The puff-specific Drosophila protein Bj6, encoded by the gene no-on transient A, shows homology to RNA-binding proteins

Nuclear functions of actin

Actin participates in several essential processes in the cell nucleus. Even though the presence of actin in the nucleus was proposed more than 30 years ago, nuclear processes that require actin have been only recently identified. Actin is part of chromatin remodeling complexes; it is associated with the transcription machineries; it becomes incorporated into newly synthesized ribonucleoproteins; and it influences long-range chromatin organization. As in the cytoplasm, nuclear actin works in conjunction with different types of actin-binding proteins that regulate actin function and bridge interactions between actin and other nuclear components.

Studies on the role of NonA in mRNA biogenesis

The NonA protein of Drosophila melanogaster is an abundant nuclear protein that belongs to the DBHS (Drosophila behavior, human splicing) protein family. The DBHS proteins bind both DNA and RNA in vitro and have been involved in different aspects of gene expression, including pre-mRNA splicing, transcription regulation and nuclear retention of mRNA. We have used double-stranded RNA interference in Drosophila S2 cells to silence the expression of NonA and to investigate its role in mRNA biogenesis. We show that knockdown of NonA does not affect transcription nor splicing. We demonstrate that NonA forms a complex with the essential nuclear export factor NXF1 in an RNA-dependent manner. We have constructed stable S2 cell lines that express full-length and truncated NXF1 fused to GFP in order to perform fluorescence recovery after photobleaching experiments. We show that knockdown of NonA reduces the intranuclear mobility of NXF1-GFP associated with poly(A)(+) RNA in vivo, while the mobility of the truncated NXF1-GFP that does not bind RNA is not affected. Our data suggest that NonA facilitates the intranuclear mobility of mRNP particles.

The Hrp65 self-interaction is mediated by an evolutionarily conserved domain and is required for nuclear import of Hrp65 isoforms that lack a nuclear localization signal

Hrp65, an evolutionary conserved RNA-binding protein from the midge Chironomus tentans, has a conserved DBHS (Drosophila behavior, human splicing) domain that is also present in several mammalian proteins. In a yeast two-hybrid screening we found that Hrp65 can interact with itself. Here we confirm the Hrp65 self-interaction by in vitro pull-down experiments and map the sequences responsible for the interaction to a region that we refer to as the protein-binding domain located within the DBHS domain. We also show that the protein-binding domains of Drosophila NonA and human PSF, two other proteins with conserved DBHS domains, bind to Hrp65 in the yeast two-hybrid system. These observations indicate that the protein-binding domain can mediate homodimerization of Hrp65 as well as heterodimerization between different DBHS-containing proteins. Moreover, analyses of recombinant Hrp65 by gel-filtration chromatography show that Hrp65 can not only dimerize but also oligomerize into complexes of at least three to six molecules. Furthermore, we have analyzed the functional significance of the Hrp65 self-interaction in cotransfection assays, and our results suggest that the interaction between different Hrp65 isoforms is crucial for their intracellular localization.

Evidence for a posttranscriptional role of a TFIIICalpha-like protein in Chironomus tentans

We have cloned and sequenced a cDNA that encodes for a nuclear protein of 238 kDa in the dipteran Chironomus tentans. This protein, that we call p2D10, is structurally similar to the alpha subunit of the general transcription factor TFIIIC. Using immunoelectron microscopy we have shown that a fraction of p2D10 is located at sites of transcription, which is consistent with a possible role of this protein in transcription initiation. We have also found that a large fraction of p2D10 is located in the nucleoplasm and in the nuclear pore complexes. Using gel filtration chromatography and coimmunoprecipitation methods, we have identified and characterized two p2D10-containing complexes that differ in molecular mass and composition. The heavy p2D10-containing complex contains at least one other component of the TFIIIC complex, TFIIIC-epsilon. Based on its molecular mass and composition, the heavy p2D10-containing complex may be the Pol III holoenzyme. The light p2D10-containing complex contains RNA together with at least two proteins that are thought to be involved in mRNA trafficking, RAE1 and hrp65. The observations reported here suggest that this new TFIIIC-alpha-like protein is involved in posttranscriptional steps of premRNA metabolism in Chironomus tentans.

