Testis/brain RNA-binding protein attaches translationally repressed and transported mRNAs to microtubules

On the origin and nature of nongenetic information in eumetazoans

Nongenetic information implies all the forms of biological information not related to genes and DNA in general. Despite the deep scientific relevance of the concept, we currently lack reliable knowledge about its carriers and origins; hence, we still do not understand its true nature. Given that genes are the targets of nongenetic information, it appears that a parsimonious approach to find the ultimate source of that information is to trace back the sequential steps of the causal chain upstream of the target genes up to the ultimate link as the source of the nongenetic information. From this perspective, I examine seven nongenetically determined phenomena: placement of locus-specific epigenetic marks on DNA and histones, changes in snRNA expression patterns, neural induction of gene expression, site-specific alternative gene splicing, predator-induced morphological changes, and cultural inheritance. Based on the available evidence, I propose a general model of the common neural origin of all these forms of nongenetic information in eumetazoans.

miR-181a negatively modulates synaptic plasticity in hippocampal cultures and its inhibition rescues memory deficits in a mouse model of Alzheimer's disease

MicroRNAs play a pivotal role in rapid, dynamic, and spatiotemporal modulation of synaptic functions. Among them, recent emerging evidence highlights that microRNA-181a (miR-181a) is particularly abundant in hippocampal neurons and controls the expression of key plasticity-related proteins at synapses. We have previously demonstrated that miR-181a was upregulated in the hippocampus of a mouse model of Alzheimer's disease (AD) and correlated with reduced levels of plasticity-related proteins. Here, we further investigated the underlying mechanisms by which miR-181a negatively modulated synaptic plasticity and memory. In primary hippocampal cultures, we found that an activity-dependent upregulation of the microRNA-regulating protein, translin, correlated with reduction of miR-181a upon chemical long-term potentiation (cLTP), which induced upregulation of GluA2, a predicted target for miR-181a, and other plasticity-related proteins. Additionally, Aβ treatment inhibited cLTP-dependent induction of translin and subsequent reduction of miR-181a, and cotreatment with miR-181a antagomir effectively reversed the effects elicited by Aβ but did not rescue translin levels, suggesting that the activity-dependent upregulation of translin was upstream of miR-181a. In mice, a learning episode markedly decreased miR-181a in the hippocampus and raised the protein levels of GluA2. Lastly, we observed that inhibition of miR-181a alleviated memory deficits and increased GluA2 and GluA1 levels, without restoring translin, in the 3xTg-AD model. Taken together, our results indicate that miR-181a is a major negative regulator of the cellular events that underlie synaptic plasticity and memory through AMPA receptors, and importantly, Aβ disrupts this process by suppressing translin and leads to synaptic dysfunction and memory impairments in AD.

Rapid coupling between gravitational forces and the transcriptome in human myelomonocytic U937 cells

The gravitational force has been constant throughout Earth's evolutionary history. Since the cell nucleus is subjected to permanent forces induced by Earth's gravity, we addressed the question, if gene expression homeostasis is constantly shaped by the gravitational force on Earth. We therefore investigated the transcriptome in force-free conditions of microgravity, determined the time frame of initial gravitational force-transduction to the transcriptome and assessed the role of cation channels. We combined a parabolic flight experiment campaign with a suborbital ballistic rocket experiment employing the human myelomonocytic cell line U937 and analyzed the whole gene transcription by microarray, using rigorous controls for exclusion of effects not related to gravitational force and cross-validation through two fully independent research campaigns. Experiments with the wide range ion channel inhibitor SKF-96365 in combination with whole transcriptome analysis were conducted to study the functional role of ion channels in the transduction of gravitational forces at an integrative level. We detected profound alterations in the transcriptome already after 20 s of microgravity or hypergravity. In microgravity, 99.43% of all initially altered transcripts adapted after 5 min. In hypergravity, 98.93% of all initially altered transcripts adapted after 75 s. Only 2.4% of all microgravity-regulated transcripts were sensitive to the cation channel inhibitor SKF-96365. Inter-platform comparison of differentially regulated transcripts revealed 57 annotated gravity-sensitive transcripts. We assume that gravitational forces are rapidly and constantly transduced into the nucleus as omnipresent condition for nuclear and chromatin structure as well as homeostasis of gene expression.

Learning induces the translin/trax RNase complex to express activin receptors for persistent memory

Long-lasting forms of synaptic plasticity and memory require de novo protein synthesis. Yet, how learning triggers this process to form memory is unclear. Translin/trax is a candidate to drive this learning-induced memory mechanism by suppressing microRNA-mediated translational silencing at activated synapses. We find that mice lacking translin/trax display defects in synaptic tagging, which requires protein synthesis at activated synapses, and long-term memory. Hippocampal samples harvested from these mice following learning show increases in several disease-related microRNAs targeting the activin A receptor type 1C (ACVR1C), a component of the transforming growth factor-β receptor superfamily. Furthermore, the absence of translin/trax abolishes synaptic upregulation of ACVR1C protein after learning. Finally, synaptic tagging and long-term memory deficits in mice lacking translin/trax are mimicked by ACVR1C inhibition. Thus, we define a new memory mechanism by which learning reverses microRNA-mediated silencing of the novel plasticity protein ACVR1C via translin/trax.

Conformational transitions in human translin enable nucleic acid binding

Translin is a highly conserved RNA- and DNA-binding protein that plays essential roles in eukaryotic cells. Human translin functions as an octamer, but in the octameric crystallographic structure, the residues responsible for nucleic acid binding are not accessible. Moreover, electron microscopy data reveal very different octameric configurations. Consequently, the functional assembly and the mechanism of nucleic acid binding by the protein remain unclear. Here, we present an integrative study combining small-angle X-ray scattering (SAXS), site-directed mutagenesis, biochemical analysis and computational techniques to address these questions. Our data indicate a significant conformational heterogeneity for translin in solution, formed by a lesser-populated compact octameric state resembling the previously solved X-ray structure, and a highly populated open octameric state that had not been previously identified. On the other hand, our SAXS data and computational analyses of translin in complex with the RNA oligonucleotide (GU)₁₂ show that the internal cavity found in the octameric assemblies can accommodate different nucleic acid conformations. According to this model, the nucleic acid binding residues become accessible for binding, which facilitates the entrance of the nucleic acids into the cavity. Our data thus provide a structural basis for the functions that translin performs in RNA metabolism and transport.

Low-resolution structure of Drosophila translin

Crystals of native Drosophila melanogaster translin diffracted to 7 Å resolution. Reductive methylation of the protein improved crystal quality. The native and methylated proteins showed similar profiles in size-exclusion chromatography analyses but the methylated protein displayed reduced DNA-binding activity. Crystals of the methylated protein diffracted to 4.2 Å resolution at BM14 of the ESRF synchrotron. Crystals with 49% solvent content belonged to monoclinic space group P21 with eight protomers in the asymmetric unit. Only 2% of low-resolution structures with similar low percentage solvent content were found in the PDB. The crystal structure, solved by molecular replacement method, refined to R work (R free) of 0.24 (0.29) with excellent stereochemistry. The crystal structure clearly shows that drosophila protein exists as an octamer, and not as a decamer as expected from gel-filtration elution profiles. The similar octameric quaternary fold in translin orthologs and in translin-TRAX complexes suggests an up-down dimer as the basic structural subunit of translin-like proteins. The drosophila oligomer displays asymmetric assembly and increased radius of gyration that accounts for the observed differences between the elution profiles of human and drosophila proteins on gel-filtration columns. This study demonstrates clearly that low-resolution X-ray structure can be useful in understanding complex biological oligomers.

