Glorund, a Drosophila hnRNP F/H homolog, is an ovarian repressor of nanos translation

The chloroplast RNA-binding protein CP29A supports rbcL expression during cold acclimation

The chloroplast genome encodes key components of the photosynthetic light reaction machinery as well as the large subunit of the enzyme central for carbon fixation, Ribulose-1,5-bisphosphat-carboxylase/-oxygenase (RuBisCo). Its expression is predominantly regulated posttranscriptionally, with nuclear-encoded RNA-binding proteins (RBPs) playing a key role. Mutants of chloroplast gene expression factors often exhibit impaired chloroplast biogenesis, especially in cold conditions. Low temperatures pose a challenge for plants as this leads to electron imbalances and oxidative damage. A well-known response of plants to this problem is to increase the production of RuBisCo and other Calvin Cycle enzymes in the cold, but how this is achieved is unclear. The chloroplast RBP CP29A has been shown to be essential for cold resistance in growing leaf tissue of Arabidopsis thaliana. Here, we examined CP29A-RNA interaction sites at nucleotide resolution. We found that CP29A preferentially binds to the 5'-untranslated region of rbcL, downstream of the binding site of the pentatricopeptide repeat protein MATURATION OF RBCL 1 (MRL1). MRL1 is an RBP known to be necessary for the accumulation of rbcL. In Arabidopsis mutants lacking CP29A, we were unable to observe significant effects on rbcL, possibly due to CP29A's restricted role in a limited number of cells at the base of leaves. In contrast, CRISPR/Cas9-induced mutants of tobacco NtCP29A exhibit cold-dependent photosynthetic deficiencies throughout the entire leaf blade. This is associated with a parallel reduction in rbcL mRNA and RbcL protein accumulation. Our work indicates that a chloroplast RNA-binding protein contributes to cold acclimation of RbcL production.

The role of HnrnpF/H as a driver of oligoteratozoospermia

Male subfertility or infertility is a common condition often characterized by men producing a low number of sperm with poor quality. To gain insight into this condition, we performed a quantitative proteomic analysis of semen samples obtained from infertile and fertile men. At least 6 proteins showed significant differences in regulation of alternatively spliced isoforms. To investigate this link between aberrant alternative splicing and production of poor-quality spermatozoa, we overexpressed the hnrnpH/F-orthologue Glorund (Glo) in Drosophila, which was also found to be abundant in poor quality human sperm. Transgenic animals produced low numbers of morphologically defective spermatozoa and aberrant formation of the "dense body," an organelle akin to the mammalian manchette. Furthermore, fertility trials demonstrated that transgenic flies were either completely infertile or highly subfertile. These findings suggest that dysregulation of hnrnpH/F is likely to result in the production of low-quality semen, leading to subfertility or infertility in men.

The translational repressor Glorund uses interchangeable RNA recognition domains to recognize Drosophila nanos

The Drosophila melanogaster protein Glorund (Glo) represses nanos (nos) translation and uses its quasi-RNA recognition motifs (qRRMs) to recognize both G-tract and structured UA-rich motifs within the nos translational control element (TCE). We showed previously that each of the three qRRMs is multifunctional, capable of binding to G-tract and UA-rich motifs, yet if and how the qRRMs combine to recognize the nos TCE remained unclear. Here we determined solution structures of a nos TCEI_III RNA containing the G-tract and UA-rich motifs. The RNA structure demonstrated that a single qRRM is physically incapable of recognizing both RNA elements simultaneously. In vivo experiments further indicated that any two qRRMs are sufficient to repress nos translation. We probed interactions of Glo qRRMs with TCEI_III RNA using NMR paramagnetic relaxation experiments. Our in vitro and in vivo data support a model whereby tandem Glo qRRMs are indeed multifunctional and interchangeable for recognition of TCE G-tract or UA-rich motifs. This study illustrates how multiple RNA recognition modules within an RNA-binding protein may combine to diversify the RNAs that are recognized and regulated.

Novel roles for RNA binding proteins squid, hephaesteus, and Hrb27C in Drosophila oogenesis

BACKGROUND

Reproductive capacity in many organisms is maintained by germline stem cells (GSCs). A complex regulatory network influences stem cell fate, including intrinsic factors, local signals, and hormonal and nutritional cues. Posttranscriptional regulatory mechanisms ensure proper cell fate transitions, promoting germ cell differentiation to oocytes. As essential RNA binding proteins with constitutive functions in RNA metabolism, heterogeneous nuclear ribonucleoproteins (hnRNPs) have been implicated in GSC function and axis specification during oocyte development. HnRNPs support biogenesis, localization, maturation, and translation of nascent transcripts. Whether and individual hnRNPs specifically regulate GSC function has yet to be explored.

RESULTS

We demonstrate that hnRNPs are expressed in distinct patterns in the Drosophila germarium. We show that three hnRNPs, squid, hephaestus, and Hrb27C are cell-autonomously required in GSCs for their maintenance. Although these hnRNPs do not impact adhesion of GSCs to adjacent cap cells, squid and hephaestus (but not Hrb27C) are necessary for proper bone morphogenetic protein signaling in GSCs. Moreover, Hrb27C promotes proper GSC proliferation, whereas hephaestus promotes cyst division.

CONCLUSIONS

We find that hnRNPs are independently and intrinsically required in GSCs for their maintenance in adults. Our results support the model that hnRNPs play unique roles in stem cells essential for their self-renewal and proliferation.

The Drosophila hnRNP F/H homolog Glorund recruits dFMRP to inhibit nanos translation elongation

Translational control of maternal mRNAs generates spatial and temporal patterns of protein expression necessary to begin animal development. Translational repression of unlocalized nanos (nos) mRNA in late-stage Drosophila oocytes by the hnRNP F/H homolog, Glorund (Glo), is important for embryonic body patterning. While previous work has suggested that repression occurs at both the translation initiation and elongation phases, the molecular mechanism by which Glo regulates nos translation remains elusive. Here, we have identified the Drosophila fragile X mental retardation protein, dFMRP, as a Glo interaction partner with links to the translational machinery. Using an oocyte-based in vitro translation system, we confirmed that Glo regulates both initiation and elongation of a nos translational reporter and showed that dFMRP specifically represses translation elongation and promotes ribosome stalling. Furthermore, we combined mutational analysis and in vivo and in vitro binding assays to show that Glo's qRRM2 domain specifically and directly interacts with dFMRP. Our findings suggest that Glo regulates nos translation elongation by recruiting dFMRP and that Glo's RNA-binding domains can also function as protein-protein interaction interfaces critical for its regulatory functions. Additionally, they reveal a mechanism for targeting dFMRP to specific transcripts.

