Human hnRNP Q re-localizes to cytoplasmic granules upon PMA, thapsigargin, arsenite and heat-shock treatments

VAP-A and its binding partner CERT drive biogenesis of RNA-containing extracellular vesicles at ER membrane contact sites

RNA transfer via extracellular vesicles (EVs) influences cell phenotypes; however, lack of information regarding biogenesis of RNA-containing EVs has limited progress in the field. Here, we identify endoplasmic reticulum membrane contact sites (ER MCSs) as platforms for the generation of RNA-containing EVs. We identify a subpopulation of small EVs that is highly enriched in RNA and regulated by the ER MCS linker protein VAP-A. Functionally, VAP-A-regulated EVs are critical for miR-100 transfer between cells and in vivo tumor formation. Lipid analysis of VAP-A-knockdown EVs revealed reductions in the EV biogenesis lipid ceramide. Knockdown of the VAP-A-binding ceramide transfer protein CERT led to similar defects in EV RNA content. Imaging experiments revealed that VAP-A promotes luminal filling of multivesicular bodies (MVBs), CERT localizes to MVBs, and the ceramide-generating enzyme neutral sphingomyelinase 2 colocalizes with VAP-A-positive ER. We propose that ceramide transfer via VAP-A-CERT linkages drives the biogenesis of a select RNA-containing EV population.

hnRNP Q and hnRNP A1 Regulate the Translation of Cofilin in Response to Transient Oxygen-Glucose Deprivation in Hippocampal Neurons

Protein aggregates of cofilin and actin have been found in neurons under oxygen-glucose deprivation. However, the regulatory mechanism behind the expression of Cfl1 during oxygen-glucose deprivation remains unclear. Here, we found that heterogeneous nuclear ribonucleoproteins (hnRNP) Q and hnRNP A1 regulate the translation of Cfl1 mRNA, and formation of cofilin-actin aggregates. The interaction between hnRNP A1 and Cfl1 mRNA was interrupted by hnRNP Q under normal conditions, while the changes in the expression and localization of hnRNP Q and hnRNP A1 increased such interaction, as did the translation of Cfl1 mRNA under oxygen-glucose deprived conditions. These findings reveal a new translational regulatory mechanism of Cfl1 mRNA in hippocampal neurons under oxygen-glucose deprivation.

Role of RNA-binding proteins during the late stages of Flavivirus replication cycle

The genus Flavivirus encompasses several worldwide-distributed arthropod-borne viruses including, dengue virus, Japanese encephalitis virus, West Nile virus, yellow fever virus, Zika virus, and tick-borne encephalitis virus. Infection with these viruses manifest with symptoms ranging from febrile illness to life- threatening hypotensive shock and encephalitis. Therefore, flaviviruses pose a great risk to public health. Currently, preventive measures are falling short to control epidemics and there are no antivirals against any Flavivirus.Flaviviruses carry a single stranded positive-sense RNA genome that plays multiple roles in infected cells: it is translated into viral proteins, used as template for genome replication, it is the precursor of the subgenomic flaviviral RNA and it is assembled into new virions. Furthermore, viral RNA genomes are also packaged into extracellular vesicles, e.g. exosomes, which represent an alternate mode of virus dissemination.Because RNA molecules are at the center of Flavivirus replication cycle, viral and host RNA-binding proteins (RBPs) are critical determinants of infection. Numerous studies have revealed the function of RBPs during Flavivirus infection, particularly at the level of RNA translation and replication. These proteins, however, are also critical participants at the late stages of the replication cycle. Here we revise the function of host RBPs and the viral proteins capsid, NS2A and NS3, during the packaging of viral RNA and the assembly of new virus particles. Furthermore, we go through the evidence pointing towards the importance of host RBPs in mediating cellular RNA export with the idea that the biogenesis of exosomes harboring Flavivirus RNA would follow an analogous pathway.

