Xenopus laevis (Daudin, 1802) as a Model Organism for Bioscience: A Historic Review and Perspective

In vitro systems have been mainly promoted by authorities to sustain research by following the 3Rs principle, but continuously increasing amounts of evidence point out that in vivo experimentation is also of extreme relevance. Xenopus laevis, an anuran amphibian, is a significant model organism in the study of evolutionary developmental biology, toxicology, ethology, neurobiology, endocrinology, immunology and tumor biology; thanks to the recent development of genome editing, it has also acquired a relevant position in the field of genetics. For these reasons, X. laevis appears to be a powerful and alternative model to the zebrafish for environmental and biomedical studies. Its life cycle, as well as the possibility to obtain gametes from adults during the whole year and embryos by in vitro fertilization, allows experimental studies of several biological endpoints, such as gametogenesis, embryogenesis, larval growth, metamorphosis and, of course, the young and adult stages. Moreover, with respect to alternative invertebrate and even vertebrate animal models, the X. laevis genome displays a higher degree of similarity with that of mammals. Here, we have reviewed the main available literature on the use of X. laevis in the biosciences and, inspired by Feymann's revised view, "Plenty of room for biology at the bottom", suggest that X. laevis is a very useful model for all possible studies.

Methods for Visualization of RNA and Cytoskeletal Elements in the Early Zebrafish Embryo

Zebrafish embryos, with their large size (>0.5 mm) and accessibility, are valuable tools for investigating core cellular processes. Many of those processes, such as cell division, asymmetric inheritance of cellular components, and structural dynamics involved in cell motility and morphology rely on cytoskeletal rearrangements and associated macromolecules. In addition to the protein-rich cytoskeleton, the early embryo is packed with maternally deposited RNA, which serves essential roles in establishing cell polarity, cell fate, and cell organization. Here, we present methods for visualizing endogenous RNA along with cytoskeletal structures, including microtubules and filamentous actin (F-actin) in the context of an intact vertebrate embryo. Each of the four protocols described herein (embryo fixation, RNA probe design/synthesis, double fluorescent in situ hybridization with tubulin immunofluorescence, and fluorescent in situ hybridization with phalloidin labeling of F-actin) are intended for optimal preservation and visualization of both the cytoskeleton and RNAs of interest. These methods can also be modified and applied to a broad range of other uses.

A guiding torch at the poles: the multiple roles of spindle microtubule-organizing centers during cell division

The spindle constitutes the cellular machinery that enables the segregation of the chromosomes during eukaryotic cell division. The microtubules that form this fascinating and complex genome distribution system emanate from specialized structures located at both its poles and known as microtubule-organizing centers (MTOCs). Beyond their structural function, the spindle MTOCs play fundamental roles in cell cycle control, the activation and functionality of the mitotic checkpoints and during cellular aging. This review highlights the pivotal importance of spindle-associated MTOCs in multiple cellular processes and their central role as key regulatory hubs where diverse intracellular signals are integrated and coordinated to ensure the successful completion of cell division and the maintenance of the replicative lifespan.

HuR translocation to the cytoplasm of cancer cells in actin-independent manner

Human antigen R (HuR) is a RNA-binding protein, which binds to the AU-rich element (ARE) in the 3'-untranslated region (3'-UTR) of certain mRNA and is involved in the export and stabilization of ARE-mRNA. HuR constitutively relocates to the cytoplasm in many cancer cells, however the mechanism of intracellular HuR trafficking is poorly understood. To address this question, we examined the functional role of the cytoskeleton in HuR relocalization. We tested the effect of actin depolymerizing macrolide latrunculin A or myosin II ATPase activity inhibitor blebbistatin for HuR relocalization induced by the vasoactive hormone Angiotensin II in cancer and control normal cells. Western blot and confocal imaging data revealed that both inhibitors attenuated the cytoplasmic HuR in normal cells but no such alteration was observed in cancer cells. Concomitant with changes in intracellular HuR localization, both inhibitors markedly decreased the accumulation and half-lives of HuR target ARE-mRNAs in normal cells, whereas no change was observed in cancer cells. Furthermore, co-immunoprecipitation experiments with HuR proteins revealed clear physical interaction with ß-actin only in normal cells. The current study is the first to verify that cancer cells can implicate a microfilament independent HuR transport. We hypothesized that when cytoskeleton structure is impaired, cancer cells can acquire an alternative HuR trafficking strategy.

Xenopus laevis oocyte as a model for the study of the cytoskeleton

At the beginning of diplotene, the oocyte of Xenopus laevis is a cell of about 10-20 microns destined to increase 10,000-fold its size when the oocyte becomes filled with yolk platelets and has accumulated a great number of pigment granules in a half of its periphery. Its internal architecture is gradually accomplished during growth because of several factors, especially because of cytoskeletal changes. In the fully-grown oocyte, the cytoskeleton appears to sustain the eccentrically located germinal vesicle through arms radiating from the cortex to the germinal vesicle, a unique organization not to be found in other Amphibians. In this report, we summarized and analysed steps of cytoskeletal proteins and related mRNAs organization and function throughout diplotene stage, highlighting our studies in this animal model. The cytoskeletal proteins appear to exploit their activity with respect to ribosomal 60S subunit maturation and during translation. Most importantly, the polarity of the oocyte is achieved through a sophisticated and highly organized localization of mRNAs and cytoskeletal proteins in one side of the cell. This asymmetry will start the construction of the oocyte polarity that is instrumental for determining the characteristic of this cell, which will become an embryo. Moreover, in the same time membrane composition, conditioned by the underlying cytoskeletal organization, will acquire the prerequisites for sperm binding and fusion.

The Expansion Segments of 28S Ribosomal RNA Extensively Match Human Messenger RNAs

Eukaryote ribosomal RNAs (rRNAs) have expanded in the course of phylogeny by addition of nucleotides in specific insertion areas, the expansion segments. These number about 40 in the larger (25–28S) rRNA (up to 2,400 nucleotides), and about 12 in the smaller (18S) rRNA (<700 nucleotides). Expansion of the larger rRNA shows a clear phylogenetic increase, with a dramatic rise in mammals and especially in hominids. Substantial portions of expansion segments in this RNA are not bound to ribosomal proteins, and may engage extraneous interactants, including messenger RNAs (mRNAs). Studies on the ribosome-mRNA interaction have focused on proteins of the smaller ribosomal subunit, with some examination of 18S rRNA. However, the expansion segments of human 28S rRNA show much higher density and numbers of mRNA matches than those of 18S rRNA, and also a higher density and match numbers than its own core parts. We have studied that with frequent and potentially stable matches containing 7–15 nucleotides. The expansion segments of 28S rRNA average more than 50 matches per mRNA even assuming only 5% of their sequence as available for such interaction. Large expansion segments 7, 15, and 27 of 28S rRNA also have copious long (≥10-nucleotide) matches to most human mRNAs, with frequencies much higher than in other 28S rRNA parts. Expansion segments 7 and 27 and especially segment 15 of 28S rRNA show large size increase in mammals compared to other metazoans, which could reflect a gain of function related to interaction with non-ribosomal partners. The 28S rRNA expansion segment 15 shows very high increments in size, guanosine, and cytidine nucleotide content and mRNA matching in mammals, and especially in hominids. With these segments (but not with other 28S rRNA or any 18S rRNA expansion segments) the density and number of matches are much higher in 5′-terminal than in 3′-terminal untranslated mRNA regions, which may relate to mRNA mobilization via 5′ termini. Matches in the expansion segments 7, 15, and 27 of human 28S rRNA appear as candidates for general interaction with mRNAs, especially those associated with intracellular matrices such as the endoplasmic reticulum.

