Association of rapidly-labelled RNAs with actin in nuclear matrix from mouse L5178Y cells

Cell and molecular biology of nuclear actin

Actin is a highly conserved protein and one of the major components of the cytoplasm and the nucleus in eukaryotic cells. In the nucleus, actin is involved in a variety of nuclear processes that include transcription and transcription regulation, RNA processing and export, intranuclear movement, and structure maintenance. Recent advances in the field of nuclear actin have established that functions of actin in the nucleus are versatile, complex, and interconnected. It also has become increasingly evident that the cytoplasmic and nuclear pools of actin are functionally linked. However, while the biological significance of nuclear actin has become clear, we are only beginning to understand the mechanisms that lie behind the regulation of nuclear actin. This review provides an overview of our current understanding of the functions of actin in the nucleus.

A novel role of the actin-nucleating Arp2/3 complex in the regulation of RNA polymerase II-dependent transcription

It has been well documented that actin is present in the nucleus and involved in numerous nuclear functions including regulation of transcription. The actin-nucleating Arp2/3 complex is an essential, evolutionarily conserved seven-subunit protein complex that promotes actin cytoskeleton assembly in the cytoplasm upon stimulation by WASP family proteins. Our recent study indicates that the nuclear localized neural Wiskott-Aldrich syndrome protein (N-WASP) can induce de novo actin polymerization in the nucleus, and this function is important for the role of N-WASP in the regulation of RNA polymerase II-dependent transcription. Here, we have presented evidence to show that the Arp2/3 complex is also localized in the nucleus and plays an essential role in mediating nuclear actin polymerization induced by N-WASP. We have also demonstrated that the Arp2/3 complex physically associates with RNA polymerase II and is involved in the RNA polymerase II-dependent transcriptional regulation both in vivo and in vitro. Together, these data provide strong support for the hypothesis that N-WASP and the Arp2/3 complex regulate transcription, at least in part, through the regulation of nuclear actin polymerization in a manner similar to their function in the cytoplasm.

Emp is a component of the nuclear matrix of mammalian cells and undergoes dynamic rearrangements during cell division

Emp, originally detected in erythroblastic islands, is expressed in numerous cell types and tissues suggesting a functionality not limited to hematopoiesis. To study the function of Emp in non-hematopoietic cells, an epitope-tagged recombinant human Emp was expressed in HEK cells. Preliminary studies revealed that Emp partitioned into both the nuclear and Triton X-100-insoluble cytoskeletal fractions in approximately a 4:1 ratio. In this study, we report investigations of Emp in the nucleus. Sequential extractions of interphase nuclei showed that recombinant Emp was present predominantly in the nuclear matrix. Immunofluorescence microscopy showed that Emp was present in typical nuclear speckles enriched with the spliceosome assembly factor SC35 and partially co-localized with actin staining. Coimmunoprecipitation and GST-pull-down assays confirmed the apparent close association of Emp with nuclear actin. During mitosis, Emp was detected at the mitotic spindle/spindle poles, as well as in the contractile ring during cytokinesis. These results suggest that Emp undergoes dynamic rearrangements within the nuclear architecture that are correlated with cell division.

Nuclear distribution of actin and myosin I depends on transcriptional activity of the cell

As previous studies suggested, nuclear myosin I (NMI) and actin have important roles in DNA transcription. In this study, we characterized the dynamics of these two proteins during transcriptional activation in phytohemagglutinin (PHA) stimulated human lymphocytes. The stimulation led to strong up-regulation of NMI both on the mRNA and protein level, while actin was relatively stably expressed. The intranuclear distribution of actin and NMI was evaluated using immunogold labeling. In nucleoli of resting cells, actin was localized predominantly to fibrillar centers (FCs), while NMI was located mainly to the dense fibrillar component (DFC). Upon stimulation, FCs remained the main site of actin localization, however, an accumulation of both actin and NMI in the DFC and in the granular component was observed. In the nucleoplasm of resting lymphocytes, both actin and NMI were localized mostly in condensed chromatin. Following stimulation, the majority of both proteins shifted towards the decondensed chromatin. In transcriptionally active cells, both actin and NMI colocalized with nucleoplasmic transcription sites. These results demonstrate that actin and NMI are compartmentalized in the nuclei where they can dynamically translocate depending on transcriptional activity of the cells.

