Cytoplasmic retention of Xenopus nuclear factor 7 before the mid blastula transition uses a unique anchoring mechanism involving a retention domain and several phosphorylation sites

Control of zygotic genome activation in Xenopus

The fertilized frog egg contains all the materials needed to initiate development of a new organism, including stored RNAs and proteins deposited during oogenesis, thus the earliest stages of development do not require transcription. The onset of transcription from the zygotic genome marks the first genetic switch activating the gene regulatory network that programs embryonic development. Zygotic genome activation occurs after an initial phase of transcriptional quiescence that continues until the midblastula stage, a period called the midblastula transition, which was first identified in Xenopus. Activation of transcription is programmed by maternally supplied factors and is regulated at multiple levels. A similar switch exists in most animals and is of great interest both to developmental biologists and to those interested in understanding nuclear reprogramming. Here we review in detail our knowledge on this major switch in transcription in Xenopus and place recent discoveries in the context of a decades old problem.

Nuclear import and DNA binding of the ZHD5 transcription factor is modulated by a competitive peptide inhibitor in Arabidopsis

Competitive inhibition of transcription factors by small proteins is an intriguing component of gene regulatory networks in both animals and plants. The small interfering proteins possess limited sequence homologies to specific transcription factors but lack one or more protein motifs required for transcription factor activities. They interfere with the activities of transcription factors, such as DNA binding and transcriptional activation, by forming nonfunctional heterodimers. A potential example is the Arabidopsis MIF1 (mini zinc finger 1) protein consisting of 101 residues. It has a zinc finger domain but lacks other protein motifs normally present in transcription factors. In this work, we show that MIF1 and its functional homologues physically interact with a group of zinc finger homeodomain (ZHD) transcription factors, such as ZHD5, that regulate floral architecture and leaf development. Gel mobility shift assays revealed that MIF1 blocks the DNA binding activity of ZHD5 homodimers by competitively forming MIF1-ZHD5 heterodimers. Accordingly, the transcriptional activation activity of ZHD5 was significantly suppressed by MIF1 coexpressed transiently in Arabidopsis protoplasts. Notably, MIF1 also prevents ZHD5 from nuclear localization. Although ZHD5 was localized exclusively in the nucleus, it was scattered throughout the cytoplasm when MIF1 was coexpressed. Transgenic plants overexpressing the ZHD5 gene (35S:ZHD5) exhibited accelerated growth with larger leaves. Consistent with the negative regulation of ZHD5 by MIF1, the 35S:ZHD5 phenotypes were diminished by MIF1 coexpression. These observations indicate that MIF1 regulates the ZHD5 activities in a dual step manner: nuclear import and DNA binding.

Non-random subcellular distribution of variant EKLF in erythroid cells

EKLF protein plays a prominent role during erythroid development as a nuclear transcription factor. Not surprisingly, exogenous EKLF quickly localizes to the nucleus. However, using two different assays we have unexpectedly found that a substantial proportion of endogenous EKLF resides in the cytoplasm at steady state in all erythroid cells examined. While EKLF localization does not appear to change during either erythroid development or terminal differentiation, we find that the protein displays subtle yet distinct biochemical and functional differences depending on which subcellular compartment it is isolated from, with PEST sequences possibly playing a role in these differences. Localization is unaffected by inhibition of CRM1 activity and the two populations are not differentiated by stability. Heterokaryon assays demonstrate that EKLF is able to shuttle out of the nucleus although its nuclear re-entry is rapid. These studies suggest there is an unexplored role for EKLF in the cytoplasm that is separate from its well-characterized nuclear function.

Multiple pathways regulate intracellular shuttling of MoKA, a co-activator of transcription factor KLF7

MoKA is a novel F-box containing protein that interacts with and stimulates the activity of transcription factor KLF7, a regulator of neuronal differentiation. MoKA accumulates throughout the cell and predominantly in the cytosol, consistent with the presence of several putative nuclear localization and export signals (NLSs and NESs). The present study was designed to refine the identity and location of the sequences responsible for MoKA intracellular shuttling and transcriptional activity. Forced expression of fusion proteins in mammalian cells demonstrated that only one of three putative NLSs potentially recognized by karyopherin receptors is involved in nuclear localization of MoKA. By contrast, three distinct sequences were found to participate in mediating cytoplasmic accumulation. One of them is structurally and functionally related to the leucine-rich export signal that interacts with the exportin 1 (CRM1) receptor. The other two export signals instead display either a novel leucine-rich sequence or an undefined peptide motif, and both appear to act through CRM1-independent pathways. Finally, transcriptional analyses using the chimeric GAL4 system mapped the major activation domain of MoKA to a highly acidic sequence that resides between the NLS and NES clusters.

Xnf7 contributes to spindle integrity through its microtubule-bundling activity

Regulation of microtubule dynamics and organization in mitosis by a number of microtubule-associated proteins (MAPs) is required for proper bipolar spindle assembly, yet the precise mechanisms by which many MAPs function are poorly understood. One interesting class of MAPs is known to localize to the nucleus during interphase yet fulfill important spindle functions during mitosis. We have identified Xenopus nuclear factor 7 (Xnf7), a developmental regulator of dorsal-ventral patterning, as a microtubule-binding protein that also associates with the nuclear import receptor importin alpha/beta. Xnf7 localized to interphase nuclei and metaphase spindles both in Xenopus egg extracts and cultured cells. Xnf7-depleted spindles were hypersensitive to microtubule-depolymerizing agents. Functional characterization of Xnf7 revealed that it binds directly to microtubules, exhibits RING-finger-dependent E3-ubiquitin-ligase activity, and has C-terminal-dependent microtubule-bundling activity. The minimal microtubule-bundling domain of Xnf7 was sufficient to rescue the spindle-hypersensitivity phenotype. Thus, we have identified Xnf7 as a nuclear MAP whose microtubule-bundling activity, but not E3-ligase activity, contributes to microtubule organization and spindle integrity. Characterization of the multiple activities of Xnf7 may have implications for understanding human diseases caused by mutations in related proteins.