Translational Regulation by hnRNP H/F Is Essential for the Proliferation and Survival of Glioblastoma

Deregulation of mRNA translation is a widespread characteristic of glioblastoma (GBM), aggressive malignant brain tumors that are resistant to conventional therapies. RNA-binding proteins (RBPs) play a critical role in translational regulation, yet the mechanisms and impact of these regulations on cancer development, progression and response to therapy remain to be fully understood. Here, we showed that hnRNP H/F RBPs are potent regulators of translation through several mechanisms that converge to modulate the expression and/or the activity of translation initiation factors. Among these, hnRNP H/F regulate the phosphorylation of eIF4E and its translational targets by controlling RNA splicing of the A-Raf kinase mRNA, which in turn modulates the MEK-ERK/MAPK signaling pathway. The underlying mechanism involves RNA G-quadruplex (RG4s), RNA structures whose modulation phenocopies hnRNP H/F translation regulation in GBM cells. Our results highlighted that hnRNP H/F are essential for key functional pathways regulating proliferation and survival of GBM, highlighting its targeting as a promising strategy for improving therapeutic outcomes.

Post-transcriptional regulation of factors important for the germ line

Echinoderms are a major model system for many general aspects of biology, including mechanisms of gene regulation. Analysis of transcriptional regulation (Gene regulatory networks, direct DNA-binding of proteins to specific cis-elements, and transgenesis) has contributed to our understanding of how an embryo works. This chapter looks at post-transcriptional gene regulation in the context of how the primordial germ cells are formed, and how the factors essential for this process are regulated. Important in echinoderms, as in many embryos, is that key steps of fate determination are made post-transcriptionally. This chapter highlights these steps uncovered in sea urchins and sea stars, and links them to a general theme of how the germ line may regulate its fate differently than many of the embryo's somatic cell lineages.

A first glimpse into the transcriptomic changes induced by the PaV1 infection in the gut of Caribbean spiny lobsters, Panulirus argus (Latreille, 1804) (Decapoda: Achelata: Palinuridae)

The Caribbean spiny lobster, Panulirus argus (Latreille, 1804) supports important fisheries in the Caribbean region. This species is affected by a deadly virus, Panulirus argus Virus 1 (PaV1), the only known pathogenic virus for this species. As infection progresses, the effects of PaV1 on its host become systemic, with far reaching impacts on the host's physiology, including structural injuries to its gastrointestinal organs, such as the hepatopancreas and the gut. This last one becomes highly compromised in the last stages of infection. Since the gut is a key organ for the physiological stability of lobsters, we compared the transcriptomic changes in the gut of juvenile individuals of Panulirus argus naturally infected with PaV1. In the RNA-Seq analysis, we obtained a total of 485 × 10⁶ raw reads. After cleaning, reads were de novo assembled into 68,842 transcripts and 50,257 unigenes. The length of unigenes ranged from 201 bp to 28,717 bp, with a N50 length of 2079, and a GC content of 40.61%. In the differential gene expression analysis, we identified a total of 3,405 non redundant differential transcripts, of which 1,920 were up-regulated and 1,485 were down-regulated. We found alterations in transcripts encoding for proteins involved in transcriptional regulation, splicing, postraductional regulation, protein signaling, transmembrane transport, cytoskeletal regulation, and proteolysis, among others. This is the first insight into the transcriptomic regulation of PaV1-P. argus interaction. The information generated can help to unravel the molecular mechanisms that may intervene in the gut during PaV1 infection.

The heterogeneous nuclear ribonucleoprotein (hnRNP) glorund functions in the Drosophila fat body to regulate lipid storage and transport

The availability of excess nutrients in Western diets has led to the overaccumulation of these nutrients as triglycerides, a condition known as obesity. The full complement of genes important for regulating triglyceride storage is not completely understood. Genome-wide RNAi screens in Drosophila cells have identified genes involved in mRNA splicing as important lipid storage regulators. Our lab has shown that a group of splicing factors called heterogeneous nuclear ribonucleoproteins (hnRNPs) regulate lipid metabolism in the fly fat body; however, the identities of all the hnRNPs that function to control triglyceride storage are not known. Here, we used the GAL4/UAS system to induce RNAi to the hnRNP glorund (glo) in the Drosophila fat body to assess whether this hnRNP has any metabolic functions. Decreasing glo levels resulted in less triglycerides being stored throughout the fly. Interestingly, decreasing fat body glo expression resulted in increased triglyceride storage in the fat body, but blunted triglyceride storage in non-fat body tissues, suggesting a defect in lipid transport. Consistent with this hypothesis, the expression of apolipophorin (apolpp), microsomal triglyceride transfer protein (mtp), and apolipoprotein lipid transfer particle (apoltp), apolipoprotein genes important for lipid transport through the fly hemolymph, was decreased in glo-RNAi flies, suggesting that glo regulates the transport of lipids from the fly fat body to surrounding tissues. Together, these results indicate that glorund plays a role in controlling lipid transport and storage and provide additional evidence of the link between gene expression and the regulation of lipid metabolism.

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The metabolic functions of the hnRNP glorund (glo) are unknown.

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Fat body glo-RNAi increases fat body triglyceride storage.

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Fat body glo-RNAi decreases triglyceride storage in non-fat body tissues.

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Decreasing glo in the fly fat body blunts the expression of lipid transport genes.

An Interaction Network of RNA-Binding Proteins Involved in Drosophila Oogenesis

During Drosophila oogenesis, the localization and translational regulation of maternal transcripts relies on RNA-binding proteins (RBPs). Many of these RBPs localize several mRNAs and may have additional direct interaction partners to regulate their functions. Using immunoprecipitation from whole Drosophila ovaries coupled to mass spectrometry, we examined protein-protein associations of 6 GFP-tagged RBPs expressed at physiological levels. Analysis of the interaction network and further validation in human cells allowed us to identify 26 previously unknown associations, besides recovering several well characterized interactions. We identified interactions between RBPs and several splicing factors, providing links between nuclear and cytoplasmic events of mRNA regulation. Additionally, components of the translational and RNA decay machineries were selectively co-purified with some baits, suggesting a mechanism for how RBPs may regulate maternal transcripts. Given the evolutionary conservation of the studied RBPs, the interaction network presented here provides the foundation for future functional and structural studies of mRNA localization across metazoans.