Nucleotide variation at the no-on-transient A gene in Drosophila littoralis

The no-on-transient A (nonA) gene encodes a putative RNA-binding protein, and mutations in this gene are known to affect vision, male courtship song and viability in Drosophila melanogaster. Here we have sequenced the coding region of the nonA gene of Drosophila littoralis and compared it with those of Drosophila virilis and D. melanogaster. All portions of nonA appeared to be conserved between D. littoralis and D. virilis, while the 5' region of the gene of these two species showed high divergence from that of a more distantly-related species, D. melanogaster. The same was true for the glycine repeat regions. No significant deviation from neutrality was observed in the analysis of intraspecific nucleotide variation in 5' or 3' region of the nonA gene in D. littoralis population. Also, comparison of D. littoralis sequences with homologous sequence of D. virilis suggests that the gene is evolving neutrally in D. virilis group. Divergence of the 5' regions between D. virilis group species and D. melanogaster could be a result of positive selection, but this finding is obscured by the long divergence time of the species groups.

The nonA gene in Drosophila conveys species-specific behavioral characteristics

The molecular basis of species-specific differences in courtship behavior, a critical factor in preserving species boundaries, is poorly understood. Genetic analysis of all but the most closely related species is usually impossible, given the inviability of hybrids. We have therefore applied interspecific transformation of a single candidate behavioral locus, no-on-transient A (nonA), between Drosophila virilis and D. melanogaster, to investigate whether nonA, like the period gene, might encode species-specific behavioral information. Mutations in nonA can disrupt both visual behavior and the courtship song in D. melanogaster. The lovesong of nonA(diss) mutant males superficially resembles that of D. virilis, a species that diverged from D. melanogaster 40-60 mya. Transformation of the cloned D. virilis nonA gene into D. melanogaster hosts carrying a synthetic deletion of the nonA locus restored normal visual function (the phenotype most sensitive to nonA mutation). However, the courtship song of transformant males showed several features characteristic of the corresponding D. virilis signal, indicating that nonA can act as a reservoir for species-specific information. This candidate gene approach, together with interspecific transformation, can therefore provide a direct avenue to explore potential speciation genes in genetically and molecularly tractable organisms such as Drosophila.

Molecular characterization of Ct-hrp65: identification of two novel isoforms originated by alternative splicing

Hrp65, a protein with two conserved RNA-binding domains, has been identified in Chironomus tentans as a component of nuclear fibers associated with ribonucleoprotein particles in transit from the gene to the nuclear pore. We have cloned two novel hrp65 isoforms and characterized the structure of the hrp65 gene. Comparison of the hrp65 gene to the hrp65 cDNAs revealed that the multiple hrp65 isoforms, hrp65-1, hrp65-2 and hrp65-3, are generated by alternative splicing of a single pre-mRNA. The hrp65-3 mRNA is only detected in C. tentans tissue culture cells of embryonic origin, whereas hrp65-1 and hrp65-2 mRNAs appear to be constitutively expressed. The hrp65 mRNAs are generated by differential 3' splice site selection at the last exon of the gene. Thus, the three hrp65 transcripts contain different 3' UTRs and encode proteins that vary in their C-terminal ends. Interestingly, the variant C-terminal region determines the subcellular localization of the hrp65 proteins. In transient transfection assays, hrp65-1 is efficiently targetted to the nucleus, whereas hrp65-2 and hrp65-3 localize mainly to the cytoplasm. Moreover, hrp65-3 is associated with cytoplasmic actin fibers. All together, our findings suggest that the different hrp65 isoforms serve specialized roles related to mRNA localization/transport in the different cell compartments.

Comparative analysis of the nonA region in Drosophila identifies a highly diverged 5' gene that may constrain nonA promoter evolution

A genomic fragment from Drosophila virilis that contained all the no-on-transientA (nonA) coding information, plus several kilobases of upstream material, was identified. Comparisons of nonA sequences and the gene nonA-like in D. melanogaster, a processed duplication of nonA, suggest that it arose before the split between D. melanogaster and D. virilis. In both species, another gene that lies <350 bp upstream from the nonA transcription starts, and that probably corresponds to the lethal gene l(1)i19, was identified. This gene encodes a protein that shows similarities to GPI1, which is required for the biosynthesis of glycosylphosphatidylinositol (GPI), a component for anchoring eukaryotic proteins to membranes, and so we have named it dGpi1. The molecular evolution of nonA and dGpi1 sequences show remarkable differences, with the latter revealing a level of amino acid divergence that is as high as that of transformer and with extremely low levels of codon bias. Nevertheless, in D. melanogaster hosts, the D. virilis fragment rescues the lethality associated with a mutation of l(1)i19e, as well as the viability and visual defects produced by deletion of nonA(-). The presence of dGpi1 sequences so close to nonA appears to have constrained the evolution of the nonA promoter.