Heterogeneous nuclear ribonucleoproteins R and Q accumulate in pathological inclusions in FTLD-FUS

Frontotemporal lobar degeneration (FTLD) is pathologically subdivided based on the presence of particular pathological proteins that are identified in inclusion bodies observed post-mortem. The FTLD-FUS subgroup is defined by the presence of the fused in sarcoma protein (FUS) in pathological inclusions. FUS is a heterogeneous nuclear ribonucleoprotein (hnRNP) protein and a member of the FET (FUS, EWS, TAF15) protein family. It shuttles between the nucleus and cytoplasm, and has been implicated in many cellular functions including translation, splicing, and RNA transport. EWS, TAF15 and the nuclear import receptor transportin have been shown to co-accumulate with FUS in neuronal inclusions specifically in FTLD-FUS, with transportin-positive inclusions most frequently observed. Here, we report the identification of hnRNP R and hnRNP Q in neuronal cytoplasmic and intranuclear inclusions in the frontal cortex and hippocampus of FTLD-FUS patients, as frequently as transportin. hnRNP R and hnRNP Q were not found in the characteristic pathological inclusions observed in FTLD-TDP (subtypes A-C). Additionally, we studied the expression of hnRNP R in the frontal and temporal cortices from patients with FTLD and found significantly increased expression of the heterogeneous nuclear ribonucleoprotein R in several FTLD disease groups. Our identification of the frequent presence of hnRNP R and hnRNP Q in FTLD-FUS inclusions suggests a potential role for these hnRNPs in FTLD-FUS pathogenesis and supports the role of dysfunctional RNA metabolism in FTLD.

Subcellular Heterogeneity of the microRNA Machinery

Different methods have recently been developed to understand the subcellular localization and role of microRNAs (miRNAs) as well as small RNAs associated with Argonaute (AGO) proteins. The heterogeneity of the protein complexes associated with miRNAs, along with their subcellular localization, provides clues into their biochemical mechanism of function. Subcellular diversity indicates that miRNAs localized to different cellular regions could have different functions, including transcriptional regulation on chromatin or post-transcriptional control, providing global regulation of gene expression by miRNAs. Herein, I review the current knowledge and most recent discoveries relating to the subcellular function of miRNAs and other AGO-associated small RNAs, revealing the emergence of a multitude of functions of the miRNA pathway to control different steps of the gene expression program(s).

Dengue Virus Capsid Interacts with DDX3X-A Potential Mechanism for Suppression of Antiviral Functions in Dengue Infection

Dengue virus is a pathogen of global concern and has a huge impact on public health system in low- and middle-income countries. The capsid protein of dengue virus is least conserved among related flavivirus and there is very limited information on the role of cytosolic proteins that interact with dengue virus capsid. We identified DEAD (Asp-Glu-Ala-Asp) Box Helicase 3, an X-Linked (DDX3X), cytosolic ATP-dependent RNA helicase as a dengue virus capsid-interacting protein. We show that the N-terminal region of capsid is important for interaction with DDX3X, while the N-terminal domain of DDX3X seems to be involved in interaction with dengue capsid. DDX3X was down-regulated in dengue virus infected cells at later stages of infection. Our results show that DDX3X is an antiviral protein as suppression of DDX3X expression by siRNA led to an increase in viral titers and overexpression of DDX3X led to inhibition of viral replication. Knock-down of DDX3X did not affect induction of type I interferon response upon infection suggesting that the effect of DDX3X knock-down is independent of the interferon-dependent pathways that DDX3X modulates under normal conditions. Thus, our study identifies DDX3X as a dengue virus capsid interacting protein and indicates a potential link between the antiviral functions of DDX3X and dengue capsid at later stages of dengue infection.