Developmentally regulated cytoplasmic retention of the transcription factor XMI-ER1 requires sequence in the acidic activation domain

Xmi-er1 is a fibroblast growth factor regulated immediate-early gene that is activated during mesoderm induction in Xenopus embryonic explants. This gene encodes a nuclear protein with potent transcriptional regulator activity and overexpression of XMI-ER1 in Xenopus embryos inhibits mesoderm induction and leads to truncations along the anteroposterior axis. We showed previously that XMI-ER1 is retained in the cytoplasm during cleavage stages and only begins to appear in the nucleus at mid-blastula. Such developmentally regulated nuclear translocation may represent an important mechanism for regulating XMI-ER1 activity in the early embryo. Here, we investigate different mechanisms that might control nuclear translocation of XMI-ER1. Using alpha-amanitin to inhibit transcription, we show that nuclear localization is not dependent on zygotic transcription. Nor is it the result of a developmentally regulated import pathway, as the XMI-ER1 nuclear localization signal (NLS) fused to beta-galactosidase (betagal) was able to direct nuclear translocation prior to mid-blastula. Fusion of an additional, heterologous NLS to the N-terminus of XMI-ER1 was not sufficient to overcome cytoplasmic retention, indicating that retention does not involve NLS masking, but rather binding to a cytoplasmic anchor. The anchoring molecule is not an RNA, as microinjection of RNase A did not affect the timing of nuclear translocation. Western blot analysis using antibodies that recognize phosphorylated residues revealed that, while XMI-ER1 is not itself phosphorylated, it is associated with two differentially phosphorylated proteins, suggesting that the anchoring mechanism may involve interaction with a cytoplasmic protein(s). A series of XMI-ER1 deletion mutants was utilized to map the putative retention domain. Our analysis revealed that amino acids 144-175, containing the fourth acidic stretch of the acidic activation domain, are required for retention. These results suggest that XMI-ER1 is retained in the cytoplasm of the early embryo by interaction of the region containing amino acids 144-175 with a cytoplasmic anchor.

Death-associated protein kinase phosphorylates ZIP kinase, forming a unique kinase hierarchy to activate its cell death functions

The death-associated protein (DAP) kinase family includes three protein kinases, DAP kinase, DAP kinase-related protein 1, and ZIP kinase, which display 80% amino acid identity within their catalytic domains and are functionally linked to common subcellular changes occurring during cell death, such as the process of membrane blebbing. Here we show physical and functional cross talk between DAP kinase and ZIP kinase. The two kinases display strong synergistic effects on cell death when coexpressed and physically bind each other via their catalytic domains. Furthermore, DAP kinase phosphorylates ZIP kinase at six specific sites within its extracatalytic C-terminal domain. ZIP kinase localizes to both the nucleus and the cytoplasm and fractionates as monomeric and trimeric forms. Significantly, modification of the DAP kinase phosphorylation sites influences both the localization and oligomerization status of ZIP kinase. A mutant ZIP kinase construct, in which the six serine/threonine residues were mutated to aspartic acid to mimic the phosphorylated state, was found predominantly in the cytoplasm as a trimer and possessed greater cell death-inducing potency. This suggests that DAP kinase and ZIP kinase function in a biochemical pathway in which DAP kinase activates the cellular function of ZIP kinase through phosphorylation, leading to amplification of death-promoting signals.

Advantages and Disadvantages of Cytidine Deamination

Cytidine deamination of nucleic acids underlies diversification of Ig genes and inhibition of retroviral infection, and thus, it would appear to be vital to host defense. The host defense properties of cytidine deamination require two distinct but homologous cytidine deaminases-activation-induced cytidine deaminase and apolipoprotein B-editing cytidine deaminase, subunit 3G. Although cytidine deamination has clear benefits, it might well have biological costs. Uncontrolled cytidine deamination might generate misfolded polypeptides, dominant-negative proteins, or mutations in tumor suppressor genes, and thus contribute to tumor formation. How cytidine deaminases target a given nucleic acid substrate at specific sequences is not understood, and what protects cells from uncontrolled mutagenesis is not known. In this paper, I shall review the functions and regulation of activation-induced cytidine deaminase and apolipoprotein B-editing cytidine deaminase, subunit 3G, and speculate about the basis for site specificity vis-à-vis generalized mutagenesis.

Cloning and characterization of a novel RING-B-box-coiled-coil protein with apoptotic function

We have identified a novel RING-B-box-coiled-coil (RBCC) protein (MAIR for macrophage-derived apoptosis-inducing RBCC protein) that consists of an N-terminal RING finger, followed by a B-box zinc finger, a coiled-coil domain, and a B30.2 domain. MAIR mRNA was expressed widely in mouse tissues and was induced by macrophage colony-stimulating factor in murine peritoneal and bone marrow macrophages. MAIR protein initially showed a granular distribution predominantly in the cytoplasm. The addition of zinc to transfectants containing MAIR cDNA as part of a heavy metal-inducible vector caused apoptosis of the cells characterized by cell fragmentation; a reduction in mitochondrial membrane potential; activation of caspase-7, -8, and -9, but not caspase-3; and DNA degradation. We also found that the RING finger and coiled-coil domains were required for MAIR activity by analysis with deletion mutants.

The editosome for cytidine to uridine mRNA editing has a native complexity of 27S: identification of intracellular domains containing active and inactive editing factors

Apolipoprotein B mRNA cytidine to uridine editing requires the assembly of a multiprotein editosome comprised minimally of the catalytic subunit,apolipoprotein B mRNA editing catalytic subunit 1 (APOBEC-1), and an RNA-binding protein, APOBEC-1 complementation factor (ACF). A rat homolog has been cloned with 93.5% identity to human ACF (huACF). Peptide-specific antibodies prepared against huACF immunoprecipitated a rat protein of similar mass as huACF bound to apolipoprotein B (apoB) RNA in UV cross-linking reactions, thereby providing evidence that the p66, mooring sequence-selective, RNA-binding protein identified previously in rat liver by UV cross-linking and implicated in editosome assembly is a functional homolog of huACF. The rat protein (p66/ACF) was distributed in both the nucleus and cytoplasm of rat primary hepatocytes. Within a thin section, a significant amount of total cellular p66/ACF was cytoplasmic, with a concentration at the outer surface of the endoplasmic reticulum. Native APOBEC-1 co-fractionated with p66/ACF in the cytoplasm as 60S complexes. In the nucleus, the biological site of apoB mRNA editing, native p66/ACF, was localized to heterochromatin and fractionated with APOBEC-1 as 27S editosomes. When apoB mRNA editing was stimulated in rat primary hepatocytes with ethanol or insulin, the abundance of p66/ACF in the nucleus markedly increased. It is proposed that the heterogeneity in size of complexes containing editing factors is functionally significant and reflects functionally engaged editosomes in the nucleus and an inactive cytoplasmic pool of factors.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Two B or not two B? Overview of the rapidly expanding B-box family of proteins

The B-box gene family represents a large number of genes involved in functions such as axial patterning, growth control, differentiation, and transcriptional regulation. These genes possess several conserved motifs that always include a B-box zinc binding motif associated with various other motifs such as the RING zinc finger, an alpha-helical coiled-coil, the rfp or B30.2 motif, propeller domain, and the NHL motif in various combinations. Mutations or rearrangements in several B-box family members are associated with human diseases and cancers such as familial Mediterranean fever (FMF), Optiz/BBB syndrome, acute promyelocytic leukemia, mulibrey nanism, and thyroid carcinomas. This suggests that members of this gene family play important roles in fundamental biological processes. Here we discuss the known members of this rapidly expanding protein family.