Molecular Dissection of the 5' Region of no-on-transientA of Drosophila melanogaster Reveals cis-Regulation by Adjacent dGpi1 Sequences

The nonA gene of Drosophila melanogaster is important for normal vision, courtship song, and viability and lies approximately 350 bp downstream of the dGpi1 gene. Full rescue of nonA mutant phenotypes can be achieved by transformation with a genomic clone that carries approximately 2 kb of 5' regulatory material and that encodes most of the coding sequence of dGpi1. We have analyzed this 5' region by making a series of deleted fragments, fusing them to yeast GAL4 sequences, and driving UAS-nonA expression in a mutant nonA background. Regions that both silence and enhance developmental tissue-specific expression of nonA and that are necessary for generating optomotor visual responses are identified. Some of these overlap the dGpi1 sequences, revealing cis-regulation by neighboring gene sequences. The largest 5' fragment was unable to rescue the normal electroretinogram (ERG) consistently, and no rescue at all was observed for the courtship song phenotype. We suggest that sequences within the nonA introns that were missing in the UAS-nonA cDNA may carry enhancer elements for these two phenotypes. Finally, we speculate on the striking observation that some of the cis-regulatory regions of nonA appear to be embedded within the coding regions of dGpi1.

The protein Hrb57A of Drosophila melanogaster closely related to hnRNP K from vertebrates is present at sites active in transcription and coprecipitates with four RNA-binding proteins

The hnRNP K protein is among the major hnRNA-binding proteins with a strong preference for cytidine-rich sequences. We have cloned a Drosophila hnRNP protein closely related to this vertebrate protein. The protein first identified by the monoclonal antibody Q18 is encoded by a gene located in 57A on polytene chromosomes and has been consequently named Hrb57A. The amino acid sequence of the Hrb57A KH domains and their overall organisation in the protein are remarkably similar to the vertebrate proteins. As the hnRNP K in vertebrates the M(r) 55 000 Drosophila Hrb57A/Q18 protein strongly binds to poly(C) in vitro and is ubiquitously present in nuclei active in transcription. On polytene chromosomes it is found in many puffs and minipuffs. Hrb57A/Q18 specifically coprecipitates four other proteins: Hrb87F/P11 a Drosophila hnRNP A1 homologue, the hnRNA-binding protein S5, the RNA recognition motif-containing protein NonA and the RNA-binding zinc finger-containing protein on ecdysone puffs PEP/X4.

Electron tomography reveals posttranscriptional binding of pre-mRNPs to specific fibers in the nucleoplasm

Using electron tomography, we have analyzed whether the Balbiani ring (BR) pre-mRNP particles in transit from the gene to the nuclear pore complex (NPC) are bound to any structure that could impair free diffusion through the nucleoplasm. We show that one-third of the BR particles are in contact with thin connecting fibers (CFs), which in some cases merge into large fibrogranular clusters. The CFs have a specific protein composition different from that of BR particles, as shown by immuno-EM. Moreover, we have identified hrp65 as one of the protein components of the CFs. The sequencing of hrp65 cDNA reveals similarities with hnRNP proteins and splicing factors. However, hrp65 is likely to have a different function because it does not bind to nascent pre-mRNA and is not part of the pre-mRNP itself. Taken together, our observations indicate that pre-mRNPs are not always freely diffusible in the nucleoplasm but interact with fibers of specific structure and composition, which implies that some of the posttranscriptional events that the pre-mRNPs undergo before reaching the NPC occur in a bound state.