The hnRNP Q-like gene is retroinserted into the B chromosomes of the cichlid fish Astatotilapia latifasciata

B chromosomes are dispensable elements observed in many eukaryotic species, including the African cichlid Astatotilapia latifasciata, which might have one or two B chromosomes. Although there have been many studies focused on the biology of these chromosomes, questions about the evolution, maintenance, and potential effects of these chromosomes remain. Here, we identified a variant form of the hnRNP Q-like gene inserted into the B chromosome of A. latifasciata that is characterized by a high copy number and intron-less structure. The absence of introns and presence of transposable elements with a reverse transcriptase domain flanking hnRNP Q-like sequences suggest that this gene was retroinserted into the B chromosome. RNA-Seq analysis did not show that the B variant retroinserted copies are transcriptionally active. However, RT-qPCR results showed variations in the canonical hnRNP Q-like copy expression levels among exons, tissues, sex, and B presence/absence. Although the patterns of transcription are not well understood, the exons of the B retrocopies were overexpressed, and a bias for female B+ expression was also observed. These results suggest that retroinsertion is an additional and important mechanism contributing to B chromosome formation. Furthermore, these findings indicate a bias towards female differential expression of B chromosome sequences, suggesting that B chromosomes and sex determination are somehow associated in cichlids.

Comparative membrane proteomics analyses of breast cancer cell lines to understand the molecular mechanism of breast cancer brain metastasis

Breast cancer is the leading type of cancer in women. Breast cancer brain metastasis is currently considered an issue of concern among breast cancer patients. Membrane proteins play important roles in breast cancer brain metastasis, involving cell adhesion and penetration of blood-brain barrier. To understand the mechanism of breast cancer brain metastasis, liquid chromatography-tandem mass spectrometry (LC-MS/MS) was employed in conjunction with enrichment of membrane proteins to analyze the proteomes from five different breast cancer and a brain cancer cell lines. Quantitative proteomic data of all cell lines were compared with MDA-MB-231BR which is a brain seeking breast cancer cell line, thus representing brain metastasis characteristics. Label-free proteomics of the six cell lines facilitates the identification of 1238 proteins and the quantification of 899 proteins of which more than 70% were membrane proteins. Unsupervised principal component analysis (PCA) of the label-free proteomics data resulted in a distinct clustering of cell lines, suggesting quantitative differences in the expression of several proteins among the different cell lines. Unique protein expressions in 231BR were observed for 28 proteins. The up-regulation of STAU1, AT1B3, NPM1, hnRNP Q, and hnRNP K and the down-regulation of TUBB4B and TUBB5 were noted in 231BR relative to 231 (precursor cell lines from which 231BR is derived). These proteins might contribute to the breast cancer brain metastasis. Ingenuity pathway analysis (IPA) supported the great brain metastatic propensity of 231BR and suggested the importance of the up-regulation of integrin proteins and down-regulation of EPHA2 in brain metastasis.

Who Regulates Whom? An Overview of RNA Granules and Viral Infections

After viral infection, host cells respond by mounting an anti-viral stress response in order to create a hostile atmosphere for viral replication, leading to the shut-off of mRNA translation (protein synthesis) and the assembly of RNA granules. Two of these RNA granules have been well characterized in yeast and mammalian cells, stress granules (SGs), which are translationally silent sites of RNA triage and processing bodies (PBs), which are involved in mRNA degradation. This review discusses the role of these RNA granules in the evasion of anti-viral stress responses through virus-induced remodeling of cellular ribonucleoproteins (RNPs).

Disseminated Tumor Cells Persist in the Bone Marrow of Breast Cancer Patients through Sustained Activation of the Unfolded Protein Response

Disseminated tumor cells (DTC), which share mesenchymal and epithelial properties, are considered to be metastasis-initiating cells in breast cancer. However, the mechanisms supporting DTC survival are poorly understood. DTC extravasation into the bone marrow may be encouraged by low oxygen concentrations that trigger metabolic and molecular alterations contributing to DTC survival. Here, we investigated how the unfolded protein response (UPR), an important cytoprotective program induced by hypoxia, affects the behavior of stressed cancer cells. DTC cell lines established from the bone marrow of patients with breast cancer (BC-M1), lung cancer, (LC-M1), and prostate cancer (PC-E1) were subjected to hypoxic and hypoglycemic conditions. BC-M1 and LC-M1 exhibiting mesenchymal and epithelial properties adapted readily to hypoxia and glucose starvation. Upregulation of UPR proteins, such as the glucose-regulated protein Grp78, induced the formation of filamentous networks, resulting in proliferative advantages and sustained survival under total glucose deprivation. High Grp78 expression correlated with mesenchymal attributes of breast and lung cancer cells and with poor differentiation in clinical samples of primary breast and lung carcinomas. In DTCs isolated from bone marrow specimens from breast cancer patients, Grp78-positive stress granules were observed, consistent with the likelihood these cells were exposed to acute cell stress. Overall, our findings provide the first evidence that the UPR is activated in DTC in the bone marrow from cancer patients, warranting further study of this cell stress pathway as a predictive biomarker for recurrent metastatic disease.