Neuronal expression, cytosolic localization, and developmental regulation of the organic solute carrier partner 1 in the mouse brain

Organic solute carrier partner 1 (OSCP1) is a mammalian, transporter-related protein that is able to facilitate the uptake of structurally diverse organic compounds into the cell when expressed in Xenopus laevis oocytes. This protein has been implicated in testicular handling of organic solutes because its mRNA expression is almost exclusive in the testis. However, in this study, we demonstrated significant expression of OSCP1 protein in mouse brain, the level of which was rather higher than that in the testis, although the corresponding mRNA expression was one-tenth of the testicular level. Immunohistochemistry revealed that OSCP1 was broadly distributed throughout the brain, and various neuronal cells were immunostained, including pyramidal cells in the cerebral cortex and hippocampus. However, there was no evidence of OSCP1 expression in glia. In primary cultures of cerebral cortical neurons, double-labeling immunofluorescence localized OSCP1 to the cytosol throughout the cell body and neurites including peri-synaptic regions. This was consistent with the subcellular fractionation of brain homogenates, in which OSCP1 was mainly recovered after centrifugation both in the cytosolic fraction and the particulate fraction containing synaptosomes. Immunoelectron microscopy of brain sections also demonstrated OSCP1 in the cytosol near synapses. In addition, it was revealed that changes in the expression level of OSCP1 correlated with neuronal maturation during postnatal development of mouse brain. These results indicate that OSCP1 may have a role in the brain indirectly mediating substrate uptake into the neurons in adult animals.

Biological roles of translin and translin-associated factor-X: RNA metabolism comes to the fore

Translin, and its binding partner protein TRAX (translin-associated factor-X) are a paralogous pair of conserved proteins, which have been implicated in a broad spectrum of biological activities, including cell growth regulation, mRNA processing, spermatogenesis, neuronal development/function, genome stability regulation and carcinogenesis, although their precise role in some of these processes remains unclear. Furthermore, translin (with or without TRAX) has nucleic-acid-binding activity and it is apparent that controlling nucleic acid metabolism and distribution are central to the biological role(s) of this protein and its partner TRAX. More recently, translin and TRAX have together been identified as enhancer components of an RNAi (RNA interference) pathway in at least one organism and this might provide critical insight into the biological roles of this enigmatic partnership. In the present review we discuss the biological and the biochemical properties of these proteins that indicate that they play a central and important role in eukaryotic cell biology.

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.

Intratesticular localization of the organic solute carrier protein, OSCP1, in spermatogenic cells in mice

Organic solute carrier protein 1 (OSCP1) is a recently described human gene that facilitates the transport of various organic solutes into the cell, when expressed in frog eggs. In this study, we cloned a mouse ortholog of OSCP1 encoding 379 amino acid protein, with 94% homology to the human counterpart. The mouse OSCP1 mRNA was predominantly expressed in the testis, in which it was attributed to the spermatogenic cells, except the spermatogonia. Immunohistochemistry confirmed that OSCP1 protein is continuously expressed during spermatogenesis in a stage- and cell type-specific manner, in the leptotene spermatocytes at stage IX through step 15 spermatids. Subcellular fractionation of mouse testis homogenates indicated that OSCP1 is a 45-kDa cytosolic protein. Moreover, when green fluorescent protein-OSCP1 fusion constructs were transfected into cultured cells, the fluorescence localized evenly in the cytoplasm. These results suggest that mouse testis OSCP1 may indirectly mediate substrate uptake into meiotic and spermiogenic germ cells, within the cytosol.

Crystal structures of Drosophila mutant translin and characterization of translin variants reveal the structural plasticity of translin proteins

Translin protein is highly conserved in eukaryotes. Human translin binds both ssDNA and RNA. Its nucleic acid binding site results from a combination of basic regions in the octameric structure. We report here the first biochemical characterization of wild-type Drosophila melanogaster (drosophila) translin and a chimeric translin, and present 3.5 A resolution crystal structures of drosophila P168S mutant translin from two crystal forms. The wild-type drosophila translin most likely exists as an octamer/decamer, and binds to the ssDNA Bcl-CL1 sequence. In contrast, ssDNA binding-incompetent drosophila P168S mutant translin exists as a tetramer. The structures of the mutant translin are identical in both crystal forms, and their C-terminal residues are disordered. The chimeric protein, possessing two nucleic acid binding motifs of drosophila translin, the C-terminal residues of human translin, and serine at position 168, attains the octameric state and binds to ssDNA. The present studies suggest that the oligomeric status of translin critically influences the DNA binding properties of translin proteins.

Molecular biological features of male germ cell differentiation

Somatic cell differentiation is required throughout the life of a multicellular organism to maintain homeostasis. In contrast, germ cells have only one specific function; to preserve the species by conveying the parental genes to the next generation. Recent studies of the development and molecular biology of the male germ cell have identified many genes, or isoforms, that are specifically expressed in the male germ cell. In the present review, we consider the unique features of male germ cell differentiation. (Reprod Med Biol 2007; 6: 1-9).

Functional characterization of Drosophila Translin and Trax

The vertebrate RNA and ssDNA-binding protein Translin has been suggested to function in a variety of cellular processes, including DNA damage response, RNA transport, and translational control. The Translin-associated factor X (Trax) interacts with Translin, and Trax protein stability depends on the presence of Translin. To determine the function of the Drosophila Translin and Trax, we generated a translin null mutant and isolated a trax nonsense mutation. translin and trax single and double mutants are viable, fertile, and phenotypically normal. Meiotic recombination rates and chromosome segregation are also not affected in translin and trax mutants. In addition, we found no evidence for an increased sensitivity for DNA double-strand damage in embryos and developing larvae. Together with the lack of evidence for their involvement in DNA double-strand break checkpoints, this argues against a critical role for Translin and Trax in sensing or repairing such DNA damage. However, Drosophila translin is essential for stabilizing the Translin interaction partner Trax, a function that is surprisingly conserved throughout evolution. Conversely, trax is not essential for Translin stability as trax mutants exhibit normal levels of Translin protein.

Rescue of p53 Blockage by the A2AAdenosine Receptor via a Novel Interacting Protein, Translin-Associated Protein X

Blockage of the p53 tumor suppressor has been found to impair nerve growth factor (NGF)-induced neurite outgrowth in PC-12 cells. We report herein that such impairment could be rescued by stimulation of the A(2A) adenosine receptor (A(2A)-R), a G protein-coupled receptor implicated in neuronal plasticity. The A(2A)-R-mediated rescue occurred in the presence of protein kinase C (PKC) inhibitors or protein kinase A (PKA) inhibitors and in a PKA-deficient PC-12 variant. Thus, neither PKA nor PKC was involved. In contrast, expression of a truncated A(2A)-R mutant harboring the seventh transmembrane domain and its C terminus reduced the rescue effect of A(2A)-R. Using the cytoplasmic tail of the A(2A)-R as bait, a novel-A(2A)-R-interacting protein [translin-associated protein X (TRAX)] was identified in a yeast two-hybrid screen. The authenticity of this interaction was verified by pull-down experiments, coimmunoprecipitation, and colocalization of these two molecules in the brain. It is noteworthy that reduction of TRAX using an antisense construct suppressed the rescue effect of A(2A)-R, whereas overexpression of TRAX alone caused the same rescue effect as did A(2A)-R activation. Results of [(3)H]thymidine and bromodeoxyuridine incorporation suggested that A(2A)-R stimulation inhibited cell proliferation in a TRAX-dependent manner. Because the antimitotic activity is crucial for NGF function, the A(2A)-R might exert its rescue effect through a TRAX-mediated antiproliferative signal. This antimitotic activity of the A(2A)-R also enables a mitogenic factor (epidermal growth factor) to induce neurite outgrowth. We demonstrate that the A(2A)-R modulates the differentiation ability of trophic factors through a novel interacting protein, TRAX.