Cytoplasmic retention of HIV-1 regulatory protein Vpr by protein-protein interaction with a novel human cytoplasmic protein VprBP

Vpr is an HIV-1 auxiliary regulatory protein packaged in the virion. It has been shown to enhance the nuclear transport of the HIV-1 pre-integration complex, activate transcription of cellular and viral promoters, and arrest the cell cycle at the G2/M check-point. We previously identified a cellular protein of 180 kDa (RIP) that interacted with HIV-1 Vpr specifically. We now rename this cellular protein as Vpr-binding protein, or VprBP. In this report, we describe the cloning of the VprBP cDNA that encodes 1507 aa residues and is identical to the previously cloned cDNA KIAA0800. We demonstrate that Vpr specifically interacts with recombinantly expressed VprBP in vitro as well as in vivo. Furthermore, Vpr interacts with the cellular endogenous VprBP in the context of the HIV-1 life cycle. Mutational analysis of VprBP suggests that the Vpr binding domain is located within the C-terminal half of VprBP, which has a Pro-rich domain and several Phe-x-x-Phe repeats. Subcellular fractionation studies show that both the endogenous VprBP and the adenovirus-expressed VprBP are distributed predominantly in the cytoplasmic fraction. Consistent with previous reports, the adenovirus-expressed Vpr is distributed in both the cytoplasmic and the nuclear fractions. However, when VprBP and Vpr are expressed together, Vpr is found almost exclusively in the cytoplasm. Expression of VprBP does not affect the nuclear transport of the adenoviral nuclear protein, pTP. VprBP expressed in insect cells also blocks the nuclear transport of a Vpr-GFP fusion protein, and VprBP mutants incapable of interacting with Vpr fail to block Vpr-GFP nuclear transport. We hypothesize that Vpr interaction with VprBP may cause changes in the host cell cytoplasm that affect HIV-1 pathogenesis as well as HIV-1 replication.

Nuclear export signal located within theDNA-binding domain of the STAT1transcription factor

Latent signal transducers and activators of transcription (STATs) reside in the cytoplasm but rapidly accumulate in the nucleus following cytokine stimulation. Nuclear accumulation requires specific tyrosine phosphorylation and STAT dimerization. The presence of STATs in the nucleus is transient, however, and within hours the STATs reappear in the cytoplasm. Results indicate that STAT1 can be dephosphorylated in the nucleus and actively exported by the chromosome region maintenance 1 (CRM1) export receptor. CRM1 recognizes a specific amino acid sequence located within the DNA-binding domain of STAT1. This region shares sequence and functional properties of characterized nuclear export signals. The location of this sequence within STAT1 suggests that it is not accessible to CRM1 when STAT1 is bound to DNA. Evidence is presented to support a model in which STAT1 is tyrosine dephosphorylated in the nucleus and dissociates from DNA, allowing recognition by CRM1 and nuclear export. The regulated export of STAT1 may contribute to silencing of the signal pathway and/or to re-establish STAT1 in the cytoplasm to monitor activity of receptor-kinase signals.

RNA-dependent cytoplasmic anchoring of a transcription factor subunit during Xenopus development

The CCAAT box transcription factor (CBTF) is a multimeric transcription factor that activates expression of the haematopoietic regulatory factor, GATA-2. The 122 kDa subunit of this complex, CBTF(122), is cytoplasmic in fertilized Xenopus eggs and subsequently translocates to the nucleus prior to activation of zygotic GATA-2 transcription at gastrulation. Here we present data suggesting both a role for CBTF(122) prior to its nuclear translocation and the mechanism that retains it in the cytoplasm before the midblastula transition (MBT). CBTF(122) and its variant CBTF(98) are associated with translationally quiescent mRNP complexes. We show that CBTF(122) RNA binding activity is both necessary and sufficient for its cytoplasmic retention during early development. The introduction of an additional nuclear localization signal to CBTF(122) is insufficient to overcome this retention, suggesting that RNA binding acts as a cytoplasmic anchor for CBTF(122). Destruction of endogenous RNA by microinjection of RNase promotes premature nuclear translocation of CBTF(122). Thus, the nuclear translocation of CBTF(122) at the MBT is likely to be coupled to the degradation of maternal mRNA that occurs at that stage.

Nuclear entry of the circadian regulator mPER1 is controlled by mammalian casein kinase I epsilon

The molecular oscillator that keeps circadian time is generated by a negative feedback loop. Nuclear entry of circadian regulatory proteins that inhibit transcription from E-box-containing promoters appears to be a critical component of this loop in both Drosophila and mammals. The Drosophila double-time gene product, a casein kinase I epsilon (CKIepsilon) homolog, has been reported to interact with dPER and regulate circadian cycle length. We find that mammalian CKIepsilon binds to and phosphorylates the murine circadian regulator mPER1. Unlike both dPER and mPER2, mPER1 expressed alone in HEK 293 cells is predominantly a nuclear protein. Two distinct mechanisms appear to retard mPER1 nuclear entry. First, coexpression of mPER2 leads to mPER1-mPER2 heterodimer formation and cytoplasmic colocalization. Second, coexpression of CKIepsilon leads to masking of the mPER1 nuclear localization signal and phosphorylation-dependent cytoplasmic retention of both proteins. CKIepsilon may regulate mammalian circadian rhythm by controlling the rate at which mPER1 enters the nucleus.

XCS-1, a maternally expressed gene product involved in regulating mitosis in Xenopus

The regulation of the cell cycle during early development is an important and complex biological process. We have cloned a cDNA, XCS-1, that may play an important role in regulating mitosis during early embryogenesis in Xenopus laevis. XCS-1 is a maternally expressed gene product that is the Xenopus homologue of the human cleavage signal protein (CS-1). XCS-1 transcripts were detected in oocytes with the titer decreasing just prior to the MBT. During development the XCS-1 protein was detected on the membrane and in the nucleus of blastomeres. It was also detected on the mitotic spindle in mitotic cells and on the centrosomes in interphase cells. Overexpression of myc-XCS-1 in Xenopus embryos resulted in abnormal mitoses with increased numbers of centrosomes, multipolar spindles, and abnormal distribution of chromosomes. Also, we observed incomplete cytokinesis resulting in multiple nuclei residing in the same cytoplasm with the daughter nuclei in different phases of the cell cycle. The phenotype depended on the presence of the N terminus of XCS-1 (aa 1–73) and a consensus NIMA kinase phosphorylation site (aa159-167). Mutations in this site affected the ability of the overexpressed XCS-1 protein to produce the phenotype. These results suggest that XCS-1 is a maternal factor playing an important role in the regulation of the cell cycle during early embryogenesis and that its function depends on its state of phosphorylation.