The RRM protein NonA from Drosophila forms a complex with the RRM proteins Hrb87F and S5 and the Zn finger protein PEP on hnRNA

The RRM protein NonA, an ubiquitous nuclear protein present in puffs on polytene chromosomes, has been immunopurified as a RNA-protein complex from Drosophila Kc cells. Three other proteins present in the complex have been identified: X4/PEP (protein on ecdysone puffs), a 100-kDa zinc finger RNA-binding protein; the 70-kDa S5 protein, an as yet uncharacterized RNA-binding protein; and P11/Hrb87F, a 38-kDa RRM protein homologous to hnRNP protein A1 from mammals. Monoclonal antibodies against any of the protein components coprecipitate all four proteins although at different ratios. NonA does not coprecipitate with the hrp40 hnRNP proteins and immunolocalizes in a pattern distinct of major hnRNP proteins. Like NonA, X4/PEP, S5, and P11/Hrb87F are present on active sites on polytene chromosomes. The precipitated NonA complex is enriched for certain protein encoding RNAs, notably, histone H3 and H4 RNA.

Prolactin induces expression of FGF-2 and a novel FGF-responsive NonO/p54nrb-related mRNA in rat lymphoma cells

The rat Nb2-11C lymphoma cell line expresses high affinity prolactin (PRL) receptors, and requires lactogenic hormones for survival and proliferation. We have applied differential display to identify genes which are differentially induced in Nb2-11C cells following PRL stimulation, or which are constitutively expressed in the PRL-independent Nb2-Sp cells. In the present study we characterized a clone (22c.2) which was expressed in Nb2-Sp cells, and in Nb2-11C cells given PRL for 3 h but not in untreated cells. The 279 bp cDNA had 95% homology with the 3' end of the murine 2.6 kb FGF-inducible gene 14 (FIN14). When clone 22c.2 was used to screen a Nb2-Sp cDNA library to obtain a longer cDNA, a unique 1039 bp clone PNR (Prolactin-responsive/ NonO-Related) was isolated, subcloned and sequenced. The deduced amino acid sequence encoded by the PNR open reading frame had significant homology with a family of RNA- and DNA-binding proteins which include the human polypyrimidine tract-binding protein (PTB)-associated splicing factor (PSF), the murine non-POU-domain-containing octamer-binding protein (NonO) and the human NonO homologue p54nrb. Nb2-11C cells expressed three PNR-related mRNA transcripts of 2.5, 3.0 and > 10 kb. Expression of the 2.5 and 3.0 kb transcripts were increased at least 4-fold within 3 h of PRL treatment. PNR expression was also significantly stimulated within 3 h by addition of FGF-2 to either Nb2-11C or Nb2-Sp cells, although alone FGF-2 was not mitogenic for either cell line. Reverse transcription-polymerase chain reaction (RT-PCR) confirmed the expression of both FGF-2 and FGF receptor mRNA in Nb2 cells. raising the possibility of an autocrine or paracrine function for FGF-2 in lymphoma cells. Furthermore, PRL rapidly stimulated the expression of FGF-2 mRNA in a time- and dose-dependent manner in both Nb2-11C and Nb2-Sp cells. FGF-2 expression was increased within 1 h and was maintained at a high level for at least 10 h following treatment with 2 ng/ml PRL. Western blotting with anti-FGF2 antisera demonstrated PRL stimulation of intracellular accumulation, but not secretion of immunoreactive FGF-2. The observation of PRL-responsive expression of FGF-2 in Nb2 cells suggests a previously unrecognized pathway for PRL action in lymphoid cells.

NonO enhances the association of many DNA-binding proteins to their targets

NonO is an unusual nucleic acid binding protein not only in that it binds both DNA and RNA but that it does so via functionally separable domains. Here we document that NonO enhances the binding of some (E47, OTF-1 and OTF-2) but not all (PEA3) conventional sequence-specific transcription factors to their recognition sites in artificial substrates as well as in an immunoglobulin VHpromoter. We also show that NonO induces the binding of the Ku complex to DNA ends. Ku has no known DNA sequence specificity. These enhancement of binding effects are NonO concentration dependent. Using the E box activity of E47 as a model, kinetic studies demonstrate that the association rate of the protein-DNA complex increases in the presence of NonO while the dissociation rate remains the same, thereby increasing the sum total of the interaction. Oligo competition experiments indicate that NonO does not contact the target DNA in order to enhance the binding activity of DNA binding proteins. Rather, methylation interference analysis reveals that the induced E47 binding-activity has the same DNA-binding sequence specificity as the normal binding. This result suggests that one of the effects of NonO is to induce a true protein-DNA interaction. In this way, it might be possible for NonO to play a crucial role in gene regulation.