Actin is part of pre-initiation complexes and is necessary for transcription by RNA polymerase II

Actin is abundant in the nucleus and has been implicated in transcription; however, the nature of this involvement has not been established. Here we demonstrate that beta-actin is critically involved in transcription because antibodies directed against beta-actin, but not muscle actin, inhibited transcription in vivo and in vitro. Chromatin immunoprecipitation assays demonstrated the recruitment of actin to the promoter region of the interferon-gamma-inducible MHC2TA gene as well as the interferon-alpha-inducible G1P3 gene. Further investigation revealed that actin and RNA polymerase II co-localize in vivo and also co-purify. We employed an in vitro system with purified nuclear components to demonstrate that antibodies to beta-actin block the initiation of transcription. This assay also demonstrates that beta-actin stimulates transcription by RNA polymerase II. Finally, DNA-binding experiments established the presence of beta-actin in pre-initiation complexes and also showed that the depletion of actin prevented the formation of pre-initiation complexes. Together, these data suggest a fundamental role for actin in the initiation of transcription by RNA polymerase II.

F-actin-dependent Insolubility of Chromatin-modifying Components

Many complexes involved in chromatin modification are difficult to isolate and commonly found associated with nuclear matrix preparations. In this study, we examine the elution properties of chromatin-modifying components under different extraction conditions. We find that most, but not all, histone acetyltransferases and histone deacetylases predominantly partition with the nuclear pellet during intermediate salt extraction. In attempts to identify a biological basis for the observed insolubility, we demonstrate that depolymerizing cellular actin, but not cellular tubulin, mobilizes a significant proportion of the insoluble pool into the intermediate salt-soluble nuclear extract. The disruption of cellular F-actin releases a specific subset of high molecular weight, active, nuclear histone deacetylase complexes that are not found under normal conditions. This study demonstrates that actin polymerization, a physiologically relevant process, is responsible for the observed insolubility of these components during nuclear extract preparation and establishes a simple strategy for isolating subsets of chromatin-modifying complexes that are otherwise depleted or absent under the same extraction conditions.

Nuclear actin and protein 4.1: essential interactions during nuclear assembly in vitro

Structural protein 4.1, which has crucial interactions within the spectrin-actin lattice of the human red cell membrane skeleton, also is widely distributed at diverse intracellular sites in nucleated cells. We previously showed that 4.1 is essential for assembly of functional nuclei in vitro and that the capacity of 4.1 to bind actin is required. Here we report that 4.1 and actin colocalize in mammalian cell nuclei using fluorescence microscopy and, by higher-resolution detergent-extracted cell whole-mount electron microscopy, are associated on nuclear filaments. We also devised a cell-free assay using Xenopus egg extract containing fluorescent actin to follow actin during nuclear assembly. By directly imaging actin under nonperturbing conditions, the total nuclear actin population is retained and visualized in situ relative to intact chromatin. We detected actin initially when chromatin and nuclear pores began assembling. As nuclear lamina assembled, but preceding DNA synthesis, actin distributed in a reticulated pattern throughout the nucleus. Protein 4.1 epitopes also were detected when actin began to accumulate in nuclei, producing a diffuse coincident pattern. As nuclei matured, actin was detected both coincident with and also independent of 4.1 epitopes. To test whether acquisition of nuclear actin is required for nuclear assembly, the actin inhibitor latrunculin A was added to Xenopus egg extracts during nuclear assembly. Latrunculin A strongly perturbed nuclear assembly and produced distorted nuclear structures containing neither actin nor protein 4.1. Our results suggest that actin as well as 4.1 is necessary for nuclear assembly and that 4.1-actin interactions may be critical.