The subcellular localization of Otx2 is cell-type specific and developmentally regulated in the mouse retina

Recent evidence implicates homeodomain-containing proteins in the specification of cell fates in the central nervous system. Here we report that in the embryonic mouse eye Otx2, a paired homeodomain transcription factor, was found in retinal pigment epithelial cells and a restricted subset of retinal neurons, including ganglion cells. In the postnatal and adult eye, however, both the cellular and subcellular distribution of the Otx2 protein were cell type-specific. Otx2 was detected only in the nuclei of retinal pigment epithelial and bipolar cells, but was present in the cytoplasm of rod photoreceptors. Immunohistochemical studies of retinal explants and transfected cell lines both suggested that the retention of Otx2 in the cytoplasm of immature rods is a developmentally regulated process. The differential distribution of Otx2 in the cytoplasm of rods and the nucleus of other cell types, suggests that subcellular localization of this transcription factor may participate cell fate determination during specific phases of retinal development.

Regulation of nuclear localization: a key to a door

Information can be transferred between the nucleus and the cytoplasm by translocating macromolecules across the nuclear envelope. Communication of extracellular or intracellular changes to the nucleus frequently leads to a transcriptional response that allows cells to survive in a continuously changing environment. Eukaryotic cells have evolved ways to regulate this movement of macromolecules between the cytoplasm and the nucleus such that the transfer of information occurs only under conditions in which a transcriptional response is required. This review focuses on the ways in which cells regulate movement of proteins across the nuclear envelope and the significance of this regulation for controlling diverse biological processes.

Xenopus Smad4beta is the co-Smad component of developmentally regulated transcription factor complexes responsible for induction of early mesodermal genes

Smad4 is defined as the common-mediator Smad (co-Smad) required for transducing signals for all TGF-beta superfamily members. This paper describes two Smad4s in Xenopus: XSmad4alpha, which is probably the Xenopus orthologue of human Smad4, and a distinct family member, XSmad4beta, which differs primarily at the extreme N-terminus and in the linker region. Both XSmad4s act as co-Smads, forming ligand-dependent complexes with receptor-regulated Smads 1 and 2 and synergizing with them to activate transcription of mesodermal genes in Xenopus embryos. The two XSmad4 genes have reciprocal temporal expression patterns in Xenopus embryos and are expressed in varying ratios in adult tissues, suggesting distinct functional roles in vivo. XSmad4beta is the predominant maternal co-Smad and we go on to demonstrate its role in the transcriptional regulation of early mesodermal genes. We have identified two distinct nuclear complexes that bind the activin-responsive element of the Xenopus Mix.2 promoter: one formed in response to high levels of activin signaling and the other activated by endogenous signaling pathways. Using specific antisera we demonstrate the presence of endogenous XSmad4beta and also XSmad2 in both of these complexes, and our data indicate that the DNA-binding components of the complexes are different. Furthermore, we show that the presence of these complexes in the nucleus perfectly correlates with the transcriptional activity of the target gene, Mix.2, and we show that one of the XSmad4beta-containing transcription factor complexes undergoes a developmentally regulated nuclear translocation.

DNA methyltransferase is actively retained in the cytoplasm during early development

The overall DNA methylation level sharply decreases from the zygote to the blastocyst stage despite the presence of high levels of DNA methyltransferase (Dnmt1). Surprisingly, the enzyme is localized in the cytoplasm of early embryos despite the presence of several functional nuclear localization signals. We mapped a region in the NH(2)-terminal, regulatory domain of Dnmt1 that is necessary and sufficient for cytoplasmic retention during early development. Altogether, our results suggest that Dnmt1 is actively retained in the cytoplasm, which prevents binding to its DNA substrate in the nucleus and thereby contributes to the erasure of gamete-specific epigenetic information during early mammalian development.

Nuclear transport and transcriptional regulation

Studies over the past 10 years have provided major insights into the molecular mechanisms responsible for active transport of macromolecules in and out of the nucleus. Nucleocytoplasmic transport pathways correspond to active and signal-mediated processes that involve substrates, adaptors and receptors. Regulation of both nuclear import and nuclear export is mainly exerted at the level of transport complex formation and has emerged as one of the most efficient mechanisms to adapt gene expression to the cell environment by restricting the access of transcriptional regulators to their target genes.

Developmental changes in RNA polymerase I and TATA box-binding protein during early Xenopus embryogenesis

Xenopus early embryos are transcriptionally quiescent until the midblastula transition (MBT). We have examined the question of whether the absence of rRNA synthesis is related to a deficiency in the RNA polymerase I (pol I) transcription machinery. Previously we have demonstrated that the maternally provided pol I transcription factor UBF already binds to the inactive rRNA genes of pre-MBT embryos (P. Bell et al., 1997, J. Cell Sci. 110, 2053-2063). Here we have analyzed the fate of pol I and the TATA box-binding protein (TBP) through immunofluorescence and immunoblotting experiments. Pol I stockpiled in the egg is taken up by in vitro assembled pronuclei and concentrated into numerous distinct nuclear domains. Comparable storage sites of template-free pol I are also seen in nuclei of blastula to neurula stage embryos. In contrast, the amount of TBP is relatively low in oocytes and eggs but increases dramatically during the cleavage stages. Most of the newly synthesized TBP colocalizes with the stored form of pol I in the extranucleolar domains of blastula/gastrula embryos. The amount of TBP per embryo reaches peak values at the blastula/gastrula stage and then rapidly declines to normal somatic levels. The positive correlation of maximal TBP levels with the timing of the MBT suggests that overproduction of TBP is required for the formation of productive transcription complexes.

The Opitz syndrome gene product, MID1, associates with microtubules

Opitz syndrome (OS) is a genetically heterogeneous disorder characterized by defects of the ventral midline, including hypertelorism, cleft lip and palate, heart defects, and mental retardation. We recently identified the gene responsible for X-linked OS. The ubiquitously expressed gene product, MID1, is a member of the RING finger family. These proteins are characterized by an N-terminal tripartite protein-protein interaction domain and a conserved C terminus of unknown function. Unlike other RING finger proteins for which diverse cellular functions have been proposed, the function of MID1 is as yet undefined. By using the green fluorescent protein as a tag, we show here that MID1 is a microtubule-associated protein that influences microtubule dynamics in MID1-overexpressing cells. We confirm this observation by demonstrating a colocalization of MID1 and tubulin in subcellular fractions and the association of endogenous MID1 with microtubules after in vitro assembly. Furthermore, overexpressed MID1 proteins harboring mutations described in OS patients lack the capability to associate with microtubules, forming cytoplasmic clumps instead. These data give an idea of the possible molecular pathomechanism underlying the OS phenotype.