Regulation by 3'-Untranslated Regions

3'-untranslated regions (3'-UTRs) are the noncoding parts of mRNAs. Compared to yeast, in humans, median 3'-UTR length has expanded approximately tenfold alongside an increased generation of alternative 3'-UTR isoforms. In contrast, the number of coding genes, as well as coding region length, has remained similar. This suggests an important role for 3'-UTRs in the biology of higher organisms. 3'-UTRs are best known to regulate diverse fates of mRNAs, including degradation, translation, and localization, but they can also function like long noncoding or small RNAs, as has been shown for whole 3'-UTRs as well as for cleaved fragments. Furthermore, 3'-UTRs determine the fate of proteins through the regulation of protein-protein interactions. They facilitate cotranslational protein complex formation, which establishes a role for 3'-UTRs as evolved eukaryotic operons. Whereas bacterial operons promote the interaction of two subunits, 3'-UTRs enable the formation of protein complexes with diverse compositions. All of these 3'-UTR functions are accomplished by effector proteins that are recruited by RNA-binding proteins that bind to 3'-UTR cis-elements. In summary, 3'-UTRs seem to be major players in gene regulation that enable local functions, compartmentalization, and cooperativity, which makes them important tools for the regulation of phenotypic diversity of higher organisms.

Cytoskeletal proteins associate with components of the ribosomal maturation and translation apparatus in Xenopus stage I oocytes

Actin-based cytoskeleton (CSK) and microtubules may bind to RNAs and related molecules implicated in translation. However, many questions remain to be answered regarding the role of cytoskeletal components in supporting the proteins involved in steps in the maturation and translation processes. Here, we performed co-immunoprecipitation and immunofluorescence to examine the association between spectrins, keratins and tubulin and proteins involved in 60S ribosomal maturation and translation in Xenopus stage I oocytes, including ribosomal rpl10, eukaryotic initiation factor 6 (Eif6), thesaurins A/B, homologs of the eEF1α elongation factor, and P0, the ribosomal stalk protein. We found that rpl10 and eif6 cross-reacted with the actin-based CSK and with tubulin. rpl10 co-localizes with spectrin, particularly in the perinuclear region. eif6 is similarly localized. Given that upon ribosomal maturation, the insertion of rpl10 into the 60S subunit occurs simultaneously with the release of eif6, one can hypothesise that actin-based CSK and microtubules provide the necessary scaffold for the insertion/release of these two molecules and, subsequently, for eif6 transport and binding to the mature 60S subunit. P0 and thesaurins cross-reacted with only spectrin and cytokeratins. Thesaurins aggregated at the oocyte periphery, rendering this a territory favourable site for protein synthesis; the CSK may support the interaction between thesaurins and sites of the translating ribosome. Moreover, given that the assembly of the ribosome stalk, where P0 is located, to the 60S subunit is essential for the release of eif6, it can be hypothesised that the CSK can facilitate the binding of the stalk to the 60S.

α -Actinin TvACTN3 of Trichomonas vaginalis is an RNA-binding protein that could participate in its posttranscriptional iron regulatory mechanism

Trichomonas vaginalis is a sexually transmitted flagellated protist parasite responsible for trichomoniasis. This parasite is dependent on high levels of iron, favoring its growth and multiplication. Iron also differentially regulates some trichomonad virulence properties by unknown mechanisms. However, there is evidence to support the existence of gene regulatory mechanisms at the transcriptional and posttranscriptional levels that are mediated by iron concentration in T. vaginalis. Thus, the goal of this study was to identify an RNA-binding protein in T. vaginalis that interacts with the tvcp4 RNA stem-loop structure, which may participate in a posttranscriptional iron regulatory mechanism mediated by RNA-protein interactions. We performed RNA electrophoretic mobility shift assay (REMSA) and supershift, UV cross-linking, Northwestern blot, and western blot (WB) assays using cytoplasmic protein extracts from T. vaginalis with the tvcp4 RNA hairpin structure as a probe. We identified a 135-kDa protein isolated by the UV cross-linking assays as α-actinin 3 (TvACTN3) by MALDI-TOF-MS that was confirmed by LS-MS/MS and de novo sequencing. TvACTN3 is a cytoplasmic protein that specifically binds to hairpin RNA structures from trichomonads and humans when the parasites are grown under iron-depleted conditions. Thus, TvACTN3 could participate in the regulation of gene expression by iron in T. vaginalis through a parallel posttranscriptional mechanism similar to that of the IRE/IRP system.

RNA-dependent association with myosin IIA promotes F-actin-guided trafficking of the ELAV-like protein HuR to polysomes

The role of the mRNA-binding protein human antigen R (HuR) in stabilization and translation of AU-rich elements (ARE) containing mRNAs is well established. However, the trafficking of HuR and bound mRNA cargo, which comprises a fundamental requirement for the aforementioned HuR functions is only poorly understood. By administering different cytoskeletal inhibitors, we found that the protein kinase Cδ (PKCδ)-triggered accumulation of cytoplasmic HuR by Angiotensin II (AngII) is an actin-myosin driven process functionally relevant for stabilization of ARE-bearing mRNAs. Furthermore, we show that the AngII-induced recruitment of HuR and its bound mRNA from ribonucleoprotein particles to free and cytoskeleton bound polysomes strongly depended on an intact actomyosin cytoskeleton. In addition, HuR allocation to free and cytoskeletal bound polysomes is highly sensitive toward RNase and PPtase and structurally depends on serine 318 (S318) located within the C-terminal RNA recognition motif (RRM3). Conversely, the trafficking of the phosphomimetic HuRS318D, mimicking HuR phosphorylation at S318 by the PKCδ remained PPtase resistant. Co-immunoprecipitation experiments with truncated HuR proteins revealed that the stimulus-induced association of HuR with myosin IIA is strictly RNA dependent and mediated via the RRM3. Our data implicate a microfilament dependent transport of HuR, which is relevant for stimulus-induced targeting of ARE-bearing mRNAs from translational inactive ribonucleoprotein particles to polysomes.