Behavioral and neurochemical alterations in mice lacking the RNA-binding protein translin

Synapse-specific local protein synthesis is thought to be important for neurodevelopment and plasticity and involves neuronal RNA-binding proteins that regulate the transport and translation of dendritically localized transcripts. The best characterized of these RNA-binding proteins is the fragile X mental retardation protein (FMRP). Mutations affecting the expression or function of FMRP cause fragile X syndrome in humans, and targeted deletion of the gene encoding FMRP results in developmental and behavioral alterations in mice. Translin is an RNA-binding protein that regulates mRNA transport and translation in mouse male germ cells and is proposed to play a similar role in neurons. Like FMRP, translin is present in neuronal dendrites, binds dendritically localized RNA, and associates with microtubules and motor proteins. We reported previously the production of viable homozygous translin knock-out mice, which demonstrate altered expression of multiple mRNA transcripts in the brain and mild motor impairments. Here, we report that translin knock-out mice also exhibit sex-specific differences in tests of learning and memory, locomotor activity, anxiety-related behavior, and sensorimotor gating, as well as handling-induced seizures and alterations in monoamine neurotransmitter levels in several forebrain regions. Similar behavioral and neurochemical alterations have been observed in mice lacking FMRP, suggesting that both proteins may act within the same neuronal systems and signaling pathways. Our results in mice indicate that mutations in translin may contribute to fragile X-like syndromes, mental retardation, attention deficit hyperactivity disorder, epilepsy, and autism spectrum disorders in humans.

RNA transport and local control of translation

In eukaryotes, the entwined pathways of RNA transport and local translational regulation are key determinants in the spatio-temporal articulation of gene expression. One of the main advantages of this mechanism over transcriptional control in the nucleus lies in the fact that it endows local sites with independent decision-making authority, a consideration that is of particular relevance in cells with complex cellular architecture such as neurons. Localized RNAs typically contain codes, expressed within cis-acting elements, that specify subcellular targeting. Such codes are recognized by trans-acting factors, adaptors that mediate translocation along cytoskeletal elements by molecular motors. Most transported mRNAs are assumed translationally dormant while en route. In some cell types, especially in neurons, it is considered crucial that translation remains repressed after arrival at the destination site (e.g., a postsynaptic microdomain) until an appropriate activation signal is received. Several candidate mechanisms have been suggested to participate in the local implementation of translational repression and activation, and such mechanisms may target translation at the level of initiation and/or elongation. Recent data indicate that untranslated RNAs may play important roles in the local control of translation.

Cloning and characterization of the Schizosaccharomyces pombe homologs of the human protein Translin and the Translin-associated protein TRAX

Translin is a human octameric protein that specifically binds the single-stranded microsatellite repeats d(GT)_n and the corresponding transcripts (GU)_n. It also binds, with lesser affinities, other single-stranded G-rich DNA and RNA sequences. TRAX is a human protein that bears a homology to Translin and interacts with it. Translin and TRAX have been proposed to be involved in DNA recombination, chromosomal translocation and mRNA transport and translation. Both proteins are highly conserved in eukaryotes, including the fission yeast Schizosaccharomyces pombe, which is amenable to genetic analysis. Here, we report the first study of the S.pombe Translin and TRAX homologs. We have deleted the genes encoding Translin and TRAX in S.pombe and found that the proliferation of the mutant cells was slightly stimulated, suggesting that these genes are not essential for the fission yeast. We have also shown that the S.pombe Translin and TRAX interact. Biochemical analysis of the S.pombe Translin, which was cloned and expressed in Escherichia coli, revealed that it is octameric and that it selectively binds d(GT)_n and d(GTT)_n microsatellite repeats. However, unlike the human protein, it has much higher affinities for the homologous RNA sequences (GU)_n and (GUU)_n. These data suggest that the S.pombe Translin is primarily involved in functions related to RNA metabolism.

Meiotic messenger RNA and noncoding RNA targets of the RNA-binding protein Translin (TSN) in mouse testis

In postmeiotic male germ cells, TSN, formerly known as testis brain-RNA binding protein, is found in the cytoplasm and functions as a posttranscriptional regulator of a group of genes transcribed by the transcription factor CREM-tau. In contrast, in pachytene spermatocytes, TSN is found predominantly in nuclei. Tsn-null males show a reduced sperm count and high levels of apoptosis in meiotic cells, suggesting a critical function for TSN during meiosis. To identify meiotic target RNAs that associate in vivo with TSN, we reversibly cross-linked TSN to RNA in testis extracts from 17-day-old and adult mice and immunoprecipitated the complexes with an affinity-purified TSN antibody. Extracts from Tsn-null mice were used as controls. Cloning and sequencing the immunoprecipitated RNAs, we identified four new TSN target mRNAs, encoding diazepam-binding inhibitor-like 5, arylsulfatase A, a tetratricopeptide repeat structure-containing protein, and ring finger protein 139. In contrast to the population of postmeiotic translationally delayed mRNAs that bind TSN, these four mRNAs are initially expressed in pachytene spermatocytes. In addition, anti-TSN also precipitated a nonprotein-coding RNA (ncRNA), which is abundant in nuclei of pachytene spermatocytes and has a putative polyadenylation signal, but no open reading frame. A second similar ncRNA is adjacent to a GGA repeat, a motif frequently associated with recombination hot spots. RNA gel-shift assays confirm that the four new target mRNAs and the ncRNA specifically bind to TSN in testis extracts. These studies have, for the first time, identified both mRNAs and a ncRNA as TSN targets expressed during meiosis.

KIF2Abeta: A kinesin family member enriched in mouse male germ cells, interacts with translin associated factor-X (TRAX)

Translin associated factor X (TRAX) is a binding partner of TB-RBP/Translin. A cDNA encoding the 260 C-terminal amino acids of KIF2Abeta was isolated from mouse testis cDNAs in a yeast two-hybrid library screen for specific TRAX-interacting proteins. KIF2Abeta was expressed predominantly in the mouse testis and enriched in germ cells. The interaction of full-length KIF2Abeta or its C-terminus with TRAX was verified using in vitro synthesized fusion proteins. Deletion mapping of the TRAX-binding region of KIF2Abeta indicated that amino acids 514-659 were necessary and sufficient for the interaction in vivo. Confocal microscopy studies using GFP-fusion proteins demonstrated that KIF2Abeta colocalizes with TRAX in a perinuclear location. KIF2Abeta does not interact with TB-RBP, suggesting that either TRAX can function as an adaptor molecule for motor proteins and TB-RBP, or that this interaction reveals an undescribed role for TRAX in germ cells. The interaction with KIF2Abeta suggests a role for TRAX in microtubule-based functions during spermatogenesis.