Modulation of nuclear localization of the influenza virus nucleoprotein through interaction with actin filaments

The influenza virus genome is transcribed in the nuclei of infected cells but assembled into progeny virions in the cytoplasm. This is reflected in the cellular distribution of the virus nucleoprotein (NP), a protein which encapsidates genomic RNA to form ribonucleoprotein structures. At early times postinfection NP is found in the nucleus, but at later times it is found predominantly in the cytoplasm. NP contains several sequences proposed to act as nuclear localization signals (NLSs), and it is not clear how these are overridden to allow cytoplasmic accumulation of the protein. We find that NP binds tightly to filamentous actin in vitro and have identified a cluster of residues in NP essential for the interaction. Complexes containing RNA, NP, and actin could be formed, suggesting that viral ribonucleoproteins also bind actin. In cells, exogenously expressed NP when expressed at a high level partitioned to the cytoplasm, where it associated with F-actin stress fibers. In contrast, mutants unable to bind F-actin efficiently were imported into the nucleus even under conditions of high-level expression. Similarly, nuclear import of NLS-deficient NP molecules was restored by concomitant disruption of F-actin binding. We propose that the interaction of NP with F-actin causes the cytoplasmic retention of influenza virus ribonucleoproteins.

The Oct-1 POU domain directs developmentally regulated nuclear translocation in Xenopus embryos

Early embryonic development in Xenopus is characterized by transcriptional repression which is relieved at the mid-blastula stage. Here we show that most of the maternally inherited POU domain transcription factor Oct-1 is retained in the cytoplasm during early development, and that it gradually translocates to the nucleus around the mid-blastula transition. Overexpressed epitope-tagged Oct-1 exhibits highly similar localization properties compared to endogenous protein. The amino acid sequence that directs this developmentally regulated nuclear translocation resides in the POU domain. Our findings may suggest that cytoplasmic retention of Oct-1 facilitates or contributes to the repression of Oct-1 target genes before the mid-blastula transition.

Nucleocytoplasmic trafficking of steroid-free glucocorticoid receptor

Glucocorticoid receptor (GR) recycles between an inactive form complexed with heat shock proteins (hsps) and localized to the cytoplasm and a free liganded form that regulates specific gene transcription in the nucleus. We report here that, contrary to previous assumptions, association of GR into hsp-containing complexes is not sufficient to prevent the shuttling or trafficking of the GR across the nuclear membrane. Following the withdrawal of treatment with cortisol or the hormone antagonist RU486, GRs recycled rapidly into hsp-associated, hormone-responsive complexes. However, cortisol-withdrawn receptors redistributed to the cytoplasm very slowly (t(1)/(2) = 8-9 h) and RU486-withdrawn receptors not at all. Persistent localization of these GRs to the nucleus was not due to a gross defect in export, since in both instances the complexed nuclear GRs transferred efficiently between heterokaryon nuclei. Moreover, the addition of a nuclear retention signal to the N terminus of GR induced the transfer of naive receptor to the nucleus in the absence of steroid. These results suggest that the localization of GR to the cytoplasm is determined by fine control of the rates of transfer of GR across the nuclear membrane and/or by active retention that occurs independently from the association of GR with hsps.

The novel estrogen-responsive B-box protein (EBBP) gene is tamoxifen-regulated in cells expressing an estrogen receptor DNA-binding domain mutant

We have identified a 2.6-kb mRNA whose steady state levels are increased 2- to 4-fold by treatment of human mammary epithelial cells (HMEC) stably expressing an estrogen receptor (ER) transgene with either estrogen (E) or the antiestrogen, 4-hydroxy-tamoxifen (HT). The cDNA corresponding to this mRNA encodes a 564-amino acid protein, named estrogen-responsive B box protein (EBBP), that is a new member of a subfamily within the B box zinc finger protein family, which includes transcription factors (e.g. TIF1), tumor suppressor proteins (e.g. PML), and proteins implicated in development (e.g. ret finger protein, XNF7). The EBBP mRNA is detectable by Northern blot analysis in most tissues, with the exception of liver and peripheral blood lymphocytes, and the gene has been mapped to human chromosome 17p11.2. In contrast to most B box family members, EBBP has a predominantly cytoplasmic localization. Studies of the estrogenic regulation of EBBP expression demonstrated that the E-dependent increase in EBBP mRNA levels in the ER-transfected HMEC is an early, ER-mediated, and cycloheximide-insensitive process. In HMEC stably transfected with an ER mutant containing a deletion in the second zinc finger of the DNA-binding domain, E and HT had different effects on EBBP gene expression; EBBP regulation by E was dramatically reduced while the effects of HT were augmented. These data indicate that HT can modulate EBBP mRNA expression through a mutated ER, which has little activity when bound by E, and suggest that different molecular mechanisms control the E and HT responsiveness of the EBBP gene.

The maternal CCAAT box transcription factor which controls GATA-2 expression is novel and developmentally regulated and contains a double-stranded-RNA-binding subunit

The transcription factor GATA-2 is expressed at high levels in the nonneural ectoderm of the Xenopus embryo at neurula stages, with lower amounts of RNA present in the ventral mesoderm and endoderm. The promoter of the GATA-2 gene contains an inverted CCAAT box conserved among Xenopus laevis, humans, chickens, and mice. We have shown that this sequence is essential for GATA-2 transcription during early development and that the factor binding it is maternal. The DNA-binding activity of this factor is detectable in nuclei and chromatin bound only when zygotic GATA-2 transcription starts. Here we report the characterization of this factor, which we call CBTF (CCAAT box transcription factor). CBTF activity mainly appears late in oogenesis, when it is nuclear, and the complex has multiple subunits. We have identified one subunit of the factor as p122, a Xenopus double-stranded-RNA-binding protein. The p122 protein is perinuclear during early embryonic development but moves from the cytoplasm into the nuclei of embryonic cells at stage 9, prior to the detection of CBTF activity in the nucleus. Thus, the accumulation of CBTF activity in the nucleus is a multistep process. We show that the p122 protein is expressed mainly in the ectoderm. Expression of p122 mRNA is more restricted, mainly to the anterior ectoderm and mesoderm and to the neural tube. Two properties of CBTF, its dual role and its cytoplasm-to-nucleus translocation, are shared with other vertebrate maternal transcription factors and may be general properties of these proteins.