RNA in centrosomes: structure and possible functions

A novel RNA was detected in the centrosomes of Spisula solidissima mollusk oocytes in 2006. This RNA was named centrosomal RNA (cnRNA); five different cnRNAs were described. During the sequencing of the first transcript, cnRNA 11, it was discovered that the transcript contained a conserved structure--a reverse transcriptase domain. In a 2005 study, we speculated about several possible mechanisms for determining the most important functions of centrosomal structures and referred to one of them as an "RNA-dependent mechanism". The discovery of RNA specific to the centrosome is indirect evidence of the centrosomal hypothesis of cellular aging and differentiation. The presence of a reverse transcriptase domain in this type of RNA, together with its uniqueness and specificity, makes the centrosome a place of information storage and reproduction.

ELAV-GAP43 pathway activation following combined exposure to cocaine and stress

RATIONALE

An increasing body of evidence suggests that drug addiction engages circuits also associated with memory processes. In particular, in the hippocampus, a substantial similarity seems to exist between the changes yielded by drugs of abuse and those induced by hippocampal-dependent learning.

OBJECTIVES

Considering the key involvement of neuronal Embryonic Lethal Abnormal Vision (nELAV) proteins in memory processes occurring within the hippocampus and the critical role of stress for compulsive drug use and relapse, we investigated the effect of cocaine and stress challenges on the activation of the nELAV cascade.

MATERIALS AND METHODS

Rats were treated subcutaneously with vehicle or cocaine hydrochloride (20 mg/kg, once a day for 2 weeks). Three days later, half of them were also subjected to a single stress exposure. Western blotting and real-time polymerase chain reaction (PCR) experiments were performed on the hippocampi.

RESULTS

Our results show that the combination of repeated exposure to cocaine and acute stress significantly enhances nELAV expression and phosphorylation in the hippocampus with a concomitant increase of GAP43 expression (a specific nELAV target), an effect that seems to involve, upstream, protein kinase C alpha (PKCα). The activation of this pathway occurs independently from widespread neuronal activation since no alterations were observed in the expression of the immediate early gene Arc (a widely established index of neuronal activity), suggesting that the activation of the nELAV-GAP43 cascade reflects a targeting of specific processes rather than a global interference with hippocampal homeostasis.

CONCLUSIONS

Based on our results, we speculate that cocaine and stress may recruit such a pathway, crucial for physiological learning, potentially contributing to the aberrant engagement of learning mechanisms observed in drug addiction behavior.

Kinesin molecular motor Eg5 functions during polypeptide synthesis

The microtubule motor Eg5 is well known for its functions during mitosis. It is shown that during interphase, Eg5 associates with ribosomes and is required for efficient protein synthesis.

The kinesin-related molecular motor Eg5 plays roles in cell division, promoting spindle assembly. We show that during interphase Eg5 is associated with ribosomes and is required for optimal nascent polypeptide synthesis. When Eg5 was inhibited, ribosomes no longer bound to microtubules in vitro, ribosome transit rates slowed, and polysomes accumulated in intact cells, suggesting defects in elongation or termination during polypeptide synthesis. These results demonstrate that the molecular motor Eg5 associates with ribosomes and enhances the efficiency of translation.

RNA binding targets aminoacyl-tRNA synthetases to translating ribosomes

Here, we examine tRNA-aminoacyl synthetase (ARS) localization in protein synthesis. Proteomics reveals that ten of the twenty cytosolic ARSs associate with ribosomes in sucrose gradients: phenylalanyl-RS (FRS), and the 9 ARSs that form the multi-ARS complex (MSC). Using the ribopuromycylation method (RPM) for localizing intracellular translation, we show that FRS and the MSC, and to a lesser extent other ARSs, localize to translating ribosomes, most strikingly when translation is restricted to poxvirus or alphavirus factories in infected cells. Immunoproximity fluorescence indicates close proximity between MSC and the ribosome. Stress induced-translational shutdown recruits the MSC to stress-granules, a depot for mRNA and translation components. MSC binding to mRNA provides a facile explanation for its delivery to translating ribosomes and stress granules. These findings, along with the abundance of the MSC (9 × 10(6) copies per cell, roughly equimolar with ribosomes), is consistent with the idea that MSC specificity, recently reported to vary with cellular stress (Netzer, N., Goodenbour, J. M., David, A., Dittmar, K. A., Jones, R. B., Schneider, J. R., Boone, D., Eves, E. M., Rosner, M. R., Gibbs, J. S., Embry, A., Dolan, B., Das, S., Hickman, H. D., Berglund, P., Bennink, J. R., Yewdell, J. W., and Pan, T. (2009) Nature 462, 522-526) can be modulated at the level of individual mRNAs to modify decoding of specific gene products.

Human G3BP1 interacts with beta-F1-ATPase mRNA and inhibits its translation

The post-transcriptional regulation of nuclear mRNAs that encode core components of mitochondria has relevant implications in cell physiology. The mRNA that encodes the catalytic subunit of the mitochondrial H(+)-ATP synthase subunit beta (ATP5B, beta-F1-ATPase) is localized in a large ribonucleoprotein (RNP) complex (beta-F1-RNP), which is subjected to stringent translational control during development and the cell cycle, and in carcinogenesis. Because downregulation of beta-F1-ATPase is a conserved feature of most prevalent human carcinomas, we have investigated the molecular composition of the human beta-F1-RNP. By means of an improved affinity-chromatography procedure and protein sequencing we have identified nine RNA-binding proteins (RNABPs) of the beta-F1-RNP. Immunoprecipitation assays of Ras-GAP SH3 binding protein 1 (G3BP1) and fluorescent in-situ hybridization of mRNA indicate a direct interaction of the endogenous G3BP1 with mRNA of beta-F1-ATPase (beta-F1 mRNA). RNA-bridged trimolecular fluorescence complementation (TriFC) assays confirm the interaction of G3BP1 with the 3'-UTR of beta-F1 mRNA in cytoplasmic RNA-granules. Confocal and high-resolution immunoelectron-microscopy experiments suggest that the beta-F1-RNP is sorted to the periphery of mitochondria. Molecular and functional studies indicate that the interaction of G3BP1 with beta-F1 mRNA inhibits its translation at the initiation level, supporting a role for G3BP1 in the glycolytic switch that occurs in cancer.