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

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

Translational Control during Developmental Transitions

The many steps of gene expression, from the transcription of a gene to the production of its protein product, are well understood. Yet, transcriptional regulation has been the focal point for the study of gene expression during development. However, quantitative studies reveal that messenger RNA (mRNA) levels are not necessarily good predictors of the respective proteins' levels in a cell. This discrepancy is, at least in part, the result of developmentally regulated, translational mechanisms that control the spatiotemporal regulation of gene expression. In this review, we focus on translational regulatory mechanisms mediating global transitions in gene expression: the shift from the maternal to the embryonic developmental program in the early embryo and the switch from the self-renewal of stem cells to differentiation in the adult.

The Drosophila hnRNP F/H Homolog Glorund Uses Two Distinct RNA-Binding Modes to Diversify Target Recognition

The Drosophila hnRNP F/H homolog, Glorund (Glo), regulates nanos mRNA translation by interacting with a structured UA-rich motif in the nanos 3' untranslated region. Glo regulates additional RNAs, however, and mammalian homologs bind G-tract sequences to regulate alternative splicing, suggesting that Glo also recognizes G-tract RNA. To gain insight into how Glo recognizes both structured UA-rich and G-tract RNAs, we used mutational analysis guided by crystal structures of Glo's RNA-binding domains and identified two discrete RNA-binding surfaces that allow Glo to recognize both RNA motifs. By engineering Glo variants that favor a single RNA-binding mode, we show that a subset of Glo's functions in vivo is mediated solely by the G-tract binding mode, whereas regulation of nanos requires both recognition modes. Our findings suggest a molecular mechanism for the evolution of dual RNA motif recognition in Glo that may be applied to understanding the functional diversity of other RNA-binding proteins.

Translational Control of Germ Cell Decisions

Germline poses unique challenges to gene expression control at the transcriptional level. While the embryonic germline maintains a global hold on new mRNA transcription, the female adult germline produces transcripts that are not translated into proteins until embryogenesis of subsequent generation. As a consequence, translational control plays a central role in governing various germ cell decisions including the formation of primordial germ cells, self-renewal/differentiation decisions in the adult germline, onset of gametogenesis and oocyte maturation. Mechanistically, several common themes such as asymmetric localization of mRNAs, conserved RNA-binding proteins that control translation by 3' UTR binding, translational activation by the cytoplasmic elongation of the polyA tail and the assembly of mRNA-protein complexes called mRNPs have emerged from the studies on Caenorhabditis elegans, Xenopus and Drosophila. How mRNPs assemble, what influences their dynamics, and how a particular 3' UTR-binding protein turns on the translation of certain mRNAs while turning off other mRNAs at the same time and space are key challenges for future work.

Poly(ADP-Ribosyl)ation of hnRNP A1 Protein Controls Translational Repression in Drosophila

Poly(ADP-ribosyl)ation of heterogeneous nuclear ribonucleoproteins (hnRNPs) regulates the posttranscriptional fate of RNA during development. Drosophila hnRNP A1, Hrp38, is required for germ line stem cell maintenance and oocyte localization. The mRNA targets regulated by Hrp38 are mostly unknown. We identified 428 Hrp38-associated gene transcripts in the fly ovary, including mRNA of the translational repressor Nanos. We found that Hrp38 binds to the 3' untranslated region (UTR) of Nanos mRNA, which contains a translation control element. We have demonstrated that translation of the luciferase reporter bearing the Nanos 3' UTR is enhanced by dsRNA-mediated Hrp38 knockdown as well as by mutating potential Hrp38-binding sites. Our data show that poly(ADP-ribosyl)ation inhibits Hrp38 binding to the Nanos 3' UTR, increasing the translation in vivo and in vitro hrp38 and Parg null mutants showed an increased ectopic Nanos translation early in the embryo. We conclude that Hrp38 represses Nanos translation, whereas its poly(ADP-ribosyl)ation relieves the repression effect, allowing restricted Nanos expression in the posterior germ plasm during oogenesis and early embryogenesis.

Nanos-mediated repression of hid protects larval sensory neurons after a global switch in sensitivity to apoptotic signals

Dendritic arbor morphology is a key determinant of neuronal function. Once established, dendrite branching patterns must be maintained as the animal develops to ensure receptive field coverage. The translational repressors Nanos (Nos) and Pumilio (Pum) are required to maintain dendrite growth and branching of Drosophila larval class IV dendritic arborization (da) neurons, but their specific regulatory role remains unknown. We show that Nos-Pum-mediated repression of the pro-apoptotic gene head involution defective (hid) is required to maintain a balance of dendritic growth and retraction in class IV da neurons and that upregulation of hid results in decreased branching because of an increase in caspase activity. The temporal requirement for nos correlates with an ecdysone-triggered switch in sensitivity to apoptotic stimuli that occurs during the mid-L3 transition. We find that hid is required during pupariation for caspase-dependent pruning of class IV da neurons and that Nos and Pum delay pruning. Together, these results suggest that Nos and Pum provide a crucial neuroprotective regulatory layer to ensure that neurons behave appropriately in response to developmental cues.

Localized Translation of gurken/TGF-α mRNA during Axis Specification Is Controlled by Access to Orb/CPEB on Processing Bodies

In Drosophila oocytes, gurken/TGF-α mRNA is essential for establishing the future embryonic axes. gurken remains translationally silent during transport from its point of synthesis in nurse cells to its final destination in the oocyte, where it associates with the edge of processing bodies. Here we show that, in nurse cells, gurken is kept translationally silent by the lack of sufficient Orb/CPEB, its translational activator. Processing bodies in nurse cells have a similar protein complement and ultrastructure to those in the oocyte, but they markedly less Orb and do not associate with gurken mRNA. Ectopic expression of Orb in nurse cells at levels similar to the wild-type oocyte dorso-anterior corner at mid-oogenesis is sufficient to cause gurken mRNA to associate with processing bodies and translate prematurely. We propose that controlling the spatial distribution of translational activators is a fundamental mechanism for regulating localized translation.

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gurken mRNA is not silenced by known repressors during its transport

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In nurse cells, gurken mRNA is not associated with processing bodies

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In nurse cells, lack of sufficient Orb/CPEB silences gurken mRNA translation

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In oocytes, gurken mRNA is associated with Orb on processing bodies and translated

Germ Plasm Biogenesis--An Oskar-Centric Perspective

Germ granules are the hallmark of all germ cells. These membrane-less, electron-dense structures were first observed over 100 years ago. Today, their role in regulating and processing transcripts critical for the establishment, maintenance, and protection of germ cells is well established, and pathways outlining the biochemical mechanisms and physical properties associated with their biogenesis are emerging.