Heterogeneous nuclear ribonucleoprotein Q is a novel substrate of SH2 domain-containing phosphatase-2

SH2 domain-containing phosphatase-2 (SHP2) is a protein-tyrosine phosphatase implicated in activation of cell signalling such as the Ras/extracellular signal-regulated kinase pathway. The substrates of SHP2 and their roles in cell activation are not fully understood. By using the substrate-trapping method with the phosphatase-dead SHP2 mutant, in which C459 was substituted by serine, and the matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometric analysis, we found that heterogeneous nuclear ribonucleoprotein Q (hnRNP Q), a protein implicated in RNA metabolisms, was a novel substrate of SHP2. Tyrosine-phosphorylated hnRNP Q was detected in HL-60, Jurkat and human peripheral blood mononuclear cells, but not mature neutrophils, treated with pervanadate. Tyrosine-phosphorylated hnRNP Q was directly bound to SHP2 in vivo and in vitro, and dephosphorylated by SHP2 in vitro. These findings suggest that hnRNP Q is a novel substrate of SHP2 and the SHP2 activity may be also involved in RNA metabolisms via dephosphorylation of hnRNP Q.

Mapping the LINE1 ORF1 protein interactome reveals associated inhibitors of human retrotransposition

LINE1s occupy 17% of the human genome and are its only active autonomous mobile DNA. L1s are also responsible for genomic insertion of processed pseudogenes and >1 million non-autonomous retrotransposons (Alus and SVAs). These elements have significant effects on gene organization and expression. Despite the importance of retrotransposons for genome evolution, much about their biology remains unknown, including cellular factors involved in the complex processes of retrotransposition and forming and transporting L1 ribonucleoprotein particles. By co-immunoprecipitation of tagged L1 constructs and mass spectrometry, we identified proteins associated with the L1 ORF1 protein and its ribonucleoprotein. These include RNA transport proteins, gene expression regulators, post-translational modifiers, helicases and splicing factors. Many cellular proteins co-localize with L1 ORF1 protein in cytoplasmic granules. We also assayed the effects of these proteins on cell culture retrotransposition and found strong inhibiting proteins, including some that control HIV and other retroviruses. These data suggest candidate cofactors that interact with the L1 to modulate its activity and increase our understanding of the means by which the cell coexists with these genomic 'parasites'.

Control of translation and miRNA-dependent repression by a novel poly(A) binding protein, hnRNP-Q

The heterogeneous nuclear ribonucleoprotein Q2 competitively binds mRNA poly(A) tails to regulate translational and miRNA-related functions of PABP.

Translation control often operates via remodeling of messenger ribonucleoprotein particles. The poly(A) binding protein (PABP) simultaneously interacts with the 3′ poly(A) tail of the mRNA and the eukaryotic translation initiation factor 4G (eIF4G) to stimulate translation. PABP also promotes miRNA-dependent deadenylation and translational repression of target mRNAs. We demonstrate that isoform 2 of the mouse heterogeneous nuclear protein Q (hnRNP-Q2/SYNCRIP) binds poly(A) by default when PABP binding is inhibited. In addition, hnRNP-Q2 competes with PABP for binding to poly(A) in vitro. Depleting hnRNP-Q2 from translation extracts stimulates cap-dependent and IRES-mediated translation that is dependent on the PABP/poly(A) complex. Adding recombinant hnRNP-Q2 to the extracts inhibited translation in a poly(A) tail-dependent manner. The displacement of PABP from the poly(A) tail by hnRNP-Q2 impaired the association of eIF4E with the 5′ m⁷G cap structure of mRNA, resulting in the inhibition of 48S and 80S ribosome initiation complex formation. In mouse fibroblasts, silencing of hnRNP-Q2 stimulated translation. In addition, hnRNP-Q2 impeded let-7a miRNA-mediated deadenylation and repression of target mRNAs, which require PABP. Thus, by competing with PABP, hnRNP-Q2 plays important roles in the regulation of global translation and miRNA-mediated repression of specific mRNAs.