Targeting of eEF1A with Amaryllidaceae isocarbostyrils as a strategy to combat melanomas

Melanomas display poor response rates to adjuvant therapies because of their intrinsic resistance to proapoptotic stimuli. This study indicates that such resistance can be overcome, at least partly, through the targeting of eEF1A elongation factor with narciclasine, an Amaryllidaceae isocarbostyril controlling plant growth. Narciclasine displays IC(50) growth inhibitory values between 30-100 nM in melanoma cell lines, irrespective of their levels of resistance to proapoptotic stimuli. Normal noncancerous cell lines are much less affected. At nontoxic doses, narciclasine also significantly improves (P=0.004) the survival of mice bearing metastatic apoptosis-resistant melanoma xenografts in their brain. The eEF1A targeting with narciclasine (50 nM) leads to 1) marked actin cytoskeleton disorganization, resulting in cytokinesis impairment, and 2) protein synthesis impairment (elongation and initiation steps), whereas apoptosis is induced at higher doses only (≥200 nM). In addition to molecular docking validation and identification of potential binding sites, we biochemically confirmed that narciclasine directly binds to human recombinant and yeast-purified eEF1A in a nanomolar range, but not to actin or elongation factor 2, and that 5 nM narciclasine is sufficient to impair eEF1A-related actin bundling activity. eEF1A is thus a potential target to combat melanomas regardless of their apoptosis-sensitivity, and this finding reconciles the pleiotropic cytostatic of narciclasine. -

The multiple roles of myelin protein genes during the development of the oligodendrocyte

It has become clear that the products of several of the earliest identified myelin protein genes perform functions that extend beyond the myelin sheath. Interestingly, these myelin proteins, which comprise proteolipid protein, 2′,3′-cyclic nucleotide 3′-phosphodiesterase and the classic and golli MBPs (myelin basic proteins), play important roles during different stages of oligodendroglial development. These non-myelin-related functions are varied and include roles in the regulation of process outgrowth, migration, RNA transport, oligodendrocyte survival and ion channel modulation. However, despite the wide variety of cellular functions performed by the different myelin genes, the route by which they achieve these many functions seems to converge upon a common mechanism involving Ca²⁺ regulation, cytoskeletal rearrangements and signal transduction. In the present review, the newly emerging functions of these myelin proteins will be described, and these will then be discussed in the context of their contribution to oligodendroglial development.

Actin: a novel interaction partner of WT1 influencing its cell dynamic properties

The Wilms' tumour suppressor, WT1, is a zinc finger protein with key roles in normal development of the genitourinary system and tumourigenesis. Mutations or deletion of WT1 result in a spectrum of developmental disorders and susceptibility to Wilms' tumour in children. Ectopic expression of Wt1 associated with oncogenic functions has been observed in a large number of malignancies, including haematological and solid cancers. Although Wt1 is predominantly a nuclear protein in normal tissues, it is mostly cytoplasmic in the majority of Wt1-expressing tumours. Actin was identified in this study as a new WT1 interaction partner both in the nucleus and in the cytoplasm. We confirmed this interaction both in vitro and in vivo and started to explore its functional significance. Perturbation of the actin cytoskeleton moved Wt1 off the polysome fraction in the cytoplasm, cancelled its nucleo-cytoplasmic shuttling and altered Wt1 DNA- and RNA-binding abilities. These data have implications for Wt1 functions in relation to RNA metabolism and response to cytoskeletal alterations in cancer cells. Thus, our findings could shed more light on the functions of both these proteins and possibly pave way for the development of new cancer therapies.

2',3'-Cyclic nucleotide 3'-phosphodiesterase: a novel RNA-binding protein that inhibits protein synthesis

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) is one of the earliest myelin-related proteins to be specifically expressed in differentiating oligodendrocytes (ODCs) in the central nervous system (CNS) and is implicated in myelin biogenesis. CNP possesses an in vitro enzymatic activity, whose in vivo relevance remains to be defined, because substrates with 2',3,-cyclic termini have not yet been identified. To characterize CNP function better, we previously determined the structure of the CNP catalytic domain by NMR. Interestingly, the structure is remarkably similar to the plant cyclic nucleotide phosphodiesterase (CPDase) from A. thaliana and the bacterial 2'-5' RNA ligase from T. thermophilus, which are known to play roles in RNA metabolism. Here we show that CNP is an RNA-binding protein. Furthermore, by using precipitation analyses, we demonstrate that CNP associates with poly(A)(+) mRNAs in vivo and suppresses translation in vitro in a dose-dependent manner. With SELEX, we isolated RNA aptamers that can suppress the inhibitory effect of CNP on translation. We also demonstrate that CNP1 can bridge an association between tubulin and RNA. These results suggest that CNP1 may regulate expression of mRNAs in ODCs of the CNS.

The 5' cap of tobacco mosaic virus (TMV) is required for virion attachment to the actin/endoplasmic reticulum network during early infection

Almost nothing is known of the earliest stages of plant virus infections. To address this, we microinjected Cy3 (UTP)-labelled tobacco mosaic virus (TMV) into living tobacco trichome cells. The Cy3-virions were infectious, and the viral genome trafficked from cell to cell. However, neither the fluorescent vRNA pool nor the co-injected green fluorescent protein (GFP) left the injected trichome, indicating that the synthesis of (unlabelled) progeny viral (v)RNA is required to initiate cell-to-cell movement, and that virus movement is not accompanied by passive plasmodesmatal gating. Cy3-vRNA formed granules that became anchored to the motile cortical actin/endoplasmic reticulum (ER) network within minutes of injection. Granule movement on actin/ER was arrested by actin inhibitors indicating actin-dependent RNA movement. The 5' methylguanosine cap was shown to be required for vRNA anchoring to the actin/ER. TMV vRNA lacking the 5' cap failed to form granules and was degraded in the cytoplasm. Removal of the 3' untranslated region or replicase both inhibited replication but did not prevent granule formation and movement. Dual-labelled TMV virions in which the vRNA and the coat protein were highlighted with different fluorophores showed that both fluorescent signals were initially located on the same ER-bound granules, indicating that TMV virions may become attached to the ER prior to uncoating of the viral genome.

Cytoskeleton-associated large RNP complexes in tobacco male gametophyte (EPPs) are associated with ribosomes and are involved in protein synthesis, processing, and localization

The progamic phase of male gametophyte development involves activation of synthetic and catabolic processes required for the rapid growth of the pollen tube. It is well-established that both transcription and translation play an important role in global and specific gene expression patterns during pollen maturation. On the contrary, germination of many pollen species has been shown to be largely independent of transcription but vitally dependent on translation of stored mRNAs. Here, we report the first structural and proteomic data about large ribonucleoprotein particles (EPPs) in tobacco male gametophyte. These complexes are formed in immature pollen where they contain translationally silent mRNAs. Although massively activated at the early progamic phase, they also serve as a long-term storage of mRNA transported along with the translational machinery to the tip region. Moreover, EPPs were shown to contain ribosomal subunits, rRNAs and a set of mRNAs. Presented results extend our view of EPP complexes from mere RNA storage and transport compartment in particular stages of pollen development to the complex and well-organized machinery devoted to mRNA storage, transport and subsequent controlled activation resulting in protein synthesis, processing and precise localization. Such an organization is extremely useful in fast tip-growing pollen tube. There, massive and orchestrated protein synthesis, processing, and transport must take place in accurately localized regions. Moreover, presented complex role of EPPs in tobacco cytoplasmic mRNA and protein metabolism makes them likely to be active in another plant species too. Expression of vast majority of the closest orthologues of EPP proteins also in Arabidopsis male gametophyte further extends this concept from tobacco to Arabidopsis, the model species with advanced tricellular pollen.