The microtubule-associated protein tumor overexpressed gene binds to the RNA trafficking protein heterogeneous nuclear ribonucleoprotein A2

In neural cells, such as oligodendrocytes and neurons, transport of certain RNAs along microtubules is mediated by the cis-acting heterogeneous nuclear ribonucleoprotein A2 response element (A2RE) trafficking element and the cognate trans-acting heterogeneous nuclear ribonucleoprotein (hnRNP) A2 trafficking factor. Using a yeast two-hybrid screen, we have identified a microtubule-associated protein, tumor overexpressed gene (TOG)2, as an hnRNP A2 binding partner. The C-terminal third of TOG2 is sufficient for hnRNP A2 binding. TOG2, the large protein isoform of TOG, is the only isoform detected in oligodendrocytes in culture. TOG coimmunoprecipitates with hnRNP A2 present in the cytoskeleton (CSK) fraction of neural cells, and both coprecipitate with microtubule stabilized pellets. Staining with anti-TOG reveals puncta that are localized in proximity to microtubules, often at the plus ends. TOG is colocalized with hnRNP A2 and A2RE-mRNA in trafficking granules that remain associated with CSK-insoluble tissue. These data suggest that TOG mediates the association of hnRNP A2-positive granules with microtubules during transport and/or localization.

The relative levels of translin-associated factor X (TRAX) and testis brain RNA-binding protein determine their nucleocytoplasmic distribution in male germ cells

Testis brain RNA-binding protein (TB-RBP), the mouse orthologue of human translin, is an RNA and single-stranded DNA-binding protein abundant in testis and brain. Translin-associated factor X (TRAX) was identified as a protein that interacts with TB-RBP and is dependent upon TB-RBP for stabilization. Using immunohistochemistry to investigate the subcellular locations of TB-RBP and TRAX during spermatogenesis, both proteins localize in nuclei in meiotic pachytene spermatocytes and in the cytoplasm of subsequent meiotic and post-meiotic cells. An identical subcellular distribution is seen in female germ cells. Western blot analysis of germ cell protein extracts reveals an increased ratio of TRAX to TB-RBP in meiotic pachytene spermatocytes compared with the post-meiotic round and elongated spermatids. Using COS-1 cells and mouse embryonic fibroblasts derived from TB-RBP null mice as model systems to examine the shuttling of TB-RBP and TRAX, we demonstrate that TRAX contains a functional nuclear localization signal and TB-RBP contains a functional nuclear export signal. Coexpression of both proteins in COS-1 cells and TB-RBP-deficient mouse embryonic fibroblasts reveals that the ratio of TRAX to TB-RBP determines their subcellular locations, i.e. increased TRAX to TB-RBP ratios lead to nuclear localizations, whereas TRAX remains in the cytoplasm when TB-RBP levels are elevated. These subcellular distributions require interaction between TB-RBP and TRAX. We propose that the subcellular locations of TB-RBP and TRAX in male germ cells are modulated by the relative ratios of TRAX and TB-RBP.

Translin-associated factor X is post-transcriptionally regulated by its partner protein TB-RBP, and both are essential for normal cell proliferation

To determine the functions of the DNA/RNA-binding protein TB-RBP in somatic cells, we examined cultured primary mouse embryonic fibroblasts (MEFs) derived from TB-RBP-deficient mice. The TB-RBP-deficient MEFs exhibit a reduced growth rate compared with MEFs from littermates. Reintroduction of TB-RBP remedies this defect. A partner protein of TB-RBP, Translin-associated factor X (TRAX), was absent in TB-RBP-deficient MEFs, despite normal TRAX mRNA levels. TRAX is dependent upon the presence of TB-RBP and is removed from null MEFs following ubiquitination. Re-introduction of TB-RBP, but not TB-RBP lacking an oligomerization domain, into null MEFs stabilized TRAX protein without changing TRAX mRNA levels. The coordinated expression of TB-RBP and TRAX is also seen in synchronized cells, where the amount of TRAX protein but not TRAX RNA closely parallels TB-RBP levels throughout the cell cycle. In transgenic mice overexpressing TRAX in testis, total TB-RBP and TRAX levels are constant with reductions of endogenous TRAX compensating for exogenous TRAX. Using RNA interference, reductions of either TB-RBP or TRAX (without affecting TB-RBP) slow cell growth rates. We conclude that TRAX is post-transcriptionally stabilized by TB-RBP and both proteins are needed for normal cell proliferation.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.

Control of mouse hils1 gene expression during spermatogenesis: identification of regulatory element by transgenic mouse

Histone H1-like protein in spermatids 1 (Hils1) is a testis- specific histone H1-like protein exclusively expressed in haploid spermatids and should be involved in chromatin remodeling during mouse spermatogenesis. Spatial and temporal regulation of the hils1 gene expression would be critical for the formation of functional sperm, controlled at both transcriptional and translational levels. Previously, we reported that transcripts of the hils1 gene are exclusively expressed in mouse testis from 23 days of age whereas the Hils1 protein is not detected until 28 days of age, suggesting that hils1 is a member of a class of translationally regulated genes. By analyzing transgenic mice, we could demonstrate that 318-base pair (bp) 5'-proximal region corresponding to the first 70-bp proximal TATA-less promoter, and 248 bp of 5'-untranslated region is sufficient to confer testis- and spermatid-specific transcription as well as posttranscriptional control of the mouse hils1 gene in vivo.

The kinesin KIF17b and RNA-binding protein TB-RBP transport specific cAMP-responsive element modulator-regulated mRNAs in male germ cells

Testis brain RNA-binding protein (TB-RBP), the mouse orthologue of the human protein Translin, is a widely expressed and highly conserved protein with proposed functions in chromosomal translocations, mitotic cell division, and mRNA transport, stabilization, and storage. Targeted inactivation of TB-RBP leads to abnormalities in fertility and behavior. A testis-enriched kinesin KIF17b coimmunoprecipitates with TB-RBP in a RNA-protein complex containing specific cAMP-responsive element modulator (CREM)-regulated mRNAs. The specificity of this interaction is confirmed by in vivo RNA-protein crosslinking and transfections of hippocampal neurons. Combining in situ hybridization and immunohistochemistry at the electron microscope level, a temporally sequential dissociation of KIF17b and TB-RBP from specific mRNAs is detected with TB-RBP release coincident with the time of mRNA translation, indicating a separation of the processes of transport and translation. We conclude that KIF17b serves as a molecular motor component of a TB-RBP-mouse ribonucleoprotein complex transporting a group of specific CREM-regulated mRNAs in mammalian male postmeiotic germ cells. Because KIF17b has been reported to control CREM-dependent transcription in male germ cells by regulating the intracellular location of the transcriptional coactivator activator of CREM in testis, this indicates that one kinesin links the processes of transcription and transport of specific mRNAs in mammalian male germ cells.