The regulation of mRNA translation and stability is of paramount importance for almost every cellular function. In eukaryotes, the poly(A) binding protein (PABP) is a central regulator of both global and mRNA-specific translation. PABP simultaneously interacts with the 3′ poly(A) tail of the mRNA and the eukaryotic translation initiation factor 4G (eIF4G). These interactions circularize the mRNA and stimulate translation. PABP also regulates specific mRNAs by promoting miRNA-dependent deadenylation and translational repression. A key step in understanding PABP's functions is to identify factors that affect its association with the poly(A) tail. Here we show that the cytoplasmic isoform of the mouse heterogeneous nuclear ribonucleoprotein Q (hnRNP-Q2/SYNCRIP), which exhibits binding preference to poly(A), interacts with the poly(A) tail by default when PABP binding is inhibited. In addition, hnRNP-Q2 competes with PABP for binding to the poly(A) tail. Depleting hnRNP-Q2 stimulates translation in cell-free extracts and in cultured cells, in agreement with its function as translational repressor. In addition, hnRNP-Q2 impeded miRNA-mediated deadenylation and repression of target mRNAs, which requires PABP. Thus, competition from hnRNP-Q2 provides a novel mechanism by which multiple functions of PABP are regulated. This regulation could play important roles in various biological processes, such as development, viral infection, and human disease.

Relationship of other cytoplasmic ribonucleoprotein bodies (cRNPB) to GW/P bodies

GW/P body components are involved in the post-transcriptional -processing of messenger RNA (mRNA) through the RNA interference and 5' → 3' mRNA degradation pathways, as well as functioning in mRNA transport and stabilization. It is currently thought that the relevant mRNA silencing and degrading factors are partitioned to these cytoplasmic microdomains thus effecting post-transcriptional regulation and the prevention of accidental degradation of functional mRNA. Although much attention has focused on GW/P bodies, a variety of other cytoplasmic RNP bodies (cRNPB) also have highly specialized functions and have been shown to interact or co-localize with components of GW/P bodies. These cRNPB include neuronal transport RNP granules, stress granules, RNP-rich cytoplasmic germline granules or chromatoid bodies, sponge bodies, cytoplasmic prion protein-induced RNP granules, U bodies and TAM bodies. Of clinical relevance, autoantibodies directed against protein and miRNA components of GW/P bodies have been associated with autoimmune diseases, neurological diseases and cancer. Understanding the molecular function of GW/P bodies and their interactions with other cRNPB may provide clues to the etiology or pathogenesis of diseases associated with autoantibodies directed to these structures. This chapter will focus on the similarities and differences of the various cRNPB as an approach to understanding their functional relationships to GW/P bodies.

Internal ribosomal entry site-mediated translation is important for rhythmic PERIOD1 expression

The mouse PERIOD1 (mPER1) plays an important role in the maintenance of circadian rhythm. Translation of mPer1 is directed by both a cap-dependent process and cap-independent translation mediated by an internal ribosomal entry site (IRES) in the 5′ untranslated region (UTR). Here, we compared mPer1 IRES activity with other cellular IRESs. We also found critical region in mPer1 5′UTR for heterogeneous nuclear ribonucleoprotein Q (HNRNPQ) binding. Deletion of HNRNPQ binding region markedly decreased IRES activity and disrupted rhythmicity. A mathematical model also suggests that rhythmic IRES-dependent translation is a key process in mPER1 oscillation. The IRES-mediated translation of mPer1 will help define the post-transcriptional regulation of the core clock genes.