Antisense suppression of tau in cultured rat oligodendrocytes inhibits process formation

The microtubule-associated protein tau is integral to neuronal process development and has a role in the pathogenesis of several neurodegenerative conditions. We examined possible roles for tau in cultured oligodendrocyte process formation by using antisense oligonucleotide treatment. Inhibition of tau synthesis with single oligonucleotides resulted in decreased tau protein levels and significantly shorter cellular processes. Simultaneous use of two nonoverlapping oligonucleotides caused a major reduction in tau levels and severely inhibited process outgrowth. The timing of oligonucleotide addition to oligodendrocyte cultures was important, with addition of antisense at the time of plating into culture having the most significant effect on morphology through reduction of tau expression.

Translation elongation factor eEF1A binds to a novel myosin binding protein-C-like protein

Eukaryotic translation elongation factor 1A (eEF1A) is a guanine-nucleotide binding protein, which transports aminoacylated tRNA to the ribosomal A site during protein synthesis. In a yeast two-hybrid screening of a human skeletal muscle cDNA library, a novel eEF1A binding protein, immunoglobulin-like and fibronectin type III domain containing 1 (IGFN1), was discovered, and its interaction with eEF1A was confirmed in vitro. IGFN1 is specifically expressed in skeletal muscle and presents immunoglobulin I and fibronectin III sets of domains characteristic of sarcomeric proteins. IGFN1 shows sequence and structural homology to myosin binding protein-C fast and slow-type skeletal muscle isoforms. IGFN1 is substantially upregulated during muscle denervation. We propose a model in which this increased expression of IGFN1 serves to down-regulate protein synthesis via interaction with eEF1A during denervation.

YB-1 promotes microtubule assembly in vitro through interaction with tubulin and microtubules

Background

YB-1 is a major regulator of gene expression in eukaryotic cells. In addition to its role in transcription, YB-1 plays a key role in translation and stabilization of mRNAs.

Results

We show here that YB-1 interacts with tubulin and microtubules and stimulates microtubule assembly in vitro. High resolution imaging via electron and atomic force microscopy revealed that microtubules assembled in the presence of YB-1 exhibited a normal single wall ultrastructure and indicated that YB-1 most probably coats the outer microtubule wall. Furthermore, we found that YB-1 also promotes the assembly of MAPs-tubulin and subtilisin-treated tubulin. Finally, we demonstrated that tubulin interferes with RNA:YB-1 complexes.

Conclusion

These results suggest that YB-1 may regulate microtubule assembly in vivo and that its interaction with tubulin may contribute to the control of mRNA translation.

The cytoplasmic-localized, cytoskeletal-associated RNA binding protein OsTudor-SN: evidence for an essential role in storage protein RNA transport and localization

Previous studies have demonstrated that the major storage protein RNAs found in the rice endosperm are transported as particles via actomyosin to specific subdomains of the cortical endoplasmic reticulum. In this study, we examined the potential role of OsTudor-SN, a major cytoskeletal-associated RNA binding protein, in RNA transport and localization. OsTudor-SN molecules occur as high-molecular-weight forms, the integrity of which are sensitive to RNase. Immunoprecipitation followed by RT-PCR showed that OsTudor-SN binds prolamine and glutelin RNAs. Immunofluorescence studies using affinity-purified antibodies show that OsTudor-SNs exists as particles in the cytoplasm, and are distributed to both the protein body endoplasmic reticulum (ER) and cisternal ER. Examination of OsTudor-SN particles in transgenic rice plants expressing GFP-tagged prolamine RNA transport particles showed co-localization of OsTudor-SN and GFP, suggesting a role in RNA transport. Consistent with this view, GFP-tagged OsTudor-SN is observed in living endosperm sections as moving particles, a property inhibited by microfilament inhibitors. Downregulation of OsTudor-SN by antisense and RNAi resulted in a decrease in steady state prolamine RNA and protein levels, and a reduction in the number of prolamine protein bodies. Collectively, these results show that OsTudor-SN is a component of the RNA transport particle, and may control storage protein biosynthesis by regulating one or more processes leading to the transport, localization and anchoring of their RNAs to the cortical ER.

Eukaryotic elongation factor 1A interacts with Turnip mosaic virus RNA-dependent RNA polymerase and VPg-Pro in virus-induced vesicles

Eukaryotic elongation factor 1-alpha (eEF1A) was identified as an interactor of Turnip mosaic virus (TuMV) RNA-dependent RNA polymerase (RdRp) and VPg-protease (VPg-Pro) using tandem affinity purification and/or in vitro assays. Subcellular fractionation experiments revealed that the level of eEF1A substantially increased in membrane fractions upon TuMV infection. Replication of TuMV occurs in cytoplasmic membrane vesicles, which are induced by 6K-VPg-Pro. Confocal microscopy indicated that eEF1A was included in these vesicles. To confirm that eEF1A was found in replication vesicles, we constructed an infectious recombinant TuMV that contains an additional copy of the 6K protein fused to the green fluorescent protein (GFP). In cells infected with this recombinant TuMV, fluorescence emitted by 6KGFP was associated with cytoplasmic membrane vesicles that contained VPg-Pro, the eukaryotic initiation factor (iso) 4E, the poly(A)-binding protein, the heat shock cognate 70-3 protein, and eEF1A. These results suggest that TuMV-induced membrane vesicles host at least three plant translation factors in addition to the viral replication proteins.

Orientation of microtubules suggests a role in mRNA transportation in fertilized eggs of Chinese pine (Pinus tabulaeformis)

Polysomes become associated with microtubules (MTs) in egg cells of Chinese pine upon fertilization, providing direct evidence for MT-based intracellular mRNA and polysome localization. We have investigated by immunoelectron microscopy the orientation and spatial distribution of MTs and their association with polysomes in the fertilized egg cells. There is a perinuclear accumulation of MTs and polysomes in the zygote soon after fertilization. At this time, some of the MTs are perpendicular to the nuclear envelope and directly connected to the outer membrane or nuclear-pore complexes (NPC) at one end, and the other ends reach to the outer tier or cortical MTs that are parallel to the long axis of the zygote. The polysomes in the perinuclear region show the same spatial and temporal pattern as the MTs. Immunolocalization of the mRNA-binding protein hnRNP indicates that the mRNAs are loaded onto the nucleus-associated MTs immediately after their export from the nuclear-pore complexes. The polysomes and mRNAs are then transported from these MTs to the outer tier and/or cortical MTs, where they further localize to the polar region of the cell.

Cytoskeletal elements and intracellular transport

Recent advances in the understanding of the functions of various components of the cytoskeleton indicate that, besides serving a structural role, the cytoskeletal elements may regulate the transport of several proteins in the cell. Studies reveal that there are co-operative interactions between the actin and microtubule cytoskeletons including functional overlap in the transport influenced by different motor families. Multiple motors are probably involved in the control of the dynamics of many proteins and intriguing hints about how these motors are co-ordinated are appearing. It has been shown that some of the intermediate elements also participate in selected intracellular transport mechanisms. In view of the author's preoccupation with the steroid receptor systems, special attention has been given to the role of the cytoskeletal elements, particularly actin, in the intracellular transport of steroid receptors and receptor-related proteins.