Genetic dissection of sexual behavior in Drosophila melanogaster

Mating of Drosophila melanogaster is a sterotypically patterned behavior consisting of a fixed sequence of actions that are primarily under genetic control. Mutations that disrupt specific aspects of mating activities offer a starting point for exploring the molecular machineries underlying sexual behavior. Several genes, identified as causing aberrant sexual behavior when mutated, have been isolated and cloned, providing molecular probes for expression and mosaic analyses that can be used in specifying the cells responsible for the behavior. This review presents current understandings of mating behavior obtained by such molecular and cellular approaches and provides an overview of future directions of research in behavioral genetics.

Temporally manipulated rescue of visual and courtship abnormalities caused by a nonA mutation in Drosophila

The no-on-transient-A gene was identified by independently isolated mutations causing visual-response abnormalities or courtship song defects. The nonA(diss) mutant is abnormal for all of these phenotypes. The pleiotropic effects of this dissonance allele dovetail with the widespread tissue expression of the nonA products. This gene, which encodes a putative RNA-binding protein, is expressed at essentially all stages of the life cycle. To determine whether the behavioral and physiological abnormalities exhibited by nonA mutant adults have a developmental etiology or are the result of an impaired mature nervous (or perhaps neuro-muscular) system, a conditional form of this gene was constructed. Animals from the resulting hsp-nonA (cDNA) transgenic strain were subjected to heat-shock regimes such that the gene's coding sequences were activated during development only, solely in the imaginal stage, or both. Surprisingly, expression during any of these time periods effected rescue of the visual-response and the courtship-song defects.

Cabeza, a Drosophila gene encoding a novel RNA binding protein, shares homology with EWS and TLS, two genes involved in human sarcoma formation

We have previously described a partial Drosophila cDNA, clone P19, which bears homology to members of the RNA recognition motif (RRM) family of proteins [Haynes et al. (1987) Proc. Natl. Acad. Sci. USA, 84, 1819-1823]. RNA binding as well as involvement in RNA processing has been demonstrated for some RRM proteins. We report here the further characterization of P19, which we renamed cabeza (caz). caz is located on the X chromosome at position 14B. Using Northern analysis, at least four transcripts from the caz gene were observed at varying levels during development. caz mRNA and protein are enriched in the brain and central nervous system during embryogenesis. In addition, the protein is enriched in the adult head. UV crosslinking was used to demonstrate in vitro RNA binding activity for full-length recombinant caz protein and for the caz RRM domain. Sequence analysis revealed caz is related to two human genes, EWS and TLS, which are involved in chromosomal translocations. The fusion of EWS and TLS to other cellular genes results in sarcoma formation. In addition to their overall structural organization and sequence similarity, these three genes share an RRM which is divergent from typical RRMs. Therefore, it appears that these genes constitute a new sub-family of RNA binding proteins.

Multiple RNA binding domains (RBDs) just don't add up

One of the most common motifs for binding RNA in eukaryotes is the RNA binding domain (RBD) or RNA Recognition Motif (RRM). One of the more intriguing aspects of these proteins is their modular nature. Proteins have been found containing from one to four RRMs. In most instances, these domains have some basal level of non-sequence specific RNA binding affinity. In addition, many also have a higher affinity for a specific structure or sequence of RNA. In the cases of heterogenous nuclear ribonucleoprotein A1 (hnRNP A1), yeast poly-A binding protein and splicing factor U2AF65, the individual free energy of binding of the RBDs for RNA are not strictly additive. By invoking a model in which the amino acids connecting adjoining RBDs are considered to be flexible linkers with an interresidue spacing of about 3.5 A, it is possible to predict the apparent association constants for at least some multi-RBD proteins to single-stranded RNA. We have surveyed the literature and found that individual RBDs are separated by 'linker' sequences of highly variable length. These linkers provide a critical determinant of binding affinity and may modulate cis versus trans binding. A clearer understanding of multi-RBD binding is essential to critically evaluating the role of these proteins in RNA splicing, packaging and transport.

The mating of a fly

Courtship in Drosophila is influenced by a wide variety of genes, in that many different kinds of pleiotropic mutations lead to defective courtship. This may seem to be a truism, but the broad temporal and spatial expression of most of the fly's "neuro genes" makes it difficult to exclude elements of such genes' actions as materially underlying reproductive behavior. "Courtship genes" that seem to play more particular roles were originally identified as sensory, learning, or rhythm mutations; their reproductive abnormalities have been especially informative for revealing components of male or female actions that might otherwise have gone unnoticed. Further behavioral mutations seemed originally to be courtship-specific, turned out not to have that property, and have led to a broadened perspective on the nature and action of Drosophila's sex-determination genes.