RNA-Seq Analysis of Differential Splice Junction Usage and Intron Retentions by DEXSeq

Alternative splicing is an important biological process in the generation of multiple functional transcripts from the same genomic sequences. Differential analysis of splice junctions (SJs) and intron retentions (IRs) is helpful in the detection of alternative splicing events. In this study, we conducted differential analysis of SJs and IRs by use of DEXSeq, a Bioconductor package originally designed for differential exon usage analysis in RNA-seq data analysis. We set up an analysis pipeline including mapping of RNA-seq reads, the preparation of count tables of SJs and IRs as the input files, and the differential analysis in DEXSeq. We analyzed the public RNA-seq datasets generated from RNAi experiments on Drosophila melanogaster S2-DRSC cells to deplete RNA-binding proteins (GSE18508). The analysis confirmed previous findings on the alternative splicing of the trol and Ant2 (sesB) genes in the CG8144 (ps)-depletion experiment and identified some new alternative splicing events in other RNAi experiments. We also identified IRs that were confirmed in our SJ analysis. The proposed method used in our study can output the genomic coordinates of differentially used SJs and thus enable sequence motif search. Sequence motif search and gene function annotation analysis helped us infer the underlying mechanism in alternative splicing events. To further evaluate this method, we also applied the method to public RNA-seq data from human breast cancer (GSE45419) and the plant Arabidopsis (SRP008262). In conclusion, our study showed that DEXSeq can be adapted to differential analysis of SJs and IRs, which will facilitate the identification of alternative splicing events and provide insights into the molecular mechanisms of transcription processes and disease development.

Regulation of alternative splicing in Drosophila by 56 RNA binding proteins

Alternative splicing is regulated by RNA binding proteins (RBPs) that recognize pre-mRNA sequence elements and activate or repress adjacent exons. Here, we used RNA interference and RNA-seq to identify splicing events regulated by 56 Drosophila proteins, some previously unknown to regulate splicing. Nearly all proteins affected alternative first exons, suggesting that RBPs play important roles in first exon choice. Half of the splicing events were regulated by multiple proteins, demonstrating extensive combinatorial regulation. We observed that SR and hnRNP proteins tend to act coordinately with each other, not antagonistically. We also identified a cross-regulatory network where splicing regulators affected the splicing of pre-mRNAs encoding other splicing regulators. This large-scale study substantially enhances our understanding of recent models of splicing regulation and provides a resource of thousands of exons that are regulated by 56 diverse RBPs.

PINK1 and Parkin control localized translation of respiratory chain component mRNAs on mitochondria outer membrane

Mitochondria play essential roles in many aspects of biology, and their dysfunction has been linked to diverse diseases. Central to mitochondrial function is oxidative phosphorylation (OXPHOS), accomplished by respiratory chain complexes (RCCs) encoded by nuclear and mitochondrial genomes. How RCC biogenesis is regulated in metazoans is poorly understood. Here we show that Parkinson's disease (PD)-associated genes PINK1 and Parkin direct localized translation of certain nuclear-encoded RCC (nRCC) mRNAs. Translationally repressed nRCC mRNAs are localized in a PINK1/Tom20-dependent manner to mitochondrial outer membrane, where they are derepressed and activated by PINK1/Parkin through displacement of translation repressors, including Pumilio and Glorund/hnRNP-F, a Parkin substrate, and enhanced binding of activators such as eIF4G. Inhibiting the translation repressors rescued nRCC mRNA translation and neuromuscular-degeneration phenotypes of PINK1 mutant, whereas inhibiting eIF4G had opposite effects. Our results reveal previously unknown functions of PINK1/Parkin in RNA metabolism and suggest new approaches to mitochondrial restoration and disease intervention.

Ultrasensitive proteome analysis using paramagnetic bead technology

In order to obtain a systems-level understanding of a complex biological system, detailed proteome information is essential. Despite great progress in proteomics technologies, thorough interrogation of the proteome from quantity-limited biological samples is hampered by inefficiencies during processing. To address these challenges, here we introduce a novel protocol using paramagnetic beads, termed Single-Pot Solid-Phase-enhanced Sample Preparation (SP3). SP3 provides a rapid and unbiased means of proteomic sample preparation in a single tube that facilitates ultrasensitive analysis by outperforming existing protocols in terms of efficiency, scalability, speed, throughput, and flexibility. To illustrate these benefits, characterization of 1,000 HeLa cells and single Drosophila embryos is used to establish that SP3 provides an enhanced platform for profiling proteomes derived from sub-microgram amounts of material. These data present a first view of developmental stage-specific proteome dynamics in Drosophila at a single-embryo resolution, permitting characterization of inter-individual expression variation. Together, the findings of this work position SP3 as a superior protocol that facilitates exciting new directions in multiple areas of proteomics ranging from developmental biology to clinical applications.

hnRNP F complexes with tristetraprolin and stimulates ARE-mRNA decay

The tristetraprolin (TTP) family of zinc-finger proteins, TTP, BRF1 and BRF2, regulate the stability of a subset of mRNAs containing 3′UTR AU-rich elements (AREs), including mRNAs coding for cytokines, transcription factors, and proto-oncogenes. To better understand the mechanism by which TTP-family proteins control mRNA stability in mammalian cells, we aimed to identify TTP- and BRF1-interacting proteins as potential TTP-family co-factors. This revealed hnRNP F as a prominent interactor of TTP and BRF1. While TTP, BRF1 and hnRNP F are all RNA binding proteins (RBPs), the interaction of hnRNP F with TTP and BRF1 is independent of RNA. Depletion of hnRNP F impairs the decay of a subset of TTP-substrate ARE-mRNAs by a mechanism independent of the extent of hnRNP F binding to the mRNA. Taken together, these findings implicate hnRNP F as a co-factor in a subset of TTP/BRF-mediated mRNA decay and highlight the importance of RBP cooperativity in mRNA regulation.

Emerging roles for hnRNPs in post-transcriptional regulation: what can we learn from flies?

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are a highly conserved family of RNA-binding proteins able to associate with nascent RNAs in order to support their localization, maturation and translation. Research over this last decade has remarked the importance of gene regulatory processes at post-transcriptional level, highlighting the emerging roles of hnRNPs in several essential biological events. Indeed, hnRNPs are key factors in regulating gene expression, thus, having a number of roles in many biological pathways. Moreover, failure of the activities catalysed by hnRNPs affects various biological processes and may underlie several human diseases including cancer, diabetes and neurodegenerative syndromes. In this review, we summarize some of hnRNPs' roles in the model organism Drosophila melanogaster, particularly focusing on their participation in all aspects of post-transcriptional regulation as well as their conserved role and involvement in the aetiology of human pathologies.