Marlin-1 and conventional kinesin link GABAB receptors to the cytoskeleton and regulate receptor transport

The cytoskeleton and cytoskeletal motors play a fundamental role in neurotransmitter receptor trafficking, but proteins that link GABA(B) receptors (GABA(B)Rs) to the cytoskeleton have not been described. We recently identified Marlin-1, a protein that interacts with GABA(B)R1. Here, we explore the association of GABA(B)Rs and Marlin-1 to the cytoskeleton using a combination of biochemistry, microscopy and live cell imaging. Our results indicate that Marlin-1 is associated to microtubules and the molecular motor kinesin-I. We demonstrate that a fraction of Marlin-1 is mobile in dendrites of cultured hippocampal neurons and that mobility is microtubule-dependent. We also show that GABA(B)Rs interact robustly with kinesin-I and that intracellular membranes containing GABA(B)Rs are sensitive to treatments that disrupt a protein complex containing Marlin-1, kinesin-I and tubulin. Finally, we report that a kinesin-I mutant severely impairs receptor transport. We conclude that Marlin-1 and kinesin-1 link GABA(B)Rs to the tubulin cytoskeleton in neurons.

Temporal and spatial control of gene expression in early embryos of farm animals

A gradual transition from oocyte-derived mRNA and proteins to full embryonic transcription characterises early embryonic development. Messenger RNAs and proteins of maternal origin are accumulated into the oocyte throughout its growth inthe ovary. Upon fertilisation, sev eral mechanisms ar e activated that controlthe appropriate use of such material and prepare for the synthesis of new products. The present review will describe some of the mechanisms active in early embryos of domestic species. Data will be presented on the control of gene expression by the 3' untranslated regions and their interaction with specialised sequences at the 5' cap end. The process of RNA sorting and localisation, initially described in different cell types and in oocytes of lower species, will also be discussed, particularly in relation to its possible role in regulating early pig development. Finally, specific genes involved in the activation of cattle embryonic transcription will be described. This brief overview will provide some suggestions on how these different mechanisms may be integrated and cooperate to ensure the correct initiation of embryonic development.

Dynamics of filamentous viral RNPs prior to egress

The final step in the maturation of paramyxoviruses, orthomyxoviruses and viruses of several other families, entails the budding of the viral nucleocapsid through the plasma membrane of the host cell. Many medically important viruses, such as influenza, parainfluenza, respiratory syncytial virus (RSV) and Ebola, can form filamentous particles when budding. Although filamentous virions have been previously studied, details of how viral filaments bud from the plasma membrane remain largely unknown. Using molecular beacon (MB)-fluorescent probes to image the viral genomic RNA (vRNA) of human RSV (hRSV) in live Vero cells, the dynamics of assembled viral filaments was observed to consist of three primary types of motion prior to egress from the plasma membrane: (i) filament projection and rotation, (ii) migration and (iii) non-directed motion. In addition, from information gained by imaging the 3D distribution of cellular vRNA, observing and characterizing vRNA dynamics, imaging vRNA/Myosin Va colocalization, and studying the effects of cytochalasin D (actin depolymerizing agent) exposure, a model for filamentous virion egress is presented.

Targeting of TMV movement protein to plasmodesmata requires the actin/ER network: evidence from FRAP

Fluorescence recovery after photobleaching (FRAP) was used to study the mechanism by which fluorescent-protein-tagged movement protein (MP) of tobacco mosaic virus (TMV) is targeted to plasmodesmata (PD). The data show that fluorescence recovery in PD at the leading edge of an infection requires elements of the cortical actin/endoplasmic reticulum (ER) network and can occur in the absence of an intact microtubule (MT) cytoskeleton. Inhibitors of the actin cytoskeleton (latrunculin and cytochalasin) significantly inhibited MP targeting, while MT inhibitors (colchicine and oryzalin) did not. Application of sodium azide to infected cells implicated an active component of MP transfer to PD. Treatment of cells with Brefeldin A (BFA) at a concentration that caused reabsorption of the Golgi bodies into the ER (precluding secretion of viral MP) had no effect on MP targeting, while disruption of the cortical ER with higher concentrations of BFA caused significant inhibition. Our results support a model of TMV MP function in which targeting of MP to PD during infection is mediated by the actin/ER network.

Microtubule-associated proteins in higher plants

A variety of microtubule-associated proteins (MAPs) have been reported in higher plants. Microtubule (MT) polymerization starts from the gamma-tubulin complex (gammaTuC), a component of the MT nucleation site. MAP200/MOR1 and katanin regulate the length of the MT by promoting the dynamic instability of MTs and cutting MTs, respectively. In construction of different MT structures, MTs are bundled or are associated with other components--actin filaments, the plasma membrane, and organelles. The MAP65 family and some of kinesin family are important in bundling MTs. MT plus-end-tracking proteins (+TIPs) including end-binding protein 1 (EB1), Arabidopsis thaliana kinesin 5 (ATK5), and SPIRAL 1 (SPR1) localize to the plus end of MTs. It has been suggested that +TIPs are involved in binding of MT to other structures. Phospholipase D (PLD) is a possible candidate responsible for binding of MTs to the plasma membrane. Many candidates have been reported as actin-binding MAPs, for example calponin-homology domain (KCH) family kinesin, kinesin-like calmodulin-binding protein (KCBP), and MAP190. RNA distribution and translation depends on MT structures, and several RNA-related MAPs have been reported. This article gives an overview of predicted roles of these MAPs in higher plants.

Modulation of Alp4 function in Schizosaccharomyces pombe induces novel phenotypes that imply distinct functions for nuclear and cytoplasmic gamma-tubulin complexes

The gamma-tubulin complex acts as a nucleation unit for microtubule assembly. It remains unknown, however, how spatial and temporal regulation of the complex activity affects microtubule-mediated cellular processes. Alp4 is one of the essential components of the S. pombe gamma-tubulin complex. We show here that overproduction of a carboxy-terminal form of Alp4 (Alp4C) and its derivatives tagged to a nuclear localization signal or to a nuclear export signal affect localization of gamma-tubulin complexes and induces novel phenotypes that reflect distinct functions of nuclear and cytoplasmic gamma-tubulin complexes. Nuclear Alp4C induces a Wee1-dependent G2 delay, reduces the levels of the gamma-tubulin complex at the spindle pole body, and results in defects in mitotic progression including spindle assembly, cytoplasmic microtubule disassembly, and chromosome segregation. In contrast, cytoplasmic Alp4C induces oscillatory nuclear movement and affects levels of cell polarity markers, Bud6 and Tip1, at the cell ends. These results demonstrate that regulation of nuclear gamma-tubulin complex activity is essential for cell cycle progression through the G2/M boundary and M phase, whereas regulation of cytoplasmic gamma-tubulin complex activity is important for nuclear positioning and cell polarity control during interphase.