Mice deficient for testis-brain RNA-binding protein exhibit a coordinate loss of TRAX, reduced fertility, altered gene expression in the brain, and behavioral changes

Testis-brain RNA-binding protein (TB-RBP), the mouse orthologue of the human protein Translin, is a widely expressed and highly conserved protein with proposed functions in chromosomal translocations, mitotic cell division, and mRNA transport and storage. To better define the biological roles of TB-RBP, we generated mice lacking TB-RBP. Matings between heterozygotes gave rise to viable, apparently normal homozygous mutant mice at a normal Mendelian ratio. The TB-RBP-related and -interacting protein Translin-associated factor X was reduced to 50% normal levels in heterozygotes and was absent in TB-RBP-null animals. The null mice were 10 to 30% smaller than their wild-type or heterozygote littermates at birth and remained so to about 6 to 9 months of age, showed normal B- and T-cell development, and accumulated visceral fat. TB-RBP-null male mice were fertile and sired offspring but had abnormal seminiferous tubules and reduced sperm counts. Null female mice were subfertile and had reduced litter sizes. Microarray analysis of total brain RNA from null and wild-type mice revealed an altered gene expression profile with the up-regulation of 14 genes and the down-regulation of 217 genes out of 12,473 probe sets. Numerous neurotransmitter receptors and ion channels, including gamma-aminobutyric acid A receptor alpha1 and glutamate receptor alpha3, were strongly down-regulated. Behavioral abnormalities were also seen. Compared to littermates, the TB-RBP-null mice appeared docile and exhibited reduced Rota-Rod performance.

Intercellular organelle traffic through cytoplasmic bridges in early spermatids of the rat: mechanisms of haploid gene product sharing

Stable cytoplasmic bridges (or ring canals) connecting the clone of spermatids are assumed to facilitate the sharing of haploid gene products and synchronous development of the cells. We have visualized these cytoplasmic bridges under phase-contrast optics and recorded the sharing of cytoplasmic material between the spermatids by a digital time-lapse imaging system ex vivo. A multitude of small (ca. 0.5 microm) granules were seen to move continuously over the bridges, but only 28% of those entering the bridge were actually transported into other cell. The average speed of the granules decreased significantly during the passage. Immunocytochemistry revealed that some of the shared granules contained haploid cell-specific gene product TRA54. We also demonstrate the novel function for the Golgi complex in acrosome system formation by showing that TRA54 is processed in Golgi complex and is transported into acrosome system of neighboring spermatid. In addition, we propose an intercellular transport function for the male germ cell-specific organelle chromatoid body. This mRNA containing organelle, ca. 1.8 microm in diameter, was demonstrated to go over the cytoplasmic bridge from one spermatid to another. Microtubule inhibitors prevented all organelle movements through the bridges and caused a disintegration of the chromatoid body. This is the first direct demonstration of an organelle traffic through cytoplasmic bridges in mammalian spermatogenesis. Golgi-derived haploid gene products are shared between spermatids, and an active involvement of the chromatoid body in intercellular material transport between round spermatids is proposed.

Mouse testis brain RNA-binding protein/translin selectively binds to the messenger RNA of the fibrous sheath protein glyceraldehyde 3-phosphate dehydrogenase-S and suppresses its translation in vitro

The testis brain RNA-binding protein (TB-RBP/translin) is a DNA- and RNA-binding protein with multiple functions. As an RNA-binding protein, TB-RBP binds to conserved sequence elements often present in the 3' untranslated regions (UTRs) of specific mRNAs modulating their translation and transport. To identify additional mRNA targets of TB-RBP, immunoprecipitation and reverse transcription-polymerase chain reaction (RT-PCR) assays were carried out using an affinity-purified antibody to TB-RBP with testicular extracts. Gapds mRNA was found to be selectively precipitated in a TB-RBP-mRNA complex. Consistent with the delayed translation of GAPDS and the subcellular ribonucleoprotein location of TB-RBP, polysomal gradient analysis showed that most of the Gapds mRNA in adult testis extracts was present in the nonpolysomal fractions. In vitro translation assays revealed that Gapds mRNA translation was inhibited by recombinant TB-RBP or by a TB-RBP mutant protein, Nb, capable of binding RNA. No inhibition was seen with mutant forms of TB-RBP lacking domains required for RNA binding, including the TB-RBP Cb mutant and the C-terminal-truncated form of TB-RBP that disrupts the leucine zipper. As an additional indicator of the specificity of TB-RBP inhibition of Gapds mRNA translation, a putative TB-RBP binding H-element was deleted from the 5' UTR of the Gapds mRNA. No translational inhibition by recombinant TB-RBP was seen with Gapds mRNA lacking the H element. These data suggest that TB-RBP is involved in the posttranscriptional regulation of Gapds gene expression during spermiogenesis. Moreover, the Gapds mRNA is the first mRNA shown to have a functional TB-RBP binding site in its 5' UTR.

Nuclear retention of MBP mRNAs in the quaking viable mice

Quaking viable (qk(v)) mice fail to properly compact myelin in their central nervous systems. Although the defect in the qk(v) mice involves a mutation affecting the expression of the alternatively spliced qk gene products, their roles in myelination are unknown. We show that the QKI RNA binding proteins regulate the nuclear export of MBP mRNAs. Disruption of the QKI nucleocytoplasmic equilibrium in oligodendrocytes results in nuclear and perikaryal retention of the MBP mRNAs and lack of export to cytoplasmic processes, as it occurs in qk(v) mice. MBP mRNA export defect leads to a reduction in the MBP levels and their improper cellular targeting to the periphery. Our findings suggest that QKI participates in myelination by regulating the mRNA export of key protein components.

Characterization of POMT2, a novel member of the PMT protein O-mannosyltransferase family specifically localized to the acrosome of mammalian spermatids

Over the past few years it has emerged that O-mannosyl glycans are not restricted to yeasts and fungi but are also present in higher eukaryotes, including humans. They play a substantial role in the onset of muscular dystrophy and neuronal migration disorders, like muscle-eye-brain disease. Protein O-mannosyltransferase genes (PMTs) are evolutionarily conserved from yeast to human; however, little is known about these enzymes in higher eukaryotes. In this study, we cloned the first PMT2 subfamily members from human (hPOMT2), mouse (mPomt2), and Drosophila (DmPOMT2). A detailed characterization of the mammalian POMT2, with emphasis on mouse Pomt2, shows that mammalian POMT2 is predominantly expressed in testis tissue. Due to differential transcription initiation of the mPomt2 gene, two distinct mRNA species that vary in length are formed. The shorter transcript is present in all somatic tissues examined. Expression of the corresponding hPOMT2 cDNA in mammalian cells identified POMT2 as an integral membrane protein of the endoplasmic reticulum with an apparent molecular weight of 83 kDa. The longer mPomt2 transcript is restricted to testis and encodes a testis-specific mPOMT2 protein isoform. Using in situ hybridization and immunolocalization, we demonstrate that in testis tissue mPOMT2 localizes to maturing spermatids and is abundant within the acrosome, a sperm-specific organelle essential for fertilization. Our data suggest a novel and specific role for the putative protein O-mannosyltransferase POMT2 in the maturation and/or function of sperm in mammals.