The 3'UTR of nanos2 directs enrichment in the germ cell lineage of the sea urchin

Nanos is a translational regulator required for the survival and maintenance of primordial germ cells during embryogenesis. Three nanos homologs are present in the genome of the sea urchin Strongylocentrotus purpuratus (Sp), and each nanos mRNA accumulates specifically in the small micromere (sMic) lineage. We found that a highly conserved element in the 3' UTR of nanos2 is sufficient for reporter expression selectively in the sMic lineage: microinjection into a Sp fertilized egg of an RNA that contains the GFP open reading frame followed by Sp nanos2 3'UTR leads to selective reporter enrichment in the small micromeres in blastulae. The same result was seen with nanos2 from the sea urchin Hemicentrotus pulcherrimus (Hp). In both species, the 5'UTR alone is not sufficient for the sMic localization but it always increased the sMic reporter enrichment when present with the 3'UTR. We defined an element conserved between Hp and Sp in the nanos2 3'UTR which is necessary and sufficient for protein enrichment in the sMic, and refer to it as GNARLE (Global Nanos Associated RNA Lability Element). We also found that the nanos2 3'UTR is essential for the selective RNA retention in the small micromeres; GNARLE is required but not sufficient for this process. These results show that a combination of selective RNA retention and translational control mechanisms instills nanos accumulation uniquely in the sMic lineage.

microRNA-independent recruitment of Argonaute 1 to nanos mRNA through the Smaug RNA-binding protein

Argonaute (Ago) proteins are typically recruited to target messenger RNAs via an associated small RNA such as a microRNA (miRNA). Here, we describe a new mechanism of Ago recruitment through the Drosophila Smaug RNA-binding protein. We show that Smaug interacts with the Ago1 protein, and that Ago1 interacts with and is required for the translational repression of the Smaug target, nanos mRNA. The Ago1/nanos mRNA interaction does not require a miRNA, but it does require Smaug. Taken together, our data suggest a model whereby Smaug directly recruits Ago1 to nanos mRNA in a miRNA-independent manner, thereby repressing translation.

mRNA localization and translational control in Drosophila oogenesis

Localization of an mRNA species to a particular subcellular region can complement translational control mechanisms to produce a restricted spatial distribution of the protein it encodes. mRNA localization has been studied most in asymmetric cells such as budding yeast, early embryos, and neurons, but the process is likely to be more widespread. This article reviews the current state of knowledge about the mechanisms of mRNA localization and its functions in early embryonic development, focusing on Drosophila where the relevant knowledge is most advanced. Links between mRNA localization and translational control mechanisms also are examined.

From cis-regulatory elements to complex RNPs and back

Messenger RNAs (mRNAs), the templates for translation, have evolved to harbor abundant cis-acting sequences that affect their posttranscriptional fates. These elements are frequently located in the untranslated regions and serve as binding sites for trans-acting factors, RNA-binding proteins, and/or small non-coding RNAs. This article provides a systematic synopsis of cis-acting elements, trans-acting factors, and the mechanisms by which they affect translation. It also highlights recent technical advances that have ushered in the era of transcriptome-wide studies of the ribonucleoprotein complexes formed by mRNAs and their trans-acting factors.

Heterogeneous nuclear ribonucleoprotein (hnRNP) F is a novel component of oligodendroglial RNA transport granules contributing to regulation of myelin basic protein (MBP) synthesis

Myelin basic protein (MBP) is a major component of central nervous system (CNS) myelin. The absence of MBP results in the loss of almost all compact myelin in the CNS. MBP mRNA is sorted into RNA granules that are transported to the periphery of oligodendrocytes in a translationally inactive state. A central mediator of this transport process is the trans-acting factor heterogeneous nuclear ribonucleoprotein (hnRNP) A2 that binds to the cis-acting A2-response element in the 3'UTR of MBP mRNA. Recently, we found that activation of the Src family nonreceptor tyrosine kinase Fyn in oligodendrocytes leads to phosphorylation of hnRNP A2 and to increased translation of MBP mRNA. Here, we identify the RNA-binding protein hnRNP F as a novel component of MBP mRNA transport granules. It is associated with hnRNP A2 and MBP mRNA in cytoplasmic granular structures and is involved in post-transcriptional regulation of MBP expression. Fyn kinase activity results in phosphorylation of hnRNP F in the cytoplasm and its release from MBP mRNA and RNA granules. Our results define hnRNP F as a regulatory element of MBP expression in oligodendrocytes and imply an important function of hnRNP F in the control of myelin synthesis.

Translational control in oocyte development

Translational control of specific mRNAs is a widespread mechanism of gene regulation, and it is especially important in pattern formation in the oocytes of organisms in which the embryonic axes are established maternally. Drosophila and Xenopus have been especially valuable in elucidating the relevant molecular mechanisms. Here, we comprehensively review what is known about translational control in these two systems, focusing on examples that illustrate key concepts that have emerged. We focus on protein-mediated translational control, rather than regulation mediated by small RNAs, as the former appears to be predominant in controlling these developmental events. Mechanisms that modulate the ability of the specific mRNAs to be recruited to the ribosome, that regulate polyadenylation of specific mRNAs, or that control the association of particular mRNAs into translationally inert ribonucleoprotein complexes will all be discussed.

A late phase of germ plasm accumulation during Drosophila oogenesis requires lost and rumpelstiltskin

Asymmetric mRNA localization is an effective mechanism for establishing cellular and developmental polarity. Posterior localization of oskar in the Drosophila oocyte targets the synthesis of Oskar to the posterior, where Oskar initiates the assembly of the germ plasm. In addition to harboring germline determinants, the germ plasm is required for localization and translation of the abdominal determinant nanos. Consequently, failure of oskar localization during oogenesis results in embryos lacking germ cells and abdominal segments. oskar accumulates at the oocyte posterior during mid-oogenesis through a well-studied process involving kinesin-mediated transport. Through live imaging of oskar mRNA, we have uncovered a second, mechanistically distinct phase of oskar localization that occurs during late oogenesis and results in amplification of the germ plasm. Analysis of two newly identified oskar localization factors, Rumpelstiltskin and Lost, that are required specifically for this late phase of oskar localization shows that germ plasm amplification ensures robust abdomen and germ cell formation during embryogenesis. In addition, our results indicate the importance of mechanisms for adapting mRNAs to utilize multiple localization pathways as necessitated by the dramatic changes in ovarian physiology that occur during oogenesis.