Essential role for a heterogeneous nuclear ribonucleoprotein (hnRNP) in oogenesis: hrp40 is absent from the germ line in the dorsoventral mutant squid

The Drosophila melanogaster hrp40 proteins are abundant nuclear pre-mRNA-binding proteins that are similar to the heterogeneous nuclear ribonucleoprotein (hnRNP) A/B proteins of vertebrates. Recently, hrp40 has been shown to be encoded by the squid gene, which is required for dorsoventral axis formation during oogenesis. Eggs and embryos from homozygous squid mothers are severely dorsalized, and complete deletion of the squid gene results in lethality. Here we have examined the expression and localization of hrp40 in wild-type and squid mutant ovaries. Using a monoclonal antibody specific for hrp40, the same isoforms of hrp40 are detected in both wild-type and squid ovaries, but the amount of hrp40 is reduced in squid ovaries. Furthermore, immunolocalization of hrp40 in wild-type egg chambers shows that hrp40 is present in the nurse cells, oocyte, and follicle cells. In contrast, in squid mutant egg chambers, hrp40 is absent from the germ-line-derived nurse cells and oocyte, but it is detected in the somatic follicle cells. The absence of hrp40 from the germ-line-derived cells of developing egg chambers is likely to lead to the striking dorsalized phenotype of squid eggs. In addition, dramatic stage-specific changes in the cellular localization of hrp40 are seen; the protein found in the nurse cell nuclei during early stages of oogenesis migrates to the cytoplasm at later stages. These findings reveal dynamic patterns of expression and localization of hnRNP proteins during development and provide evidence for an essential role for hnRNP proteins.

Behavioral and neurobiological implications of sex-determining factors in Drosophila

The function of the central nervous system as it controls sex-specific behaviors in Drosophila has been studied with renewed intensity, in the context of genetic factors that influence the development of sexually differentiated aspects of this insect. Three categories of genetic variations that cause anomalies in courtship and mating behaviors are discussed: (1) mutants isolated with regard to courtship defects, of which putatively courtship-specific variants such as the fruitless mutant are a subset; (2) general behavioral and neurological variants (including sensory and learning mutants), whose defects include subnormal reproductive performance; and (3) mutations of genes within the sex-determination regulatory hierarchy of Drosophila, the analysis of which has included studies of reproductive behavior. Recent studies of mutations in two of these categories have provided new insights into the control of neuronally based aspects of sex-specific behavior. The doublesex gene, the final factor acting in the sex-determination hierarchy, had been previously thought to regulate all aspects of sexual differentiation. Yet, it has been recently shown that doublesex does not control at least one neuronally-determined feature of sex-specific anatomy--a muscle in the male's abdomen, whose normal development is, however, dependent on the action of fruitless. These considerations prompted us to examine further (and in some cases re-examine) the influences exerted by sex-determination hierarchy genes on behavior. Our results--notably those obtained from assessments of doublesex mutations' effects on general reproductive actions and on a particular component of the courtship sequence (male "singing" behavior)--lead to the suggestion that there is a previously unrecognized branch within the sex-determination hierarchy, which controls the differentiation of the male- and female- specific phenotypes of Drosophila. This new branch separates from the doublesex-related one immediately before the action of that gene (just after transformer and transformer-2) and appears to control as least some aspects of neuronally determined sexual differentiation of males.

NonO, a non-POU-domain-containing, octamer-binding protein, is the mammalian homolog of Drosophila nonAdiss

We have cloned the ubiquitous form of an octamer-binding, 60-kDa protein (NonO) that appears to be the mammalian equivalent of the Drosophila visual and courtship song behavior protein, no-on-transient A/dissonance (nonAdiss). A region unprecedently rich in aromatic amino acids containing two ribonuclear protein binding motifs is highly conserved between the two proteins. A ubiquitous form of NonO is present in all adult tissues, whereas lymphocytes and retina express unique forms of NonO mRNA. The ubiquitous form contains a potential helix-turn-helix motif followed by a highly charged region but differs from prototypic octamer-binding factors by lacking the POU DNA-binding domain. In addition to its conventional octamer duplex-binding, NonO binds single-stranded DNA and RNA at a site independent of the duplex site.

