synthesis regulation and this review will summarize cellular strategies to regulate mRNA translation upon stress and its impact on longevity.

Specificity factors in cytoplasmic polyadenylation

Poly(A) tail elongation after export of an messenger RNA (mRNA) to the cytoplasm is called cytoplasmic polyadenylation. It was first discovered in oocytes and embryos, where it has roles in meiosis and development. In recent years, however, has been implicated in many other processes, including synaptic plasticity and mitosis. This review aims to introduce cytoplasmic polyadenylation with an emphasis on the factors and elements mediating this process for different mRNAs and in different animal species. We will discuss the RNA sequence elements mediating cytoplasmic polyadenylation in the 3' untranslated regions of mRNAs, including the CPE, MBE, TCS, eCPE, and C-CPE. In addition to describing the role of general polyadenylation factors, we discuss the specific RNA binding protein families associated with cytoplasmic polyadenylation elements, including CPEB (CPEB1, CPEB2, CPEB3, and CPEB4), Pumilio (PUM2), Musashi (MSI1, MSI2), zygote arrest (ZAR2), ELAV like proteins (ELAVL1, HuR), poly(C) binding proteins (PCBP2, αCP2, hnRNP-E2), and Bicaudal C (BICC1). Some emerging themes in cytoplasmic polyadenylation will be highlighted. To facilitate understanding for those working in different organisms and fields, particularly those who are analyzing high throughput data, HUGO gene nomenclature for the human orthologs is used throughout. Where human orthologs have not been clearly identified, reference is made to protein families identified in man.

The role of mammalian poly(A)-binding proteins in co-ordinating mRNA turnover

The function of cytoplasmic PABPs [poly(A)-binding proteins] in promoting mRNA translation has been intensively studied. However, PABPs also have less clearly defined functions in mRNA turnover including roles in default deadenylation, a major rate-limiting step in mRNA decay, as well as roles in the regulation of mRNA turnover by cis-acting control elements and in the detection of aberrant mRNA transcripts. In the present paper, we review our current understanding of the complex roles of PABP1 in mRNA turnover, focusing on recent progress in mammals and highlighting some of the major questions that remain to be addressed.