Multiple mechanisms collaborate to repress nanos translation in the Drosophila ovary and embryo

Translational control of gene expression is essential for development in organisms that rely on maternal mRNAs. In Drosophila, translation of maternal nanos (nos) mRNA must be restricted to the posterior of the early embryo for proper patterning of the anterior-posterior axis. Spatial control of nos translation is coordinated through the localization of a small subset of nos mRNA to the posterior pole late in oogenesis, activation of this localized mRNA, and repression of the remaining unlocalized nos mRNA throughout the bulk cytoplasm. Translational repression is mediated by the interaction of a cis-acting element in the nos 3' untranslated region with two proteins, Glorund (Glo) and Smaug (Smg), that function in the oocyte and embryo, respectively. The mechanism of Glo-dependent repression is unknown. Previous work suggests that Smg represses translation initiation but this model is not easily reconciled with evidence for polysome association of repressed nos mRNA. Using an in vitro translation system, we have decoupled translational repression of nos imposed during oogenesis from repression during embryogenesis. Our results suggest that both Glo and Smg regulate translation initiation, but by different mechanisms. Furthermore, we show that, during late oogenesis, nos translation is also repressed post-initiation and provide evidence that Glo mediates this event. This post-initiation block is maintained into embryogenesis during the transition to Smg-dependent regulation. We propose that the use of multiple modes of repression ensures inactivation of nos RNA that is translated at earlier stages of oogenesis and maintenance of this inactivate state throughout late oogenesis into embryogenesis.

Transcript clearance during the maternal-to-zygotic transition

In all animals, a key event in the transition from maternal control of development to control by products of the zygotic genome is the elimination of a significant fraction of the mRNAs loaded into the egg by the mother. Clearance of these maternal mRNAs is accomplished by two activities: the first is maternally encoded while the second requires zygotic transcription. Recent advances include identification of RNA-binding proteins that function as specificity factors to direct the maternal degradation machinery to its target mRNAs; small RNAs-most notably microRNAs-that function as components of the zygotically encoded activity; signaling pathways that trigger production and/or activation of the clearance mechanism in early embryos; and mechanisms for spatial control of transcript clearance.

Xenopus germline nanos1 is translationally repressed by a novel structure-based mechanism

The translational repressor Nanos is expressed in the germline and stem cell populations of jellyfish as well as humans. Surprisingly, we observed that unlike other mRNAs, synthetic nanos1 RNA translates very poorly if at all after injection into Xenopus oocytes. The current model of simple sequestration of nanos1 within germinal granules is insufficient to explain this observation and suggests that a second level of repression must be operating. We find that an RNA secondary structural element immediately downstream of the AUG start site is both necessary and sufficient to prevent ribosome scanning in the absence of a repressor. Accordingly, repression is relieved by small in-frame insertions before this secondary structure, or translational control element (TCE), that provide the 15 nucleotides required for ribosome entry. nanos1 is translated shortly after fertilization, pointing to the existence of a developmentally regulated activator. Oocyte extracts were rendered fully competent for nanos1 translation after the addition of a small amount of embryo extract, confirming the presence of an activator. Misexpression of Nanos1 in oocytes from unlocalized RNA results in abnormal development, highlighting the importance of TCE-mediated translational repression. Although found in prokaryotes, steric hindrance as a mechanism for negatively regulating translation is novel for a eukaryotic RNA. These observations unravel a new mode of nanos1 regulation at the post-transcriptional level that is essential for normal development.

Posttranscriptional regulation in Drosophila oocytes and early embryos

Molecular asymmetries underlying embryonic axis patterning and germ cell specification are established in Drosophila largely by position-dependent translational regulation of maternally expressed messenger RNAs. Here, I review several mechanisms of posttranscriptional regulation in the Drosophila oocyte and syncytial embryo, and how they relate to embryonic patterning, with a strong emphasis on the most recent advances in the area. The review is not exhaustive, but focuses on examples that illustrate the interplay between specific regulatory events and the general metabolic machinery that governs translation and mRNA stability. Biophysical investigations into how the Bicoid gradient is formed are discussed, as are the elaborate mechanisms controlling how the Oskar and Nanos proteins become restricted to the posterior pole of the embryo. Work on Vasa, a translational activator of some germ line mRNAs and on 4EHP, a negative regulator that unproductively binds the 5' cap structure of target mRNAs, is also briefly reviewed. Finally, the emerging understanding of the role of microRNAs in regulating translation of germ line mRNAs is also discussed.

Smaug assembles an ATP-dependent stable complex repressing nanos mRNA translation at multiple levels

The nanos (nos) mRNA encodes the posterior determinant of the Drosophila embryo. Translation of the RNA is repressed throughout most of the embryo by the protein Smaug binding to Smaug recognition elements (SREs) in the 3' UTR. Translation is locally activated at the posterior pole by Oskar. This paper reports that the SREs govern the time- and ATP-dependent assembly of an exceedingly stable repressed ribonucleoprotein particle (RNP) in embryo extract. Repression can be virtually complete. Smaug and its co-repressor Cup as well as Trailer hitch and the DEAD box protein Me31B are part of the repressed RNP. The initiation factor eIF4G is specifically displaced, and 48S pre-initiation complex formation is inhibited. However, later steps in translation initiation are also sensitive to SRE-dependent inhibition. These data confirm several previously untested predictions of a current model for Cup-dependent repression but also suggest that the Cup model by itself is insufficient to explain translational repression of the nos RNA. In the embryo extract, recombinant Oskar relieves translational repression and deadenylation by preventing Smaug's binding to the SREs.

Bazooka regulates microtubule organization and spatial restriction of germ plasm assembly in the Drosophila oocyte

Localization of the germ plasm to the posterior of the Drosophila oocyte is required for anteroposterior patterning and germ cell development during embryogenesis. While mechanisms governing the localization of individual germ plasm components have been elucidated, the process by which germ plasm assembly is restricted to the posterior pole is poorly understood. In this study, we identify a novel allele of bazooka (baz), the Drosophila homolog of Par-3, which has allowed the analysis of baz function throughout oogenesis. We demonstrate that baz is required for spatial restriction of the germ plasm and axis patterning, and we uncover multiple requirements for baz in regulating the organization of the oocyte microtubule cytoskeleton. Our results suggest that distinct cortical domains established by Par proteins polarize the oocyte through differential effects on microtubule organization. We further show that microtubule plus-end enrichment is sufficient to drive germ plasm assembly even at a distance from the oocyte cortex, suggesting that control of microtubule organization is critical not only for the localization of germ plasm components to the posterior of the oocyte but also for the restriction of germ plasm assembly to the posterior pole.