NonO, a non-POU-domain-containing, octamer-binding protein, is the mammalian homolog of Drosophila nonAdiss

We have cloned the ubiquitous form of an octamer-binding, 60-kDa protein (NonO) that appears to be the mammalian equivalent of the Drosophila visual and courtship song behavior protein, no-on-transient A/dissonance (nonAdiss). A region unprecedently rich in aromatic amino acids containing two ribonuclear protein binding motifs is highly conserved between the two proteins. A ubiquitous form of NonO is present in all adult tissues, whereas lymphocytes and retina express unique forms of NonO mRNA. The ubiquitous form contains a potential helix-turn-helix motif followed by a highly charged region but differs from prototypic octamer-binding factors by lacking the POU DNA-binding domain. In addition to its conventional octamer duplex-binding, NonO binds single-stranded DNA and RNA at a site independent of the duplex site.

In vivo observation of the puff-specific protein no-on transient A (NONA) in nuclei of Drosophila embryos

The spatial distribution of no-on transient A (NONA), a protein associated with specific puffs on polytene chromosomes, was followed in nuclei of living Drosophila embryos by microinjection of fluorescently labeled monoclonal antibody to NONA. The injected antibodies remained active until the larval stage, revealing the distribution of the NONA protein throughout embryogenesis. Most injected animals completed embryonic development and hatched as normal larvae. NONA was restricted to the cytoplasm until the end of cycle 11. We document an active uptake of the NONA-antibody complex into early interphase nuclei from nuclear cycle 14 onwards, following each mitosis. Significant differences in the distribution of the protein between fixed and living embryos were apparent, particularly at high resolution. The NONA protein was localized in the nuclei of living embryos at discrete sites, most of which lay at the periphery and some of which were tightly clustered. The constellation of sites changed with time; in some nuclei these changes were fast whereas in other nuclei the pattern was quite stable. These data suggest that specific protein complexes associated with active interphase chromatin, and possibly chromatin in general, are mobile in the living organism.

Purification and cDNA cloning of HeLa cell p54nrb, a nuclear protein with two RNA recognition motifs and extensive homology to human splicing factor PSF and Drosophila NONA/BJ6

While searching for a human homolog of the S.cerevisiae splicing factor PRP18, we found a polypeptide that reacted strongly with antibodies against PRP18. We purified this polypeptide from HeLa cells using a Western blot assay, and named it p54nrb (for nuclear RNA-binding protein, 54 kDa). cDNAs encoding p54nrb were cloned with probes derived from partial sequence of the purified protein. These cDNAs have identical coding sequences but differ as a result of alternative splicing in the 5' untranslated region. The cDNAs encode a 471 aa polypeptide that contains two RNA recognition motifs (RRMs). Human p54nrb has no homology to yeast PRP18, except for a common epitope, but is instead 71% identical to human splicing factor PSF within a 320 aa region that includes both RRMs. In addition, both p54nrb and PSF are rich in Pro and Gln residues outside the main homology region. The Drosophila puff-specific protein BJ6, one of three products encoded by the alternatively spliced no-on-transient A gene (nonA), which is required for normal vision and courtship song, is 42% identical to p54nrb in the same 320 aa region. The striking homology between p54nrb, PSF, and NONA/BJ6 defines a novel phylogenetically conserved protein segment, termed DBHS domain (for Drosophila behavior, human splicing), which may be involved in regulating diverse pathways at the level of pre-mRNA splicing.

Hel-N1: an autoimmune RNA-binding protein with specificity for 3' uridylate-rich untranslated regions of growth factor mRNAs

We have investigated the RNA binding specificity of Hel-N1, a human neuron-specific RNA-binding protein, which contains three RNA recognition motifs. Hel-N1 is a human homolog of Drosophila melanogaster elav, which plays a vital role in the development of neurons. A random RNA selection procedure revealed that Hel-N1 prefers to bind RNAs containing short stretches of uridylates similar to those found in the 3' untranslated regions (3' UTRs) of oncoprotein and cytokine mRNAs such as c-myc, c-fos, and granulocyte macrophage colony-stimulating factor. Direct binding studies demonstrated that Hel-N1 bound and formed multimers with c-myc 3' UTR mRNA and required, as a minimum, a specific 29-nucleotide stretch containing AUUUG, AUUUA, and GUUUUU. Deletion analysis demonstrated that a fragment of Hel-N1 containing 87 amino acids, encompassing the third RNA recognition motif, forms an RNA binding domain for the c-myc 3' UTR. In addition, Hel-N1 was shown to be reactive with autoantibodies from patients with paraneoplastic encephalomyelitis both before and after binding to c-myc mRNA.