Crystal structure of TB-RBP, a novel RNA-binding and regulating protein

The testis/brain-RNA-binding protein (TB-RBP) spatially and temporally controls the expression of specific mRNAs in developing male germ cells and brain cells, and is implicated in DNA recombination and repair events. We report the 2.65 A crystal structure of mouse TB-RBP. The structure is predominantly alpha-helical and exhibits a novel protein fold and mode of assembly. Crystal symmetry and molecular symmetry combine to form an octet of TB-RBP monomers in the shape of an elongated spherical particle with a large cavity at its center. Amino acid residues that affect RNA and DNA binding are located on the interior surface of the assembled particle, and a putative nucleotide-binding domain that controls RNA binding is located at a dimer interface. Other modes of assembly are suggested for TB-RBP based on our structure and recently reported electron microscopic reconstructions of human TB-RBP.

A TB-RBP and Ter ATPase complex accompanies specific mRNAs from nuclei through the nuclear pores and into intercellular bridges in mouse male germ cells

The testis brain RNA-binding protein (TB-RBP) functions as an RNA-binding protein in brain and testis, binding to conserved sequence elements present in specific mRNAs, such as protamine 1 and 2. We show here by RNA gel shift assays, immunoprecipitation, and by a novel in situ hybridization immunohistochemical technique that TB-RBP binds to AKAP4 mRNA in male mouse germ cells. AKAP4 is a component of the fibrous sheath and functions as a scaffolding protein in the sperm flagellum. AKAP4 is encoded by an X-linked gene, is expressed solely in postmeiotic (haploid) male germ cells, and is an essential protein in all spermatozoa, requiring its transport between spermatids as a protein or mRNA. AKAP4 mRNA forms a complex with TB-RBP and the Ter ATPase in nuclei and remains associated with these proteins as it exits nuclei into the cytoplasm and as it passes through intercellular bridges between spermatids. A similar mRNA-TB-RBP-Ter ATPase association is seen for protamine 2 mRNA, which is stored in the cytoplasm of postmeiotic germ cells about 7 days before translation. In contrast, no association is seen with PGK-2 mRNA which is initially transcribed early in meiosis with increased transcription in postmeiotic male germ cells. Although PGK-2 mRNA is subject to translational control, it lacks TB-RBP-binding sequences in its mRNA. The AKAP4 or protamine 2 mRNA-protein complexes dissociate in late-stage male germ cells when the mRNAs are translated. We propose that TB-RBP and the Ter ATPase are part of a complex that accompanies specific mRNAs in haploid mouse male germ cells in intracellular and intercellular movement. The temporal relationship of TB-RBP binding and mRNA inactivation in conjunction with the subsequent dissociation of the mRNA-protein complex at the time of mRNA translation suggests a role in translational suppression and/or mRNA stabilization.

Identification and characterization of cDNAs encoding four novel proteins that interact with translin associated factor-X

Translin-associated factor X (TRAX) is the predominantly cytoplasmic binding partner of TB-RBP/translin in mouse testis. Four mouse testis cDNAs encoding specific TRAX-interacting proteins were isolated from a yeast two-hybrid library screen. One novel cDNA designated Tsnaxip1 (TRAX-interacting protein-1) encodes 709 amino acids. We isolated a cDNA encoding the 427 carboxy-terminal amino acids of MEA-2, a Golgi-associated, maleenhanced autoantigen; a cDNA encoding 429 amino acids with 73% homology to centrosomal Akap9; and a cDNA encoding 346 amino acids with 75% homology to SUN1, a predicted human protein that contains a SUN domain (which is present in some perinuclear proteins). Interactions were verified using in vitro synthesized fusion proteins. All four genes were expressed in the testis and enriched in germ cells. Confocal microscopy studies using green fluorescent protein fusion proteins determined that these TRAX-interacting proteins colocalize with TRAX. The data suggest that TRAX may have a function associated with perinuclear organelles during spermatogenesis.

Testis fascin (FSCN3): a novel paralog of the actin-bundling protein fascin expressed specifically in the elongate spermatid head

During spermiogenesis, significant morphological changes occur as round spermatids are remodeled into the fusiform shape of mature spermatozoa. These changes are correlated with a reorganization of microfilaments and microtubules in the head and tail regions of elongating spermatids. There is also altered expression of specialized actin- and tubulin-associated proteins. We report the characterization of a novel, spermatid-specific murine paralog of the actin-bundling protein fascin (FSCN1); this paralog is designated testis fascin or FSCN3. Testis fascin is distantly related to fascins but retains its primary sequence organization. cDNA clones of mouse testis fascin predict a 498 amino acid protein of molecular mass 56 kD that shares 29% identity with mouse fascin. Mapping of murine and human FSCN3 genes shows localization to the 7q31.3 chromosome. Northern analysis indicates that FSCN3 expression is highly specific to testis and that in situ hybridization further restricts expression to elongating spermatids. Antibodies raised against recombinant FSCN3 protein identify a band at 56 kD in testis, epididymis, and epididymal spermatozoa, suggesting that testis fascin persists in mature spermatozoa. In accord with the in situ hybridization results, immunofluorescent microscopy localizes testis fascin protein to areas of the anterior spermatid head that match known distributions of F-actin in the dorsal and ventral subacrosomal spaces. It is possible that testis fascin may function in the terminal elongation of the spermatid head and in microfilament rearrangements that accompany fertilization.

Candidate RNA-binding proteins regulating extrasomatic mRNA targeting and translation in mammalian neurons

In mammalian neurons, long-lasting changes in the efficacy of individual synapses depend on the synthesis of new proteins. To maintain specificity, neuronal cells have to ensure that these newly synthesized proteins accumulate at the appropriate subpopulation of synapses. One way that neurons have solved this challenge appears to be the local translation of extrasomatic mRNAs in dendrites and at postsynaptic sites. Mechanisms, which regulate the targeting, translation, and stability of dendritic mRNAs, involve an organized interaction between cis-acting elements of localized transcripts and trans-acting RNA-binding proteins. The molecular identity and cellular functions of trans-acting factors that are likely to play an important role in post-transcriptional processing of extrasomatic transcripts in mammalian neurons are now being elucidated.

Strategies for the purification and on-column cleavage of glutathione-S-transferase fusion target proteins

In this report, we describe a flexible, efficient and rapid protein purification strategy for the isolation and cleavage of glutathione-S-transferase (GST) fusion proteins. The purification and on-column cleavage strategy was developed to work for the purification of difficult proteins and for target proteins where efficient fusion-tag cleavage is essential for downstream processes, such as structural and functional studies. To test and demonstrate the flexibility of this method, seven diverse unrelated target proteins were assayed. A purification technique is described that can be applied to a wide range of both soluble and membrane inserted recombinant target proteins of differing function, structure and chemical nature. This strategy is performed in a single chromatographic step applying an on-column cleavage method, yielding "native" proteins in the 200 microg to 40 mg/l scale of 95-98% purity.