A glycine-rich domain of hnRNP H/F promotes nucleocytoplasmic shuttling and nuclear import through an interaction with transportin 1

Heterogeneous nuclear ribonucleoprotein (hnRNP) H and F are members of a closely related subfamily of hnRNP proteins that are implicated in many aspects of RNA processing. hnRNP H and F are alternative splicing factors for numerous U2- and U12-dependent introns. The proteins have three RNA binding domains and two glycine-rich domains and localize to both the nucleus and cytoplasm, but little is known about which domains govern subcellular localization or splicing activity. We show here that the central glycine-tyrosine-arginine-rich (GYR) domain is responsible for nuclear localization, and a nonclassical nuclear localization signal (NLS) was mapped to a short, highly conserved sequence whose activity was compromised by point mutations. Glutathione S-transferase (GST) pulldown assays demonstrated that the hnRNP H NLS interacts with the import receptor transportin 1. Finally, we show that hnRNP H/F are transcription-dependent shuttling proteins. Collectively, the results suggest that hnRNP H and F are GYR domain-dependent shuttling proteins whose posttranslational modifications may alter nuclear localization and hence function.

The Nanos3-3'UTR is required for germ cell specific NANOS3 expression in mouse embryos

BACKGROUND

The regulation of gene expression via a 3' untranslated region (UTR) plays essential roles in the discrimination of the germ cell lineage from somatic cells during embryogenesis. This is fundamental to the continuation of a species. Mouse NANOS3 is an essential protein required for the germ cell maintenance and is specifically expressed in these cells. However, the regulatory mechanisms that restrict the expression of this gene in the germ cells is largely unknown at present.

METHODOLOGY/PRINCIPAL FINDINGS

In our current study, we show that differences in the stability of Nanos3 mRNA between germ cells and somatic cells is brought about in a 3'UTR-dependent manner in mouse embryos. Although Nanos3 is transcribed in both cell lineages, it is efficiently translated only in the germ lineage. We also find that the translational suppression of NANOS3 in somatic cells is caused by a 3'UTR-mediated mRNA destabilizing mechanism. Surprisingly, even when under the control of the CAG promoter which induces strong ubiquitous transcription in both germ cells and somatic cells, the addition of the Nanos3-3'UTR sequence to the coding region of exogenous gene was effective in restricting protein expression in germ cells.

CONCLUSIONS/SIGNIFICANCE

Our current study thus suggests that Nanos3-3'UTR has an essential role in translational control in the mouse embryo.

Heterogeneous nuclear ribonucleoprotein H blocks MST2-mediated apoptosis in cancer cells by regulating A-Raf transcription

A-Raf belongs to the family of oncogenic Raf kinases that are involved in mitogenic signaling by activating the mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK pathway. Low kinase activity of A-Raf toward MEK suggested that A-Raf might have alternative functions. Here, we show that A-Raf prevents cancer cell apoptosis contingent on the expression of the heterogeneous nuclear ribonucleoprotein H (hnRNP H) splice factor, which is required for the correct transcription and expression of a-raf. Apoptosis was prevented by A-Raf through sequestration and inactivation of the proapoptotic MST2 kinase. Small interfering RNA-mediated knockdown of hnRNP H or A-Raf resulted in MST2-dependent apoptosis. In contrast, enforced expression of either hnRNP H or A-Raf partially counteracted apoptosis induced by etoposide. In vivo expression studies of colon specimens corroborated the overexpression of hnRNP H in malignant tissues and its correlation with A-Raf levels. Our findings define a novel mechanism that is usurped in tumor cells to escape naturally imposed apoptotic signals.

Translational control during early development

Translational control of specific messenger RNAs, which themselves are often asymmetrically localized within the cytoplasm of a cell, underlies many events in germline development, and in embryonic axis specification. This comprehensive, but by no means exhaustive, review attempts to present a picture of the present state of knowledge about mechanisms underlying mRNA localization and translational control of specific mRNAs that are mediated by trans-acting protein factors. While RNA localization and translational control are widespread in evolution and have been studied in many experimental systems, this article will focus mainly on three particularly well-characterized systems: Drosophila, Caenorhabditis elegans, and Xenopus. In keeping with the overall theme of this volume, instances in which translational control factors have been linked to human disease states will also be discussed.

The evolutionarily conserved RNA binding protein SMOOTH is essential for maintaining normal muscle function

The Drosophila smooth gene encodes an RNA binding protein that has been well conserved through evolution. To investigate the pleiotropic functions mediated by the smooth gene, we have selected and characterized two sm mutants, which are viable as adults yet display robust phenotypes (including a significant decrease in lifespan). Utilizing these mutants, we have made the novel observation that disruption of the smooth/CG9218 locus leads to age-dependent muscle degeneration, and motor dysfunction. Histological characterization of adult sm mutants revealed marked abnormalities in the major thoracic tubular muscle: the tergal depressor of the trochanter (TDT). Corresponding defects include extensive loss/disruption of striations and nuclei. These pathological changes are recapitulated in flies that express a smooth RNA interference construct (sm RNAi) in the mesoderm. In contrast, targeting sm RNAi constructs to motor neurons does not alter muscle morphology. In addition to examining the TDT phenotype, we explored whether other muscular abnormalities were evident. Utilizing physiological assays developed in the laboratory, we have found that the thoracic muscle defect is preceded by dysmotility of the gastrointestinal tract. SMOOTH thus joins a growing list of hnRNPs that have previously been linked to muscle physiology/pathophysiology. Our findings in Drosophila set the stage for investigating the role of the corresponding mammalian homolog, hnRNP L, in muscle function.

Selective recruitment of proteins to 5' cap complexes during the growth cycle in Arabidopsis

Translation of most mRNAs is performed in a cap-dependent manner, requiring a protein complex, the cap complex, to regulate the accessibility of the message to the 40S ribosome. The cap complex initiates protein translation by binding to the 5' cap of an mRNA and recruiting ribosomes to begin translation. Compared to animals and yeast, there are significant plant-specific differences in the regulation of cap-dependent mRNA translation, but these are poorly understood. Here, we purified proteins that bind to the 5' cap during the Arabidopsis growth cycle. The protein profile of the cap-binding complexes varies during the various stages of the growth cycle in suspension culture cells. Using Western blotting, the cap complexes of quiescent cells were found to be composed of only three major proteins: eIF4isoE, which is primarily a cytoplasmic protein, and eIF4E and CBP80, which accumulate in the nucleus. However, when cells proliferate, at least 10 major proteins bind directly or indirectly to the 5' cap. Proteomic, Western blotting and immunoprecipitation data establish that the spectrum of RNA helicases in the cap complexes also changes during the growth cycle. Cap complexes from proliferating cultures mainly contain eIF4A, which associates with at least four cap complexes, but eIF4A is replaced by additional helicases in quiescent cells. These findings suggest that the dynamic and selective recruitment of various proteins to mRNA 5' cap complexes could play an important role in the regulation of gene expression.