Cytoplasmic localization of LIM-kinase 1 is directed by a short sequence within the PDZ domain

LIM-containing protein kinase 1 (LIMK1) is a serine/threonine kinase with a structure composed of two LIM domains, a PDZ domain, and a protein kinase domain. We examined the subcellular localization of LIMK1 and its variously deleted mutants in HeLa cells by transfection with these cDNAs. Immunofluorescence analysis revealed that the full-length LIMK1 and its mutants deleted with LIM domain or protein kinase domain preferentially localized in the cytoplasm, while the mutants deleted with the PDZ domain or a 52 amino acid region (B region) within the PDZ domain localized mainly in the nucleus. When the normally nuclear cyclin A was fused with the PDZ domain or the B region of LIMK1, it was localized in the cytoplasm of transfected cells. The corresponding region of the PDZ domain of postsynaptic density protein (PSD)-95 had no such function. Additionally, the PDZ domain of LIMK1 had no potential to bind to the C-terminal S/TXV peptides, to which the PSD-95 PDZ domain can bind. Taken together these results suggest that the PDZ domain, particularly the B region, of LIMK1 has a specific function to localize the protein in the cytoplasm. When glutathione S-transferase (GST) fused with the PDZ domain of LIMK1 (GST-PDZ) or GST-PDZ deleted with the B region (GST-PDZ delta B) was microinjected into the nucleus of COS cells, GST-PDZ was almost completely excluded from the nucleus within 30 min, whereas GST-PDZ delta B remained in the nucleus. These findings suggest that the B region of LIMK1 probably has nuclear export signal activity.

Apparent continuity between the messenger transport organizer and late RNA localization pathways during oogenesis in Xenopus

The localization of RNAs at the vegetal cortex in Xenopus oocytes is a complex process, involving at least two different pathways. The early, or messenger transport organizer (METRO), pathway, localizes RNAs such as Xlsirts, Xcat2 and Xwnt11 during stages 1 and 2 of oogenesis, while the late pathway localizes RNAs such as Vg1 during stages 2-4. We demonstrate that the onset of Vg1 localization is characterized by its microtubule-independent binding to a subdomain of the endoplasmic reticulum (ER). The formation of this unique ER structure is intimately associated with the movement of the mitochondrial cloud toward the vegetal cortex. In addition, we demonstrate that the mitochondrial cloud contains a gamma-tubulin-positive structure that may function as a microtubule organizing center for establishing microtubule tracks for Vg1 localization. These data, support, although they do not prove, a model in which the development of the late pathway machinery relies upon the prior functioning of the early pathway.

XL43 and XL75: two novel RING finger-containing genes expressed during oogenesis and embryogenesis in Xenopus laevis

This article reports on the isolation of two new Xenopus genes that encode two putative zinc finger proteins. The XL43 and XL75 proteins belong to the RBCC family, and also contains the rfp-like domain. XL43 and XL75 RNAs are found in the ovary, and myc-tagged proteins are detected in mRNA injected oocytes. Whole-mount in situ hybridization of embryos revealed that these two clones are expressed exclusively in neurectodermic and mesodermic tissues. The data suggest that XL43 and XL75 genes could play an important role in the frog's early development, perhaps as a transcription factor. This RBCC family, a subclass of the RING finger family, comprises proteins with known cellular transformation properties. All members possess, besides one RING finger motif, one or two B-boxes, each having a pair of zinc fingers, and a coiled coil domain (Borden, K.L. B. et al., 1996. Proc. Natl. Acad. Sci. USA 93, 1601-1606). Among this group, some members possess, besides the RING-B box-coiled coil (RBCC) motifs, a long C-terminal domain referred to as the rfp-like domain. Although its effective role has not been elucidated yet, this last domain could play an important role by binding a ligand (Bellini, M. et al., 1993. EMBO J. 12, 107-114).

Analysis of domains affecting intracellular localization of the FMRP protein

Fragile X syndrome is the most frequent form of inherited mental retardation and it is caused by deficiency of FMRP, the protein encoded by the FMR1 gene. FMRP is a RNA binding protein of unknown function which is associated with ribosomes. FMRP is found in the cytoplasm, but it is endowed with a nuclear export signal (NES), encoded by exon 14, and a nuclear localization signal (NLS). Characterization of the FMRP NES and NLS domains is presented here. We show by site-directed mutagenesis that three leucine residues in exon 14 are functionally important for the cytoplasmic localization of FMRP. Changing these leucines to serine resulted in a nuclear localization, while another nonconservative change (leucine to tyrosine) did not show such an effect. We also show that the NLS activity is localized between residues 115 and 150, a region that lacks stretches of basic residues. Such stretches are typical of nuclear localization signals that act through the important alpha pathway. The region between residues 151 and 196 can reinforce the NLS activity. A truncated construct containing the N-terminal region of FMRP (residues 1-114) is strikingly concentrated in the nucleus. This suggests that it may contain a domain of strong affinity with a nuclear component.

Functional dissection of YA, an essential, developmentally regulated nuclear lamina protein in Drosophila melanogaster

The Drosophila YA protein is a nuclear lamina component whose function is essential to initiate embryonic development. To identify regions of YA required for its action in its normal cellular context, we made targeted mutations in the YA protein and tested their consequences in flies and embryos in vivo. We found that critical amino acids are distributed along the length of the YA molecule, with functionally important regions including the N- and the C-terminal ends, the cysteine residues in YA's two potential zinc fingers, a serine/threonine-rich region, and a potential maturation-promoting factor or mitogen-activated protein kinase phosphorylation target site, ITPIR. In addition, several Ya mutations showed intragenic complementation, with N-terminal mutations complementing C-terminal mutations, suggesting that YA proteins interact with one another. In support of this interaction, we demonstrated by immunoprecipitation that YA molecules are present in complexes with each other. Finally, we showed that the C-terminal 179 amino acids of YA are necessary to target, or retain, YA in the nuclear envelope.

Multiple protein domains determine the cell type-specific nuclear distribution of the catalytic subunit required for apolipoprotein B mRNA editing

Apolipoprotein B (apoB) mRNA editing catalyzed by apoB mRNA editing catalytic subunit 1 (APOBEC-1) has been proposed to be a nuclear process. To test this hypothesis, the subcellular distribution of hemagglutinin- (HA) tagged APOBEC-1 expressed in transiently transfected hepatoma cells was determined by indirect immunofluorescence microscopy. HA-APOBEC-1 was detected in both the nucleus and cytoplasm of rat and human hepatoma cells. Mutagenesis of APOBEC-1 demonstrated that the N-terminal 56 amino acids (1-56) were necessary for the nuclear distribution of APOBEC-1, but this region did not contain a functional nuclear localization signal (NLS). However, we identified a 24-amino acid domain in the C terminus of APOBEC-1 with characteristics of a cytoplasmic retention signal (CRS) or a nuclear export signal (NES). These data suggest, therefore, that the nuclear import of APOBEC-1 may not be mediated by a positive NLS; rather, it may be achieved by overcoming the effect of a CRS/NES. We also demonstrated that the nuclear distribution of APOBEC-1 occurred only in cell lines that were capable of editing apoB RNA. We propose that the cellular distribution of APOBEC-1 is determined by multiple domains within this protein, and a nuclear localization of the enzyme may be regulated by cell type-specific factors that render these cells uniquely editing competent.