RNA binding protein Musashi1 is expressed in sertoli cells in the rat testis from fetal life to adulthood

The Musashi1 (Msi1) gene identified in mouse is a member of a subfamily of RNA binding proteins that are highly conserved across species. Msi1 expression is highly enriched in proliferative cells within the developing central nervous system. Within the testis, proliferation and differentiation of germ cells takes place within the seminiferous epithelium, where these cells are supported physically and functionally by Sertoli cells that do not themselves proliferate following the onset of puberty. RNA binding proteins expressed in testicular germ cells are essential for normal fertility. Preliminary data suggested the mRNA for Msi1 was present in ovary; therefore, we used an Msi1-specific cRNA and monoclonal antibody to investigate whether Msi1 was expressed in the testis. Msi1 mRNA was expressed in rat testis from birth until adulthood; in situ hybridization revealed silver grains within the seminiferous epithelium. Immunohistochemical studies demonstrated that at all ages examined (from Fetal Day 14.5 until adulthood) Msi1 protein was expressed in Sertoli cells. In fetal and adult rat ovaries, Msi1 was detected in granulosa cells and their precursors. In Sertoli cells, protein was detected in both cytoplasmic and nuclear compartments; in adult testes, the immunointensity of the nuclear staining was stage dependent, with highest levels of expression in Sertoli cells at stages I-VI. In rat gonads, the RNA binding protein Msi1 is expressed in both proliferating and nonproliferating Sertoli and granulosa cells.

Identification of a rice RNA- and microtubule-binding protein as the multifunctional protein, a peroxisomal enzyme involved in the beta -oxidation of fatty acids

The control of subcellular mRNA localization and translation is often mediated by protein factors that are directly or indirectly associated with the cytoskeleton. We report the identification and characterization of a rice seed protein that possesses both RNA and microtubule binding activities. In vitro UV cross-linking assays indicated that this protein binds to all mRNA sequences tested, although there was evidence for preferential binding to RNAs that contained A-C nucleotide sequence motifs. The protein was purified to homogeneity using a two-step procedure, and amino acid sequencing identified it as the multifunctional protein (MFP), a peroxisomal enzyme known to possess a number of activities involved in the beta-oxidation of fatty acids. The recombinant version of this rice MFP binds to RNA in UV cross-linking and gel mobility shift experiments, co-sediments specifically with microtubules, and possesses at least two enzymatic activities involved in peroxisomal fatty acid beta-oxidation. Taken together these data suggest that MFP has an important role in mRNA physiology in the cytoplasm, perhaps in regulating the localization or translation of mRNAs through an interaction with microtubules, in addition to its peroxisomal function.

A new translational repression element and unusual transcriptional control regulate expression of don juan during Drosophila spermatogenesis

The Drosophila don juan (dj) gene encodes a basic protein that is expressed solely in the male germline and shows structural similarities to the linker histone H1. Don Juan is located in two different subcellular structures: in the nucleus during the phase of chromatin condensation and later in the mitochondrial derivatives starting with spermatid individualization. The don juan gene is transcribed in primary spermatocytes under the control of 23 bp upstream in combination with downstream sequences. During meiotic stages and in early spermatid stages don juan mRNA is translationally repressed for several days. Analysis of male sterile mutants which fail to undergo meiosis shows that release of dj mRNA from translational repression is independent of meiosis. In gel retardation assays 60 nucleotides at the end of the dj leader form four major complexes with proteins that were extracted from testes but not with protein extracts from ovaries. Transformation studies prove that in vivo 35 bp within that region of the dj mRNA is essential to confer translational repression. UV cross-linking studies show that a 62 kDa protein specifically binds to the same region within the 5' untranslated region. The dj translational repression element, TRE, is distinct from the translational control element, TCE, described earlier for all members of the Mst(3)CGP gene family. Moreover, expression studies in several male sterile mutants reveal that don juan mRNA is translated in earlier developmental stages during sperm morphogenesis than the Mst(3)CGP mRNAs. This proves that translational activation of dormant mRNAs in spermatogenesis occurs at different time-points which are characteristic for each gene, an essential feature for coordinated sperm morphogenesis.

Altering the GTP binding site of the DNA/RNA-binding protein, Translin/TB-RBP, decreases RNA binding and may create a dominant negative phenotype

The DNA/RNA-binding protein, Translin/Testis Brain RNA-binding protein (Translin/TB-RBP), contains a putative GTP binding site in its C-terminus which is highly conserved. To determine if guanine nucleotide binding to this site functionally alters nucleic acid binding, electrophoretic mobility shift assays were performed with RNA and DNA binding probes. GTP, but not GDP, reduces RNA binding by approximately 50% and the poorly hydrolyzed GTP analog, GTPgammaS, reduces binding by >90% in gel shift and immunoprecipitation assays. No similar reduction of DNA binding is seen. When the putative GTP binding site of TB-RBP, amino acid sequence VTAGD, is altered to VTNSD by site directed mutagenesis, GTP will no longer bind to TB-RBP(GTP) and TB-RBP(GTP) no longer binds to RNA, although DNA binding is not affected. Yeast two-hybrid assays reveal that like wild-type TB-RBP, TB-RBP(GTP) will interact with itself, with wild-type TB-RBP and with Translin associated factor X (Trax). Transfection of TB-RBP(GTP) into NIH 3T3 cells leads to a marked increase in cell death suggesting a dominant negative function for TB-RBP(GTP) in cells. These data suggest TB-RBP is an RNA-binding protein whose activity is allosterically controlled by nucleotide binding.

Electron microscopic studies of the translin octameric ring

Translin is thought to participate in a variety of cellular activities including chromosomal translocations, translational regulation of mRNA expression, and mRNA transport. It forms an octameric ring structure capable of sequence-specific binding of both DNA and RNA substrates. We have used electron microscopy and single-particle image analysis to generate a three-dimensional reconstruction of the Translin ring. The subunits appear to have two distinct domains that assemble to form an open channel with diameter of approximately 30 A at one end and approximately 50 A at the opposite end. In the presence of either DNA or RNA containing consensus binding sequences, the largest opening into the central cavity is filled with density. Strikingly, although Translin shows significant sequence homology to only one other protein, Translin-associated factor X, the quaternary organization and the dimerization of subunits in the ring are very similar to those observed for hexameric ring helicases. This suggests that many of the structures in DNA and RNA metabolism may have similar quaternary organization.

A Highly Conserved Sequence Essential for Translational Repression of the Protamine 1 Messenger RNA in Murine Spermatids1

Translational regulation of the protamine 1 mRNA is mediated by sequences in its 3' untranslated region. In this study, we demonstrate that a highly conserved sequence, the translational control element, is solely responsible for protamine 1 translational regulation. Mutation of the conserved sequence causes premature translation of a transgene containing a fusion between the human growth hormone coding sequence and the protamine 1 3' untranslated region. Temporal expression of the transgene was monitored in prepubertal animals by Northern and Western blotting and in adult animals by immunocytochemistry. Messenger RNAs lacking the translational control element sediment in the messenger ribonucleoprotein particle and ribosomal fractions of polysome gradients, suggesting that the translational control element is required for translational repression but not for incorporation of mRNAs into ribonucleoprotein particles.

Lack of Sharing of Spam1 (Ph-20) among Mouse Spermatids and Transmission Ratio Distortion1

Gametic equality is thought to exist, despite haploid gene action in mammalian spermiogenesis, because of product sharing via the intercellular bridges of conjoined spermatids. However, mice carrying different t-alleles have been known to produce functionally different sperm, leading to transmission ratio distortion (TRD), whose mechanism is unknown. The reduced Spam1 mRNA levels, previously shown to be associated with TRD and reduced fertility in mice carrying the Rb(6.16) or the Rb(6.15) Robertsonian translocation, are reflected in the levels of its encoded membrane protein (Spam1) and its accompanying insoluble hyaluronidase activity. Studies of the temporal expression pattern of Spam1 reveal that it is haploid expressed, with both the RNA and protein first appearing on Day 21.5. RNA fluorescence in situ hybridization and immunocytochemistry show both the mRNA and the protein to be compartmentalized. Compartmentalization of the mRNA along with its immediate translation and insertion of the protein in the plasma membrane suggests the nonsharing of Spam1 transcripts among spermatids, resulting in functionally different sperm in males with different Spam1 alleles. Evidence for biochemically different sperm in these heterozygous males was revealed by flow cytometry and confocal microscopy. Our findings support the notion that the Spam1 antigen is not shared, and we may have uncovered a mechanism for TRD.