Heterogeneous nuclear ribonucleoprotein A2 participates in the replication of Japanese encephalitis virus through an interaction with viral proteins and RNA

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that is kept in a zoonotic transmission cycle between pigs and mosquitoes. JEV causes infection of the central nervous system with a high mortality rate in dead-end hosts, including humans. Many studies have suggested that the flavivirus core protein is not only a component of nucleocapsids but also an important pathogenic determinant. In this study, we identified heterogeneous nuclear ribonucleoprotein A2 (hnRNP A2) as a binding partner of the JEV core protein by pulldown purification and mass spectrometry. Reciprocal coimmunoprecipitation analyses in transfected and infected cells confirmed a specific interaction between the JEV core protein and hnRNP A2. Expression of the JEV core protein induced cytoplasmic retention of hnRNP A2 in JEV subgenomic replicon cells. Small interfering RNA (siRNA)-mediated knockdown of hnRNP A2 resulted in a 90% reduction of viral RNA replication in cells infected with JEV, and the reduction was cancelled by the expression of an siRNA-resistant hnRNP A2 mutant. In addition to the core protein, hnRNP A2 also associated with JEV nonstructural protein 5, which has both methyltransferase and RNA-dependent RNA polymerase activities, and with the 5'-untranslated region of the negative-sense JEV RNA. During one-step growth, synthesis of both positive- and negative-strand JEV RNAs was delayed by the knockdown of hnRNP A2. These results suggest that hnRNP A2 plays an important role in the replication of JEV RNA through the interaction with viral proteins and RNA.

TAR DNA-binding protein 43 (TDP-43) regulates stress granule dynamics via differential regulation of G3BP and TIA-1

TAR deoxyribonucleic acid-binding protein 43 (TDP-43) is a multifunctional protein with roles in transcription, pre-messenger ribonucleic acid (mRNA) splicing, mRNA stability and transport. TDP-43 interacts with other heterogeneous nuclear ribonucleoproteins (hnRNPs), including hnRNP A2, via its C-terminus and several hnRNP family members are involved in the cellular stress response. This relationship led us to investigate the role of TDP-43 in cellular stress. Our results demonstrate that TDP-43 and hnRNP A2 are localized to stress granules (SGs), following oxidative stress, heat shock and exposure to thapsigargin. TDP-43 contributes to both the assembly and maintenance of SGs in response to oxidative stress and differentially regulates key SGs components, including TIA-1 and G3BP. The controlled aggregation of TIA-1 is disrupted in the absence of TDP-43 resulting in slowed SG formation. In addition, TDP-43 regulates the levels of G3BP mRNA, a SG nucleating factor. The disease-associated mutation TDP-43(R361S) is a loss-of-function mutation with regards to SG formation and confers alterations in levels of G3BP and TIA-1. In contrast, a second mutation TDP-43(D169G) does not impact this pathway. Thus, mutations in TDP-43 are mechanistically divergent. Finally, the cellular function of TDP-43 extends beyond splicing and places TDP-43 as a participant of the central cellular response to stress and an active player in RNA storage.

Phosphoproteomics profiling suggests a role for nuclear βΙPKC in transcription processes of undifferentiated murine embryonic stem cells

Protein kinase C (PKC) plays a key role in embryonic stem cell (ESC) proliferation, self-renewal, and differentiation. However, the function of specific PKC isoenzymes have yet to be determined. Of the PKCs expressed in undifferentiated ESCs, βIPKC was the only isoenzyme abundantly expressed in the nuclei. To investigate the role of βΙPKC in these cells, we employed a phosphoproteomics strategy and used two classical (cPKC) peptide modulators and one βIPKC-specific inhibitor peptide. We identified 13 nuclear proteins that are direct or indirect βΙPKC substrates in undifferentiated ESCs. These proteins are known to be involved in regulating transcription, splicing, and chromatin remodeling during proliferation and differentiation. Inhibiting βΙPKC had no effect on DNA synthesis in undifferentiated ESCs. However, upon differentiation, many cells seized to express βΙPKC and βΙPKC was frequently found in the cytoplasm. Taken together, our results suggest that βIPKC takes part in the processes that maintain ESCs in their undifferentiated state.