Actin-filled nuclear invaginations indicate degree of cell de-differentiation

For years the existence of nuclear actin has been heavily debated, but recent data have clearly demonstrated that actin, as well as actin-binding proteins (ABPs), are located in the nucleus. We examined live EGFP-actin-expressing cells using confocal microscopy and saw the presence of structures strongly resembling actin filaments in the nuclei of MDA-MB-231 human mammary epithelial tumor cells. Many nuclei had more than one of these filamentous structures, some of which appeared to cross the entire nucleus. Extensive analysis, including fluorescence recovery after photobleaching (FRAP), showed that all EGFP-actin in the nucleus is monomeric (G-actin) rather than filamentous (F-actin) and that the apparent filaments seen in the nucleus are invaginations of cytoplasmic monomeric actin. Immunolocalization of nuclear pore complex proteins shows that similar invaginations are seen in cells that are not overexpressing EGFP-actin. To determine whether there is a correlation between increased levels of invagination in the cell nuclei and the state of de-differentiation of the cell, we examined a variety of cell types, including live Xenopus embryonic cells. Cells that were highly de-differentiated, or cancerous, had an increased incidence of invagination, while cells that were differentiated had few nuclear invaginations. The nuclei of embryonic cells that were not yet differentiated underwent multiple shape changes throughout interphase, and demonstrated numerous transient invaginations of varying sizes and shapes. Although the function of these actin-filled invaginations remains speculative, their presence correlates with cells that have increased levels of nuclear activity.

Nuclear actin is associated with a specific subset of hnRNP A/B-type proteins

Pre-mRNP complexes were isolated from rat liver nuclei as 40S hnRNP particles, and actin-binding proteins were collected by DNase I affinity chromatography. The bound proteins were analyzed by 2D gel electrophoresis, and the following five hnRNP A/B-type proteins were identified by tandem mass spectrometry: DBP40/CBF-A (CArG binding factor A), a minor hnRNP A2 variant and three minor hnRNP A3 (mBx) variants. DBP40 was chosen for further analysis of the association of actin with the pre-mRNP complex. It was shown in vitro that purified actin binds to recombinant DBP40 suggesting that the interaction between actin and DBP40 is direct in the pre-mRNP particles. The association of actin with DBP40 was further explored in vivo. It was shown in a transfection study that DBP40 appears both in the nucleus and cytoplasm. Microinjection experiments revealed that DBP40 is exported from the nucleus to the cytoplasm. Finally, RNA-protein and protein-protein cross-linking experiments showed that DBP40 interacts with poly(A)(+) RNA as well as actin, both in the nucleus and cytoplasm. We propose that actin associated with DBP40, and perhaps with additional hnRNP A/B-type proteins, is transferred from nucleus to cytoplasm bound to mRNA.

The interaction of p53 with the nuclear matrix is mediated by F-actin and modulated by DNA damage

The tumour suppressor protein p53 is localized in the cell nucleus where it serves to initiate cellular responses to a variety of stresses, particularly DNA damage and has the capacity to transactivate stress response genes. An emerging body of evidence indicates that its action is also exerted through direct protein-protein interactions. An approach to understanding p53 function has been to analyse its positioning in relation to nuclear structures and we have shown that p53 can associate with the nuclear matrix. A potential nuclear matrix component for this association is actin. Here we show that p53 interacts with nuclear F-actin and we map the domains involved in this interaction. Using fluorescence resonance energy transfer, we demonstrate that the partition of p53 between F-actin bound and unbound forms is not constant, but is modulated by the presence of DNA damage, which increases binding. Our results indicate that the dynamic interaction of p53 with the nuclear matrix has to be considered for a full understanding of the mechanisms of the p53-mediated cellular response to DNA damage.