Opitz G/BBB syndrome, a defect of midline development, is due to mutations in a new RING finger gene on Xp22

Opitz syndrome (OS) is an inherited disorder characterized by midline defects including hypertelorism, hypospadias, lip-palate-laryngotracheal clefts and imperforate anus. We have identified a new gene on Xp22, MID1 (Midline 1), which is disrupted in an OS patient carrying an X-chromosome inversion and is also mutated in several OS families. MID1 encodes a member of the B-box family of proteins, which contain protein-protein interaction domains, including a RING finger, and are implicated in fundamental processes such as body axis patterning and control of cell proliferation. The association of MID1 with OS suggests an important role for this gene in midline development.

xnf7 functions in dorsal-ventral patterning of the Xenopus embryo

Xenopus nuclear factor 7 (xnf7) is a maternally expressed nuclear protein that is retained in the cytoplasm from oocyte maturation until the midblastula transition (MBT). Mutations of the xnf7 phosphorylation sites to glutamic acids (dnxnf7) resulted in the retention of the endogenous protein in the cytoplasm past the MBT, indicating that cytoplasmic retention is a phosphorylation dependent process. In addition, dnxnf7 acted as a dominant negative mutant by keeping the endogenous xnf7 protein in the cytoplasm past the MBT. Overexpression of dnxnf7 in future dorsal blastomeres resulted in a ventralized or posteriorized phenotype in which the embryos lacked anterior structures, while overexpression in ventral blastomeres resulted in dorsalized embryos. dnxnf7 also affected the expression of both dorsal and ventral mesodermal markers. These data suggest that xnf7 functions in dorsal/ventral patterning and that the movement of the protein from the cytoplasm to the nucleus at the MBT is critical for the execution of a genetic program conferring a dorsal or ventral identity to the mesoderm.

Mitogen-activated protein kinase and cyclin B/Cdc2 phosphorylate Xenopus nuclear factor 7 (xnf7) in extracts from mature oocytes. Implications for regulation of xnf7 subcellular localization

Xenopus nuclear factor 7 (xnf7) is a maternally expressed putative transcription factor that exhibits phosphorylation-dependent changes in subcellular localization during early Xenopus development. Xnf7 is localized to the germinal vesicle (nucleus) of immature oocytes in a hypophosphorylated state. Xnf7 is phosphorylated during oocyte maturation and released to the cytoplasm. The protein is retained in the cytoplasm during early embryonic cleavage stages but returns to nuclei at the mid-blastula transition. Xnf7 is phosphorylated at two sites during oocyte maturation, designated P1, consisting of one threonine at position 103, and P2, consisting of three clustered threonines at positions 209, 212, and 218. Phosphorylation of both sites is important in regulating xnf7 localization. The P1 site can be phosphorylated by cyclin B/Cdc2 in vitro. To further understand the mechanisms regulating subcellular localization of xnf7 during early development, kinases capable of catalyzing phosphorylation of the P2 site were purified from mature oocyte extracts. We found that mitogen-activated protein kinase phosphorylated Thr212 and cyclin B/Cdc2 phosphorylated Thr 209 and Thr212. No other kinase in mature oocyte extracts phosphorylated the xnf7 P2 site to a significant extent. These results implicate mitogen-activated protein kinase and cyclin B/Cdc2 in regulating xnf7 localization during oocyte maturation. This also suggests that localization of xnf7 may be regulated by multiple kinase activation pathways.

Nuclear localization of basonuclin in human keratinocytes and the role of phosphorylation

Basonuclin is a zinc-finger protein found in basal cells of the epidermis. In human keratinocyte cultures, basonuclin is susceptible to serine-phosphorylation and the addition of the phosphatase inhibitor, okadaic acid, promotes accumulation of basonuclin in the cytoplasm. The region of basonuclin containing the nuclear localization signal of basonuclin is necessary for nuclear localization of the protein and Ser-541, located immediately C-terminal to the nuclear localization signal, is the principal phosphorylation site in vitro. A nearly complete basonuclin transiently expressed in cultured keratinocytes localizes predominantly in the nucleus, but substitution of aspartic acid for Ser-541 promotes cytoplasmic localization. The same substitution of Ser-537 has a similar but weaker effect. Substitution of both serine residues by alanine leads to nuclear localization. These results show that nuclear localization of basonuclin depends on serine dephosphorylation, primarily of Ser-541. Different subcellular locations of basonuclin in different keratinocyte subtypes are therefore most likely to be controlled by the state of phosphorylation of Ser-541.

Cytoplasmically anchored plakoglobin induces a WNT-like phenotype in Xenopus

Plakoglobin is one of two vertebrate proteins closely related to the Drosophila segment polarity gene product armadillo. Overexpression of plakoglobin induces neural axis duplication in Xenopus and the exogenous plakoglobin is localized to nuclei (Karnovsky, A., and Klymkowsky, M. W., Proc. Natl. Acad. Sci. USA 92, 4255, 1995; Rubenstein, A., et al., Dev. Genet., 1997, in press). We have carried out a series of experiments to test whether the nuclear localization of plakoglobin is required for its inductive effects. Prior to the midblastula transition exogenous plakoglobin is cytoplasmic and concentrated in the cortical regions of blastomeres; after the midblastula transition exogenous plakoglobin accumulates in embryonic nuclei. The addition of a "nuclear localization sequence" does not change the timing of plakoglobin's nuclear localization, suggesting that it is anchored in the cytoplasm prior to the midblastula transition. Next, we constructed two "membrane-anchored" forms of plakoglobin. These are exclusively cytoplasmic; yet both were as effective at producing a "Wnt-like" axis duplication as were "free," unfettered forms of plakoglobin. Moreover, expression of anchored plakoglobins had no apparent effect on the cytoplasmic or nuclear levels of beta-catenin. These data indicate that plakoglobin can act cytoplasmically to generate a WNT-like phenotype. Taken together with the ventralizing effects of a mutant from of the XTcf-3 transcription factor, described by Molenaar et al. Cell 86, 391, 1996, we speculate that in the early Xenopus embryo, activation of plakoglobin (or beta-catenin) inhibits the activity of XTcf-3 or a XTcf-3-like factor.