Compartmentalisation and localisation of the translation initiation factor (eIF) 4F complex in normally growing fibroblasts

Previous observations of association of mRNAs and ribosomes with subcellular structures highlight the importance of localised translation. However, little is known regarding associations between eukaryotic translation initiation factors and cellular structures within the cytoplasm of normally growing cells. We have used detergent-based cellular fractionation coupled with immunofluorescence microscopy to investigate the subcellular localisation in NIH3T3 fibroblasts of the initiation factors involved in recruitment of mRNA for translation, focussing on eIF4E, the mRNA cap-binding protein, the scaffold protein eIF4GI and poly(A) binding protein (PABP). We find that these proteins exist mainly in a soluble cytosolic pool, with only a subfraction tightly associated with cellular structures. However, this "associated" fraction was enriched in active "eIF4F" complexes (eIF4E.eIF4G.eIF4A.PABP). Immunofluorescence analysis reveals both a diffuse and a perinuclear distribution of eIF4G, with the perinuclear staining pattern similar to that of the endoplasmic reticulum. eIF4E also shows both a diffuse staining pattern and a tighter perinuclear stain, partly coincident with vimentin intermediate filaments. All three proteins localise to the lamellipodia of migrating cells in close proximity to ribosomes, microtubules, microfilaments and focal adhesions, with eIF4G and eIF4E at the periphery showing a similar staining pattern to the focal adhesion protein vinculin.

Polysomes are associated with microtubules in fertilized eggs of Chinese pine (Pinus tabulaeformis)

Transmission electron microscopy of immunogold-labeled Chinese pine egg cells before and after fertilization revealed that polysomes are associated with microtubules (MTs) from fertilization to the 2-nucleate embryo stage. Ribosome aggregates of various size and shape were randomly distributed in the cytoplasm of the eggs before fertilization. Single MTs or clusters were observed to be free of polysomes at this stage. Upon fertilization, all polysomes were attached to MTs, and this association persisted until the formation of the polarized embryo. Thereafter, the polysomes spread into the cytoplasm and no polysome-MT association was observed in the embryo. Some of the polysomes were attached to one end of the MTs, while others appeared to form contacts along their entire length. No polysome-microfilament association was observed at any stage of the development. The polysome-MT association may provide a mechanism for MT-dependent mRNA localization in early embryo development of this plant.

Messages on the move: the role of the cytoskeleton in mRNA localization and translation in plant cellsThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

The cytoskeleton plays an important role in numerous cellular processes, including subcellular mRNA localization and translation. Several examples of mRNA localization have emerged in plant cells, and these appear to function in protein targeting, the establishment of polarity, and cell-to-cell trafficking. The identification of several cytoskeleton-associated RNA-binding proteins in plant cells has made available candidate proteins that mediate the interaction between mRNA and the cytoskeleton, and possibly play a role in mRNA localization and translational control. We propose a model that links mRNA–microtubule interactions to translational autoregulation, a process that may assist in the efficient and regulated binding of proteins to microtubules.

Neuronal BC1 RNA: microtubule-dependent dendritic delivery

RNA localization is an important means of post-transcriptional regulation of gene expression in many eukaryotic cell types. In neurons, select RNAs are delivered to postsynaptic dendritic microdomains, a mechanism that is considered a key underpinning in the administration of long-term synaptic plasticity. BC1 RNA is a small untranslated RNA that interacts with translation initiation factors and functions as a translational repressor by targeting assembly of 48S initiation complexes. BC1 RNA is specifically and rapidly transported to dendrites where it is found concentrated in postsynaptic microdomains. The cytoskeletal infrastructure underlying dendritic localization of BC1 RNA has not been investigated. We now report that the dendritic delivery of BC1 RNA is dependent on intact microtubules. In two neuronal cell types, hippocampal neurons and sympathetic neurons in primary culture, disruption of microtubules abolished dendritic localization of BC1 RNA. In contrast, disruption of actin filaments had no significant effect on the somatodendritic distribution of BC1 RNA. It is concluded that the long-range dendritic delivery of BC1 RNA is supported by microtubules. At the same time, a role for actin filaments, while unlikely for long-range BC1 delivery, is not ruled out for short-range local translocation and anchoring at dendritic destination sites.

Centriolar Mechanisms of Differentiation and Replicative Aging of Higher Animal Cells

The centrosome (centriole) and the cytoskeleton produced by it are structures, which probably determine differentiation, morphogenesis, and switching on the mechanism of replicative aging in all somatic cells of multicellular animals. The mechanism of such programming of the events seems to include cytoskeleton influences and small RNAs related to the centrosome. 1) If these functions are really related with centrioles, the multicellular organism's cells which: a) initially lack centrioles (e.g., higher plant cells and also zygote and early blastomeres of some animals) or cytoskeleton (e.g., embryonic stem cells); or b) generate centrioles de novo (e.g., zygote and early blastomeres of some animals), will be totipotent and lack replicative aging. Consequently, the absence (constant or temporary) of the structure determining the counting of divisions also means the absence of counting of differentiation processes. 2) Although a particular damage to centrioles or cytoskeleton (e.g., in tumor cells) fails to make the cells totipotent (because the morphogenetic status of these cells, as differentiated from that of totipotent ones, is not zero), but such a transformation can suppress the initiation of the aging mechanism induced by these structures and, thus, make such cells replicatively "immortal".

An eleven nucleotide section of the 3'-untranslated region is required for perinuclear localization of rat metallothionein-1 mRNA

Localization of mRNAs provides a novel mechanism for synthesis of proteins close to their site of function. MT1 (metallothionein-1) is a small, metal-binding protein that is largely cytoplasmic but which can be found in the nucleus. The localization of rat MT1 requires the perinuclear localization of its mRNA by a mechanism dependent on the 3'-UTR (3'-untranslated region). The present study investigates the nature of this mRNA localization signal using Chinese-hamster ovary cells transfected with gene constructs in which either MT1 or the globin coding region is linked to different sequences from the MT1 3'-UTR. Deletion, mutagenesis and antisense oligonucleotide approaches indicate that nt 45-76 of the 3'-UTR, in particular nt 66-76, are required for the localization of either MT1 mRNA or chimaeric transcripts in which a beta-globin coding region is linked to sequences from the MT1 3'-UTR. This section of the 3'-UTR contains a CACC repeat. Two mutations that are predicted to alter the secondary structure of this region also impair localization. Our hypothesis is that the perinuclear localization signal in MT1 mRNA is formed by a combination of the CACC repeat and its structural context.