Hel-N1: an autoimmune RNA-binding protein with specificity for 3' uridylate-rich untranslated regions of growth factor mRNAs

We have investigated the RNA binding specificity of Hel-N1, a human neuron-specific RNA-binding protein, which contains three RNA recognition motifs. Hel-N1 is a human homolog of Drosophila melanogaster elav, which plays a vital role in the development of neurons. A random RNA selection procedure revealed that Hel-N1 prefers to bind RNAs containing short stretches of uridylates similar to those found in the 3' untranslated regions (3' UTRs) of oncoprotein and cytokine mRNAs such as c-myc, c-fos, and granulocyte macrophage colony-stimulating factor. Direct binding studies demonstrated that Hel-N1 bound and formed multimers with c-myc 3' UTR mRNA and required, as a minimum, a specific 29-nucleotide stretch containing AUUUG, AUUUA, and GUUUUU. Deletion analysis demonstrated that a fragment of Hel-N1 containing 87 amino acids, encompassing the third RNA recognition motif, forms an RNA binding domain for the c-myc 3' UTR. In addition, Hel-N1 was shown to be reactive with autoantibodies from patients with paraneoplastic encephalomyelitis both before and after binding to c-myc mRNA.

The Rb97D gene encodes a potential RNA-binding protein required for spermatogenesis in Drosophila

Many proteins that bind RNA contain a common RNA-binding domain, the RNP motif. We have been studying two Drosophila RNP motif proteins, Hrb98DE and Hrb87F, which are hnRNA-binding proteins. We report here the characterization of the Rb97D gene, which encodes a protein that is closely related to the Hrb proteins in the RNP motif domain, but has a distinctive proline-rich C-terminal domain. The gene is located at 97D on the right arm of the third chromosome, near the rough gene. Multiple transcripts from the Rb97D gene are present at varying levels throughout development. The transcripts are generated by alternative processing in the coding and 3' untranslated regions, and can encode two protein isoforms. Analysis of a mutant containing a P element inserted into the 5' untranslated region of the gene demonstrates that Rb97D is required for male fertility. Possible models for the function of Rb97D in testes are discussed.

An extended view of nuclear lamin structure, function, and dynamics

Molecularly-based studies of nuclear lamins have progressed at a rapid rate in the last decade. However, we still have no answer to the most important question: what are the functions of lamins? In this review we describe recent experiments which challenge traditional views of lamin function and structure. These surprising results indicate that much lamin functionality remains to be discovered, and that more global approaches to lamin structure and function are especially appropriate at this time.

The Drosophila nuclear protein Bx42, which is found in many puffs on polytene chromosomes, is highly charged

The Drosophila nuclear protein Bx42 is present in a set of transcriptionally active puffs on polytene chromosomes. cDNA clones coding for this protein were isolated from a lambda gt11 expression library. The two Bx42 transcripts are ubiquitously expressed and are already detectable in early stages of development. The corresponding genomic region, in 8C7-8, was isolated and sequenced. Both transcripts direct the production of the same basic, highly charged 547 amino acid protein with a calculated 61.1 kDa molecular weight.

Physical mapping of the Esterase-6 locus of Drosophila melanogaster

The Esterase-6 gene locus of Drosophila melanogaster although well-characterized, has not been definitely mapped by in situ hybridization. In this paper, a high resolution in situ hybridization protocol using an avidin/biotinylated-horseradish peroxidase/diaminobenzidine system was adopted to refine the physical map position of the Esterase-6 locus. Clarity of signal, detail of banding pattern and absence of background allowed the assignment of a 1.8 kb cDNA encoding Esterase-6 to three bands within subsections 69 A1-A3 on the left arm of polytene chromosome 3. These data refine earlier deletion mapping and low resolution in situ hybridization results, which assigned Esterase-6 to 69A1-A5. The potential use of this high resolution in situ hybridization technique in the analysis of the physical organization of the Esterase-6 gene duplication and surrounding region is discussed.