Regulation of nuclear entry of the Drosophila clock proteins period and timeless

Two genes, period (per) and timeless (tim), are essential for circadian rhythmicity in Drosophila. The encoded proteins (PER and TIM) physically interact. Here, it is shown that TIM and PER accumulate in the cytoplasm when independently expressed in cultured (S2) Drosophila cells. However, the proteins move to the nuclei of these cells if coexpressed. Domains of PER and TIM have been identified that block nuclear localization of the monomeric proteins. In vitro protein interaction studies indicate that the sequence inhibiting the nuclear accumulation of PER forms a binding site for TIM. The results indicate a mechanism for controlled nuclear localization in which suppression of cytoplasmic localization is accomplished by direct interaction of PER and TIM. No other clock functions are required for nuclear localization. The findings suggest that a checkpoint in the circadian cycle is established by requiring cytoplasmic assembly of a PER/TIM complex as a condition for nuclear transport of either protein.

Regulation and expression of transgenes in fish -- a review

Transgenic fish, owing to a number of advantages which they offer over other species, are proving to be valuable model systems for the study of gene regulation and development genetics in addition to being useful targets for the genetic manipulation of commercially important traits. Despite having begun only a decade ago, the production of transgenic fish has become commonplace in a number of laboratories world-wide and considerable progress has been made. In this review, we initially consider the various regulatory elements and coding genes which have been used in fish, and subsequently discuss and compare both the transient and long-term fate and expression patterns of injected DNA sequences in the context of the different factors which are likely to have an effect on the expression of transgenes.

Altering cell fates in sea urchin embryos by overexpressing SpOtx, an orthodenticle-related protein

While many general features of cell fate specification in the sea urchin embryo are understood, specific factors associated with these events remain unidentified. SpOtx, an orthodenticle-related protein, has been implicated as a transcriptional activator of the aboral ectoderm-specific Spec2a gene. Here, we present evidence that SpOtx has the potential to alter cell fates. SpOtx was found in the cytoplasm of early cleavage stage embryos and was translocated into nuclei between the 60- and 120-cell stage, coincident with Spec gene activation. Eggs injected with SpOtx mRNA developed into epithelial balls of aboral ectoderm suggesting that SpOtx redirected nonaboral ectoderm cells to an aboral ectoderm fate. At least three distinct domains on SpOtx, the homeobox and regions in the N-terminal and C-terminal halves of the protein, were required for the morphological alterations. These same N-terminal and C-terminal regions were shown to be transactivation domains in a yeast transactivation assay, indicating that the biological effects of overexpressing SpOtx were due to its action as a transcription factor. Our results suggest that SpOtx is involved in aboral ectoderm differentiation by activating aboral ectoderm-specific genes and that modulating its expression can lead to changes in cell fate.

Cytoplasmic retention and nuclear import of 5S ribosomal RNA containing RNPs

Nuclear export of newly transcribed 5S ribosomal RNA in Xenopus oocytes occurs in the context of either a complex with the ribosomal protein L5 (5S RNP) or with the transcription factor IIIA (7S RNP). Here we examine nuclear import of 5S RNA, L5 and TFIIIA. The 5S RNP shuttles between nucleus and cytoplasm and only 5S RNA variants which can bind to L5 gain access to the nucleus. The 7S RNP is retained in the cytoplasm. Only TFIIIA which is not bound to 5S RNA is imported into the nucleus. As a novel mechanism for cytoplasmic retention, we propose that RNA binding masks a nuclear localization sequence in TFIIIA. In contrast to the nuclear import of L5, import of TFIIIA is sensitive towards the nuclear localization sequence (NLS) competitor p(lys)-BSA, suggesting that these two proteins make use of different import pathways.

Finely tuned regulation of cytoplasmic retention of Xenopus nuclear factor 7 by phosphorylation of individual threonine residues

Xenopus nuclear factor 7 (xnf7) is a maternal gene product that functi ons in dorsal/ventral patterning of the embryo. The xnf7 protein is stored in the oocyte nucleus germinal vesicle in a hypophosphorylated state. At oocyte maturation, xnf7 is hyperphosphorylated and released into the cytoplasm, where it is anchored until the midblastula stage, where it is dephosphorylated and enters the nucleus. We demonstrated that cytoplasmic anchoring of xnf7 was regulated by changes in the phosphorylation status of four threonines within two sites, site 1 (Thr-103) and site 2 (Thr-209, Thr-212, and Thr-218), which function in an additive manner. A mutant form of xnf7 (xnf7thr-glu) in which the threonines at sites 1 and 2 were mutated to glutamic acids to mimic a permanent state of phosphorylation was retained in the cytoplasm in oocytes and embryos through the gastrula stage. The cytoplasmic form of xnf7 was detected in a large 670-kDa protein complex probably consisting of xnf7 and several other unknown protein components. Anchoring of xnf7 was not dependent on association with either microtubule or microfilament components of the cytoskeleton, since treatment with cytochalasin B and nocodazole did not affect cytoplasmic retention. Both wild-type xnf7 and xnf7thr-glu form dimers in the yeast two-hybrid system; however, homodimerization was not required for cytoplasmic retention. We suggest that the cytoplasmic retention of xnf7 depends on the phosphorylation state of the protein whereas the cytoplasmic anchoring machinery appears to be constitutively present in oocytes and throughout development until the gastrula stage.

Alternative splicing of exon 14 determines nuclear or cytoplasmic localisation of fmr1 protein isoforms

Impaired expression of the FMR1 gene is responsible for the fragile X mental retardation syndrome. The FMR1 gene encodes a cytoplasmic protein with RNA-binding properties. Its complex alternative splicing leads to several isoforms, whose abundance and specific functions in the cell are not known. We have cloned in expression vectors, cDNAs corresponding to several isoforms. Western blot comparison of the pattern of endogenous FMR1 proteins with these transfected isoforms allowed the tentative identification of the major endogenous isoform as ISO 7 and of a minor band as an isoform lacking exon 14 sequences (ISO 6 or ISO 12), while some other isoforms (ISO 4, ISO 5) were not expressed at detectable levels. Surprisingly, in immunofluorescence studies, the transfected splice variants that exclude exon 14 sequences (and have alternate C-terminal regions) were shown to be nuclear. Such differential localisation was however not seen in subcellular fractionation studies. Analysis of various deletion mutants suggests the presence of a cytoplasmic retention domain encoded in exon 14 and of a nuclear association domain encoded within the first eight exons that appear however to lack a typical nuclear localisation signal.