Nuclear DNA helicase II/RNA helicase A binds to filamentous actin

Nuclear DNA helicase II (NDH II), also designated RNA helicase A, is a multifunctional protein involved in transcription, RNA processing, and transport. Here we report that NDH II binds to F-actin. NDH II was partially purified from HeLa nuclear extracts by ion-exchange chromatography on Bio-Rex 70 and DEAE-Sepharose. Upon gel-filtration chromatography on Sepharose 4B, partially purified NDH II resolved into two distinct peaks. The first NDH II peak, corresponding to the void volume of Sepharose 4B, displayed coelution with an abundant 42-kDa protein that was subsequently identified as actin. Several nuclear proteins such as RNA polymerase II, the U5 small nuclear ribonucleoprotein (RNP)-associated WD40 protein, and heterogeneous nuclear RNPs (hnRNPs) copurified with NDH II. However, only hnRNPs A1 and C were found together with NDH II and actin polymers during gel filtration. NDH II and hnRNP C from the HeLa nuclear extract coeluted with F-actin on Sepharose 4B in an RNase-resistant manner, whereas hnRNP A1 was nearly completely removed from F-actin-associated hnRNP complexes following RNA digestion. The association of NDH II and hnRNP C with F-actin was abolished by gelsolin, an F-actin-depolymerizing protein that fragments actin polymers into oligomers or monomers. Furthermore, NDH II co-immunoprecipitated with F-actin and hnRNP C, respectively. In vitro translated NDH II coeluted with F-actin on Sepharose 4B, whereas no coelution with F-actin was observed for in vitro translated hnRNP A1 or C1. Binding to F-actin requires an intact C terminus of NDH II and most likely a native protein conformation. Electron microscopy indicated a close spatial proximity among NDH II, hnRNP C, and F-actin within the HeLa nucleus. These results suggest an important function of NDH II in mediating the attachment of hnRNP-mRPP RNP complexes to the actin nucleoskeleton for RNA processing, transport, or other actin-related processes.

Actin bound to the heterogeneous nuclear ribonucleoprotein hrp36 is associated with Balbiani ring mRNA from the gene to polysomes

In the salivary glands of the dipteran Chironomus tentans, a specific messenger ribonucleoprotein (mRNP) particle, the Balbiani ring (BR) granule, can be visualized during its assembly on the gene and during its nucleocytoplasmic transport. We now show with immunoelectron microscopy that actin becomes associated with the BR particle concomitantly with transcription and is present in the particle in the nucleoplasm. DNase I affinity chromatography experiments with extracts from tissue culture cells indicate that both nuclear and cytoplasmic actin are bound to the heterogeneous RNP (hnRNP) protein hrp36, but not to the hnRNP proteins hrp23 and hrp45. The interaction is likely to be direct as purified actin binds to recombinant hrp36 in vitro. Furthermore, it is demonstrated by cross linking that nuclear as well as cytoplasmic actin are bound to hrp36 in vivo. It is known that hrp36 is added cotranscriptionally along the BR mRNA molecule and accompanies the RNA through the nuclear pores and into polysomes. We conclude that actin is likely to be bound to the BR transcript via hrp36 during the transfer of the mRNA from the gene all the way into polysomes.

Experimental observations of a nuclear matrix

Nuclei are intricately structured, and nuclear metabolism has an elaborate spatial organization. The architecture of the nucleus includes two overlapping and nucleic-acid-containing structures - chromatin and a nuclear matrix. The nuclear matrix is observed by microscopy in live, fixed and extracted cells. Its ultrastructure and composition show it to be, in large part, the ribonucleoprotein (RNP) network first seen in unfractionated cells more than 30 years ago. At that time, the discovery of this RNP structure explained surprising observations that RNA, packaged in proteins, is attached to an intranuclear, non-chromatin structure. Periodic and specific attachments of chromatin fibers to the nuclear matrix create the chromatin loop domains that can be directly observed by microscopy or inferred from biochemical experiments. The ultrastructure of the nuclear matrix is well characterized and consists of a nuclear lamina and an internal nuclear network of subassemblies linked together by highly structured fibers. These complex fibers are built on an underlying scaffolding of branched 10-nm filaments that connect to the nuclear lamina. The structural proteins of the nuclear lamina have been well characterized, but the structural biochemistry of the internal nuclear matrix has received less attention. Many internal matrix proteins have been identified, but far less is known about how these proteins assemble to make the fibers, filaments and other assemblies of the internal nuclear matrix. Correcting this imbalance will require the combined application of biochemistry and electron microscopy. The central problem in trying to define nuclear matrix structure is to identify the proteins that assemble into the 10-nm filaments upon which the interior architecture of the nucleus is constructed. Only by achieving a biochemical characterization of the nuclear matrix will we advance beyond simple microscopic observations of structure to a better understanding of nuclear matrix function, regulation and post-mitotic assembly.