Differential translation and fragile X syndrome

Fragile X syndrome (FXS) is caused by the transcriptional silencing of the Fmr1 gene, which encodes a protein (FMRP) that can act as a translational suppressor in dendrites, and is characterized by a preponderance of abnormally long, thin and tortuous dendritic spines. According to a current theory of FXS, the loss of FMRP expression leads to an exaggeration of translation responses linked to group I metabotropic glutamate receptors. Such responses are involved in the consolidation of a form of long-term depression that is enhanced in Fmr1 knockout mice and in the elongation of dendritic spines, resembling synaptic phenotypes over-represented in fragile X brain. These observations place fragile X research at the heart of a long-standing issue in neuroscience. The consolidation of memory, and several distinct forms of synaptic plasticity considered to be substrates of memory, requires mRNA translation and is associated with changes in spine morphology. A recent convergence of research on FXS and on the involvement of translation in various forms of synaptic plasticity has been very informative on this issue and on mechanisms underlying FXS. Evidence suggests a general relationship in which the receptors that induce distinct forms of efficacy change differentially regulate translation to produce unique spine shapes involved in their consolidation. We discuss several potential mechanisms for differential translation and the notion that FXS represents an exaggeration of one 'channel' in a set of translation-dependent consolidation responses.

Direct visualization of mRNA colocalization with mitochondria in living cells using molecular beacons

The intracellular localization and specific organelle association of mRNA may reflect essential functions, stages, and stability of mRNA. We report the direct visualization of subcellular localization of K-ras and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNAs in live HDF cells using molecular beacons together with membrane-permeabilization and peptide-based delivery. Unexpectedly, we found that both K-ras and GAPDH mRNAs colocalize with mitochondria. Extensive control studies are performed, including the use of fluorescence in-situ hybridization (FISH), negative-control beacons, and the detection of colocalization of 28S ribosomal RNA with the rough endoplasmic reticulum (ER), suggesting that the mRNA localization and colocalization patterns observed in our study are true and specific. Our observation reveals intriguing subcellular associations of mRNA with organelles such as mitochondria, which may provide new insight into the transport, dynamics, and functions of mRNA and mRNA-protein interactions.

Identification of proteins binding the native tubulin dimer

Microtubules play an essential role in eukaryotic cells, where they perform a wide variety of functions. In this paper, we describe the characterization of proteins associated to tubulin dimer in its native form, using affinity chromatography and mass spectrometry. We used an immunoaffinity column with coupled-monoclonal antibody directed against the alpha-tubulin C-terminus. Tubulin was first loaded onto the column, then interphase and mitotic cell lysates were chromatographed. Tubulin-binding proteins were eluted using a peptide mimicking the alpha-tubulin C-terminus. Elution fractions were analyzed by SDS-PAGE, and a total of 14 proteins were identified with high confidence by mass spectrometry. These proteins could be grouped in four classes: known tubulin-binding proteins, one microtubule-associated protein, heat shock proteins, and proteins that were not shown previously to bind tubulin dimer or microtubules.

Genetic interactions of DST1 in Saccharomyces cerevisiae suggest a role of TFIIS in the initiation-elongation transition

TFIIS promotes the intrinsic ability of RNA polymerase II to cleave the 3'-end of the newly synthesized RNA. This stimulatory activity of TFIIS, which is dependent upon Rpb9, facilitates the resumption of transcription elongation when the polymerase stalls or arrests. While TFIIS has a pronounced effect on transcription elongation in vitro, the deletion of DST1 has no major effect on cell viability. In this work we used a genetic approach to increase our knowledge of the role of TFIIS in vivo. We showed that: (1) dst1 and rpb9 mutants have a synthetic growth defective phenotype when combined with fyv4, gim5, htz1, yal011w, ybr231c, soh1, vps71, and vps72 mutants that is exacerbated during germination or at high salt concentrations; (2) TFIIS and Rpb9 are essential when the cells are challenged with microtubule-destabilizing drugs; (3) among the SDO (synthetic with Dst one), SOH1 shows the strongest genetic interaction with DST1; (4) the presence of multiple copies of TAF14, SUA7, GAL11, RTS1, and TYS1 alleviate the growth phenotype of dst1 soh1 mutants; and (5) SRB5 and SIN4 genetically interact with DST1. We propose that TFIIS is required under stress conditions and that TFIIS is important for the transition between initiation and elongation in vivo.

mRNA localization and the cytoskeleton

mRNA localization is a widespread post-transcriptional mechanism for targeting protein synthesis to specific cellular sites. It is involved in the generation of cell polarity, asymmetric segregation of cell fate determinants and germ cell specification. Actin and microtubule filaments have key functions during RNA localization, especially during transport of mRNAs and anchoring at target sites. Recent advances in understanding the role of motors and filament systems have mainly resulted from the contribution of live imaging of mRNA movement and from the purification of putative localization ribonucleoproteins. There have also been new findings on the role of centrosomes in RNA localization.

Keratins modulate c-Flip/extracellular signal-regulated kinase 1 and 2 antiapoptotic signaling in simple epithelial cells

Among the large family of intermediate filament proteins, the keratin 8 and 18 (K8/K18) pair constitutes a hallmark for all simple epithelial cells, such as hepatocytes and mammary cells. Functional studies with different cell models have suggested that K8/K18 are involved in simple epithelial cell resistance to several forms of stress that may lead to cell death. We have reported recently that K8/K18-deprived hepatocytes from K8-null mice are more sensitive to Fas-mediated apoptosis. Here we show that upon Fas, tumor necrosis factor alpha receptor, or tumor necrosis factor alpha-related apoptosis-inducing ligand receptor stimulation, an inhibition of extracellular signal-regulated kinase 1 and 2 (ERK1/2) activation sensitizes wild-type but not K8-null mouse hepatocytes to apoptosis and that a much weaker ERK1/2 activation occurs in K8-null hepatocytes. In turn, this impaired ERK1/2 activation in K8-null hepatocytes is associated with a drastic reduction in c-Flip protein, an event that also holds in a K8-null mouse mammary cell line. c-Flip, along with Raf-1, is part of a K8/K18-immunoisolated complex from wild-type hepatocytes, and Fas stimulation leads to further c-Flip and Raf-1 recruitment in the complex. This points to a new regulatory role of simple epithelium keratins in the c-Flip/ERK1/2 antiapoptotic signaling pathway.

Biochemical evidence for the association of fragile X mental retardation protein with brain polyribosomal ribonucleoparticles

Fragile X syndrome is caused by the absence of the fragile X mental retardation protein (FMRP). This RNA-binding protein is widely expressed in human and mouse tissues, and it is particularly abundant in the brain because of its high expression in neurons, where it localizes in the cell body and in granules throughout dendrites. Although FMRP is thought to regulate trafficking of repressed mRNA complexes and to influence local protein synthesis in synapses, it is not known whether it has additional functions in the control of translation in the cell body. Here, we have used recently developed approaches to investigate whether FMRP is associated with the translation apparatus. We demonstrate that, in the brain, FMRP is present in actively translating polyribosomes, and we show that this association is acutely sensitive to the type of detergent required to release polyribosomes from membranous structures. In addition, proteomic analyses of purified brain polyribosomes reveal the presence of several RNA-binding proteins that, similarly to FMRP, have been previously localized in neuronal granules. Our findings highlight the complex roles of FMRP both in actively translating polyribosomes and in repressed trafficking ribonucleoparticle granules.