Bam and Bgcn antagonize Nanos-dependent germ-line stem cell maintenance

The balance between germ-line stem cell (GSC) self-renewal and differentiation in Drosophila ovaries is mediated by the antagonistic relationship between the Nanos (Nos)-Pumilio translational repressor complex, which promotes GSC self-renewal, and expression of Bam, a key differentiation factor. Here, we find that Bam and Nos proteins are expressed in reciprocal patterns in young germ cells. Repression of Nos in Bam-expressing cells depends on sequences in the nos 3'-UTR, suggesting that Nos is regulated by translational repression. Ectopic Bam causes differentiation of GSCs, and this activity depends on the endogenous nos 3'-UTR sequence. Previous evidence showed that Bgcn is an obligate factor for the ability of Bam to drive differentiation, and we now report that Bam forms a complex with Bgcn, a protein related to the RNA-interacting DExH-box polypeptides. Together, these observations suggest that Bam-Bgcn act together to antagonize Nos expression; thus, derepressing cystoblast-promoting factors. These findings emphasize the importance of translational repression in balancing stem cell self-renewal and differentiation.

The translational repressors Nanos and Pumilio have divergent effects on presynaptic terminal growth and postsynaptic glutamate receptor subunit composition

Pumilio (Pum) is a translational repressor that binds selectively to target mRNAs and recruits Nanos (Nos) as a corepressor. In the larval neuromuscular system, Pum represses expression of the translation factor eIF-4E and the glutamate receptor subunit GluRIIA. Here, we show that Nos, like Pum, is expressed at the neuromuscular junction (NMJ) and in neuronal cell bodies. Surprisingly, however, Nos and Pum have divergent functions on both the presynaptic and postsynaptic sides of the NMJ. In nos mutant and nos RNA interference larvae, the number of NMJ boutons is increased, whereas loss of Pum reduces the bouton number. On the postsynaptic side, Nos acts in opposition to Pum in regulating the subunit composition of the glutamate receptor. NMJ active zones are associated with GluRIIA- and GluRIIB-containing receptor clusters. Loss of Nos causes downregulation of GluRIIA and increases the levels of GluRIIB. Consistent with this finding, the electrophysiological properties of NMJs lacking postsynaptic Nos suggest that they use primarily GluRIIB-containing receptors. Nos can regulate GluRIIB in the absence of GluRIIA, suggesting that the effects of Nos on GluRIIB levels are at least partially independent of synaptic competition between GluRIIA and GluRIIB. Nos is a target for Pum repression, and Pum binds selectively to the 3' untranslated regions of the nos and GluRIIA mRNAs. Our results suggest a model in which regulatory interplay among Pum, Nos, GluRIIA, and GluRIIB could cause a small change in Pum activity to be amplified into a large shift in the balance between GluRIIA and GluRIIB synapses.

Glorund interactions in the regulation of gurken and oskar mRNAs

Precise temporal and spatial regulation of gene expression during Drosophila oogenesis is essential for patterning the anterior-posterior and dorsal-ventral body axes. Establishment of the anterior-posterior axis requires posterior localization and translational control of both oskar and nanos mRNAs. Establishment of the dorsal-ventral axis depends on the precise restriction of gurken mRNA and protein to the dorsal-anterior corner of the oocyte. We have previously shown that Glorund, the Drosophila hnRNP F/H homolog, contributes to anterior-posterior axis patterning by regulating translation of nanos mRNA, through a direct interaction with its 3' untranslated region. To investigate the pleiotropy of the glorund mutant phenotype, which includes dorsal-ventral and nuclear morphology defects, we searched for proteins that interact with Glorund. Here we show that Glorund is part of a complex containing the hnRNP protein Hrp48 and the splicing factor Half-pint and plays a role both in mRNA localization and nurse cell chromosome organization, probably by regulating alternative splicing of ovarian tumor. We propose that Glorund is a component of multiple protein complexes and functions both as a translational repressor and splicing regulator for anterior-posterior and dorsal-ventral patterning.

A splicer that represses (translation)

Regulated translation and subcellular localization of maternal mRNAs underlies establishment of the antero-posterior axis in the Drosophila oocyte. In this issue of Genes & Development, Besse et al. (pp. 195-207) show that a molecule better known as a regulator of alternative splicing in the nucleus, polypyrimidine tract-binding protein (PTB), is required for repression of oskar mRNA in the cytoplasm. Their work suggests that PTB need not engage oskar mRNA in the nucleus for efficient repression, providing an important counterexample to the increasingly popular idea that cytoplasmic regulation initiates in the nucleus.

RNA recognition motifs: boring? Not quite

The RNA recognition motif (RRM) is one of the most abundant protein domains in eukaryotes. While the structure of this domain is well characterized by the packing of two alpha-helices on a four-stranded beta-sheet, the mode of protein and RNA recognition by RRMs is not clear owing to the high variability of these interactions. Here we report recent structural data on RRM-RNA and RRM-protein interactions showing the ability of this domain to modulate its binding affinity and specificity using each of its constitutive elements (beta-strands, loops, alpha-helices). The extreme structural versatility of the RRM interactions explains why RRM-containing proteins have so diverse biological functions.

Spatial regulation of nanos is required for its function in dendrite morphogenesis

Spatial control of mRNA translation can generate cellular asymmetries and functional specialization of polarized cells like neurons. A requirement for the translational repressor Nanos (Nos) in the Drosophila larval peripheral nervous system (PNS) implicates translational control in dendrite morphogenesis [1]. Nos was first identified by its requirement in the posterior of the early embryo for abdomen formation [2]. Nos synthesis is targeted to the posterior pole of the oocyte and early embryo through translational repression of unlocalized nos mRNA coupled with translational activation of nos mRNA localized at the posterior pole [3, 4]. Abolishment of nos localization prevents abdominal development, whereas translational derepression of unlocalized nos mRNA suppresses head/thorax development, emphasizing the importance of spatial regulation of nos mRNA [3, 5]. Loss and overexpression of Nos affect dendrite branching complexity in class IV dendritic arborization (da) neurons, suggesting that nos also might be regulated in these larval sensory neurons [1]. Here, we show that localization and translational control of nos mRNA are essential for da neuron morphogenesis. RNA-protein interactions that regulate nos translation in the oocyte and early embryo also regulate nos in the PNS. Live imaging of nos mRNA shows that the cis-acting signal responsible for posterior localization in the oocyte/embryo mediates localization to the processes of class IV da neurons but suggests a different transport mechanism. Targeting of nos mRNA to the processes of da neurons may reflect a local requirement for Nos protein in dendritic translational control.