Peripheral nuclear matrix actin forms perinuclear shells

Perinuclear actin shells have been reported in a variety of organisms. The shells have been identified by staining perinuclear material with fluorescently-labelled phalloidin, but have not been localized to a specific subcellular compartment at the ultrastructural level. We show here that the shells of 3T3 cells lie in the peripheral nuclear matrix. Nuclear shells and matrix actin in other parts of the nucleus are not usually detected by immunohistochemical staining because they are inaccessible to antibodies or to phalloidin. Immunohistochemical detection of nuclear actin is only possible during its deposition at the end of mitosis, or in interphase nuclei that have been extracted with detergent, digested with nucleases and washed with high salt buffers.

Immunofluorescent localization of actin in relation to transcription sites in mouse pronuclei

Previous biochemical and morphological studies have shown the presence of actin in the nucleus of different cell types where its role remains unclear. In this work, through fluorescence microscopy we studied the localization of actin in the nuclei of early mouse embryos with particular attention to its possible involvement in the onset of transcription occurring at the late one-cell stage. Fluorescent labelling of embryo sections showed that nuclear actin in abundant, in a non-filamentous state, in the whole nucleoplasm excluding the nucleolar precursor bodies. Immunofluorescence on permeabilized embryos revealed that insoluble nuclear actin accumulates in a few large aggregates in transcriptionally inert early one-cell embryos and progressively redistributes into many small aggregates in transcriptionally active late one-cell embryos. Interestingly, these actin aggregates clearly colocalize with transcription sites. Treatment of late one-cell embryos with cytochalasin D induces the formation of actin bundles network in the nucleoplasm but has no apparent effect on the transcriptional activity. In addition, the inhibition of transcription by alpha-amanitin does not modify the nuclear actin distribution. Hence, there does not appear to be a direct causal relationship between transcriptional activity and nuclear actin organization at the one-cell stage although nuclear actin aggregates appear associated with transcription sites.

The architectural organization of nuclear metabolism

Nucleic acid metabolism is structurally organized in the nucleus. DNA replication and transcription have been localized to particular nuclear domains. Additional domains have been identified by their morphology or by their composition; for example, by their high concentration of factors involved in RNA splicing. The domain organization of the nucleus is maintained by the nuclear matrix, a nonchromatin nuclear scaffolding that holds most nuclear RNA and organizes chromatin into loops. The nuclear matrix is built on a network of highly branched core filaments that have an average diameter of 10 nm. Many of the intermediates and the regulatory and catalytic factors of nucleic acid metabolism are retained in nuclear matrix preparations, suggesting that nucleic acid synthesis and processing are structure-bound processes in cells. Tissue-specific and malignancy-induced variations in nuclear structure and metabolism may result from altered matrix architecture and composition.

Single mRNAs visualized by ultrastructural in situ hybridization are principally localized at actin filament intersections in fibroblasts

Considerable evidence indicates that mRNA associates with structural filaments in the cell (cytoskeleton). This relationship would be an important mechanism to effect mRNA sorting since specific mRNAs could be sequestered at sites within the cell. In addition, it can provide a mechanism for spatial regulation of mRNA expression. However, the precise structural interactions between mRNA and the cytoskeleton have yet to be defined. An objective of this work was to visualize "individual" poly(A) mRNA molecules in situ by electron microscopy to identify their relationship to individual filaments. Poly(A) RNA and filaments were identified simultaneously using antibodies to detect hybridized probe and filaments or actin-binding proteins. In human fibroblasts, most of the poly(A) mRNA (72%) was localized within 5 nm of orthogonal networks of F-actin filaments. Poly(A) mRNA also colocalized with vimentin filaments (29%) and microtubules (< 10%). The sites of mRNA localization were predominantly at filament intersections. The majority of poly(A) mRNA and polysomes colocalized with the actin crosslinking proteins, filamin, and alpha-actinin, and the elongation factor, EF-1 alpha (actin-binding protein; ABP-50). Evidence that intersections contained single mRNA molecules was provided by using a labeled oligo dT probe to prime the synthesis of cDNA in situ using reverse transcriptase. Both the poly(A) and cis sequences of the same mRNA molecule could then be visualized independently. We propose that the cytoskeletal intersection is a mRNA receptor and serves as a "microdomain" where mRNA is attached and functionally expressed.