Trax (translin-associated factor X), a primarily cytoplasmic protein, inhibits the binding of TB-RBP (translin) to RNA

Trax (Translin-associated factor X) has been shown to interact with TB-RBP/Translin by its coimmunoprecipitation and in yeast two-hybrid assays. Here we demonstrate that Trax is widely expressed, does not bind to DNA or RNA, but forms heterodimers with TB-RBP under reducing conditions. The heterodimer of TB-RBP and Trax inhibits TB-RBP binding to RNA, but enhances TB-RBP binding to specific single stranded DNA sequences. The in vitro interactions between TB-RBP and Trax are confirmed by similar interactions in the yeast two-hybrid system. Cell fractionation and confocal microscope studies reveal that Trax is predominantly cytoplasmic. In contrast, TB-RBP is present in both the nuclei and cytoplasm of transfected cells and uses a highly conserved nuclear export signal to exit nuclei. In addition to a leucine zipper, two basic domains in TB-RBP are essential for RNA binding, but only one of these domains is needed for DNA binding. Trax restores DNA binding to TB-RBP containing an altered form of this domain. These data suggest that Trax-TB.RBP interactions modulate the DNA- and RNA-binding activity of TB-RBP.

Targeting of mRNAs within the glial cell cytoplasm: how to hide the message along the journey

The subcellular targeting of mRNAs encoding myelin proteins to the oligodendrocyte processes is an accepted fact in myelin formation. How these messengers are kept silent during their movement to the subcellular domain where they are turned on remains a mystery. This review focuses on aspects of mRNA targeting and speculates on possible molecular mechanisms for the translational control of myelin-located mRNAs.

The single-stranded DNA- and RNA-binding proteins pur alpha and pur beta link BC1 RNA to microtubules through binding to the dendrite-targeting RNA motifs

Neural BC1 RNA is distributed in neuronal dendrites as RNA-protein complexes (BC1 RNPs) containing Translin. In this study, we demonstrated that the single-stranded DNA- and RNA-binding protein pur alpha and its isoform, pur beta, which have been implicated in control of DNA replication and transcription, linked BC1 RNA to microtubules (MTs). The binding site was within the 5' proximal region of BC1 RNA containing putative dendrite-targeting RNA motifs rich in G and U residues, suggesting that in the cytoplasm of neurons, these nuclear factors are involved in the BC1 RNA transport along dendritic MTs. The pur proteins were not components of BC1 RNP but appeared to associate with MTs in brain cells. Therefore, it is suggested that they may transiently interact with the RNP during transport. In this respect, the interaction of pur proteins with BC1 RNA could be regulated by the Translin present within the RNP, because the binding mode of these two classes of proteins (pur proteins and Translin) to the dendrite-targeting RNA motifs was mutually exclusive. As the motifs are well conserved in microtubule-associated protein 2a/b mRNA as well, the pur proteins may also play a role(s) in the dendritic transport of a subset of mRNAs.

Neural BC1 RNA associates with pur alpha, a single-stranded DNA and RNA binding protein, which is involved in the transcription of the BC1 RNA gene

BC1 RNA is preferentially expressed in neural cells by RNA polymerase III (Pol III) and forms ribonucleoprotein particles (RNP) in the somatodendritic domain of neurons. Our previous studies have suggested that, in the nucleus, BC1 RNA forms an RNP containing a nuclear protein(s) that participates in the transcription of the BC1 RNA gene. In this study, we have shown that newly synthesized BC1 RNA in purified brain nuclear extracts is immunoprecipitated by an antibody against Pur alpha. Pur alpha is a protein that binds single-stranded DNA and RNA and is known to regulate transcription of Pol II system. Although BC1 RNA is transcribed by Pol III, the BC1 RNA gene has two putative Pur alpha binding sites, which Pur alpha specifically recognizes. Point mutations within these sites reduced transcriptional activity in vitro. Furthermore, transcription was inhibited by depletion of Pur alpha from the nuclear extracts, either by the coexistence of its binding region of BC1 RNA or by the antibody that was able to precipitate the nuclear BC1 RNP. These observations suggest that BC1 RNA associates with Pur alpha which is involved in the transcription of the BC1 RNA gene.

Part of Xenopus translin is localized in the centrosomes during mitosis

During oogenesis, maternal mRNAs are synthesised and stored in a translationally dormant form due to the presence of regulatory elements at the 3' untranslated regions (3'UTR). In Xenopus oocytes, several studies have described the presence of RNA-binding proteins capable to repress maternal-mRNA translation. The testis-brain RNA-binding protein (TB-RBP/Translin) is a single-stranded DNA- and RNA-binding protein which can bind the 3' UTR regions (Y and H elements) of stored mRNAs and can suppress in vitro translation of the mRNAs that contain these sequences. Here we report the cloning of the Xenopus homologue of the TB-RBP/Translin protein (X-translin) as well as its expression, its localisation, and its biochemical association with the protein named Translin associated factor X (Trax) in Xenopus oocytes. The fact that this protein is highly present in the cytoplasm from stage VI oocytes until 48 h embryos and that it has been described as capable to inhibit paternal mRNA translation, indicates that it could play an important role in maternal mRNA translation control during Xenopus oogenesis and embryogenesis. Moreover, we investigated X-translin localisation during cell cycle in XTC cells. In interphase, although a weak and diffuse nuclear staining was observed, X-translin was mostly present in the cytoplasm where it exhibited a prominent granular staining. Interestingly, part of X-translin underwent a remarkable redistribution throughout mitosis and associated with centrosomes, which may suggest a new unknown role for this protein in cell cycle.

Identification of a negative regulatory DNA element for neuronal BC1 RNA expression by RNA polymerase III

BC1 RNA is a neuronal cell-specific RNA polymerase III (Pol III) transcript. The BC1 RNA gene has plural types of Pol III promoters, in addition to which an E-box sequence (E2 site) acts as a transcriptional activator, which is recognized by a brain-specific protein(s). Using an in vitro transcription system, we found that the upstream region of the BC1 RNA gene contained a sequence that interfered with the activity of the E-box element in a distance-independent manner. A tandem repeat within this sequence, which was weakly homologous with the neuron-restrictive silencer element (NRSE) found in the Pol II system, was recognized by a brain nuclear protein. Consistently, the transcriptional activity increased by deleting the tandem repeat sequence. We called this BC1 RNA-repressing element BCRE. The DNA-binding specificities of BCRE-binding protein differed from that of NRSE-binding protein (NRSF). A similar protein with an ability to bind to BCRE was also found in liver and kidney. Furthermore, the glutamate analog kainic acid increased the DNA-binding of both E2 site-binding protein and BCRE-binding protein, and then the levels of BC1 RNA also increased transiently. Our results suggested that both positive and negative regulatory elements contribute to neuronal BC1 RNA expression.