Turnover of AU-rich-containing mRNAs during stress: a matter of survival

Cells undergo various adaptive measures in response to stress. Among these are specific changes in the posttranscriptional regulation of various genes. In particular, the turnover of mRNA is modified to either increase or decrease the abundance of certain target messages. Some of the best-studied mRNAs that are affected by stress are those that contain adenine/uridine-rich elements (AREs) in their 3'-untranslated regions. ARE-containing mRNAs are involved in many important cellular processes and are normally labile, but in response to stress they are differentially regulated through the concerted efforts of ARE-binding proteins (AUBPs) such as HuR, AUF1, tristetraprolin, BRF1, and KSRP, along with microRNA-mediated effects. Additionally, the fate of ARE-containing mRNAs is modified by inducing their localization to stress granules or mRNA processing bodies. Coordination of these various mechanisms controls the turnover of ARE-containing mRNAs, and thereby enables proper responses to cellular stress. In this review, we discuss how AUBPs regulate their target mRNAs in response to stress, along with the involvement of cytoplasmic granules in this process.

On track with P-bodies

P-bodies (processing bodies) are cytoplasmic foci visible by light microscopy in somatic cells of vertebrate and invertebrate origin as well as in yeast, plants and trypanosomes. At the molecular level, P-bodies are dynamic aggregates of specific mRNAs and proteins that serve a dual function: first, they harbour mRNAs that are translationally silenced, and such mRNA can exit again from P-bodies to re-engage in translation. Secondly, P-bodies recruit mRNAs that are targeted for deadenylation and degradation by the decapping/Xrn1 pathway. Whereas certain proteins are core constituents of P-bodies, others involved in recognizing short-lived mRNAs can only be trapped in P-bodies when mRNA decay is attenuated. This reflects the very transient interactions by which many proteins associate with P-bodies. In the present review, we summarize recent findings on the function, assembly and motility of P-bodies. An updated list of proteins and RNAs that localize to P-bodies will help in keeping track of this fast-growing field.

Regulation of translation by stress granules and processing bodies

Stress necessitates rapid reprogramming of translation in order to facilitate an adaptive response and promote survival. Cytoplasmic stress granules (SGs) and processing bodies (PBs) are dynamic structures that form in response to stress-induced translational arrest. PBs are linked to mRNA silencing and decay, while SGs are more closely linked to translation and the sorting of specific mRNAs for different fates. While they share some components and can interact physically, SGs and PBs are regulated independently, house separate functions, and contain unique markers. SG formation is associated with numerous disease states, and the expanding list of SG-associated proteins integrates SG formation with other processes such as transcription, splicing, and survival. Growing evidence suggests that SG assembly is initiated by translational arrest, and mediates cross talk with many other signaling pathways.

Functional association of human Ki-1/57 with pre-mRNA splicing events

The cytoplasmic and nuclear protein Ki-1/57 was first identified in malignant cells from Hodgkin's lymphoma. Despite studies showing its phosphorylation, arginine methylation, and interaction with several regulatory proteins, the functional role of Ki-1/57 in human cells remains to be determined. Here, we investigated the relationship of Ki-1/57 with RNA functions. Through immunoprecipitation assays, we verified the association of Ki-1/57 with the endogenous splicing proteins hnRNPQ and SFRS9 in HeLa cell extracts. We also found that recombinant Ki-1/57 was able to bind to a poly-U RNA probe in electrophoretic mobility shift assays. In a classic splicing test, we showed that Ki-1/57 can modify the splicing site selection of the adenoviral E1A minigene in a dose-dependent manner. Further confocal and fluorescence microscopy analysis revealed the localization of enhanced green fluorescent proteinKi-1/57 to nuclear bodies involved in RNA processing and or small nuclear ribonucleoprotein assembly, depending on the cellular methylation status and its N-terminal region. In summary, our findings suggest that Ki-1/57 is probably involved in cellular events related to RNA functions, such as pre-mRNA splicing.