Sequential rearrangement and nuclear polymerization of actin in baculovirus-infected Spodoptera frugiperda cells

Proper assembly of nucleocapsids of the baculovirus Autographa californica nuclear polyhedrosis virus is prevented by cytochalasin D, a drug that interferes with actin microfilament function. To investigate the involvement of microfilaments in A. californica nuclear polyhedrosis virus replication, a fluorescence microscopy study was conducted that correlated changes in distribution of microfilaments with events in the life cycle of the virus. Tetramethylrhodamine isothiocyanate-labeled phalloidin was used to label microfilaments, and monoclonal antibody was used to label p39, the major viral capsid protein. Three microfilament arrangements were found in infected cells. During uptake of virus, thick cables were formed. These were insensitive to cycloheximide, indicating that this configuration was a rearrangement of preexisting cellular actin mediated by a component of the viral inoculum. At the time of cell rounding and before viral DNA replication, ventral aggregates of actin were observed. These were sensitive to cycloheximide but not to aphidicolin, indicating that an early viral gene mediated this actin rearrangement. Ventral aggregates did not result from the rounding process itself. Uninfected cells prerounded with colchicine did not form ventral aggregates. Cells prerounded with colchicine and then infected did form aggregates. At the time of exponential production of progency virus, microfilaments were found in the nucleus surrounding the virogenic stroma. In this area (where nucleocapsid assembly is known to take place) microfilaments colocalized with p39. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western immunoblot analysis identified p39 among proteins retained on an f-actin affinity column. We postulate that microfilaments in the nucleus provide a scaffold to position capsids for proper assembly and filling with DNA.

Identification of a nuclear antigen with molecular weight of 48,000 differentially expressed in tumour and normal cells

A non-histone protein with mol. wt of 48,000 differentially expressed in normal and tumour cells was identified using immunological criteria. Antibodies were raised against a component specific for Kirkman-Robbins hepatoma of mol. wt about 48,000 separated from hepatoma non-histone proteins by preparative electrophoresis in polyacrylamide gel. It was demonstrated by immunoblotting that Morris hepatoma 7777 and Ehrlich ascites cells share an antigenic non-histone protein with Kirman-Robbins hepatoma. Tumour cells when compared with normal cells, i.e. hamster and rat liver, are characterized by significant enrichment of this component. Intracellular distribution of the polypeptide with mol. wt 48,000 suggests that this component may be a structural protein the biosynthesis of which increases or the antigenic determinants of which change in tumour cells.

Association of actin with DNA and nuclear matrix from Guerin ascites tumour cells

The protein composition of nuclear matrices containing different amount of DNA was examined. It was found that, in matrices containing 2% to 80% of total DNA, the quantity of DNA-bound proteins remains relatively constant varying from 10% to 15% of total nuclear proteins. Electrophoretic patterns do not differ substantially, but autoradiograms with in vitro 125I labelled proteins show quantitative variations in the actin content. Application of radioimmunoassay (RIA) enabled to determine the exact content of actin in GAT nuclei and nuclear matrices - 5 micrograms/ml in nuclei, of which 50% are bound to DNA and 30% being a component of the protein part of the nuclear matrix. These results are supported by electron microscopic data, where immunogold technique was performed on thin sections and spread material. The applied methods suggest that part of the nuclear actin is tightly bound (resistant to 2 M NaCl) to DNA and represents a component of the internal nuclear matrix.

Cytochalasin B selectively releases ovalbumin mRNA precursors but not the mature ovalbumin mRNA from hen oviduct nuclear matrix

Hen oviduct nuclear matrix-bound mature ovalbumin mRNA is released from the matrix in the presence of ATP, while the ovalbumin mRNA precursors remain bound to this structure. Detachment of the mature mRNA from the matrix by ATP as well as ATP-dependent efflux of mRNA from isolated nuclei were found to be inhibited by cytochalasin B. On the other hand, in the absence of ATP, cytochalasin B exclusively caused the release (and nucleocytoplasmic efflux) of the ovalbumin messenger precursors, but not of the mature mRNA. After cytochalasin B treatment, actin could be detected in the matrix supernatant. Phalloidin which stabilizes actin filaments did not cause RNA liberation in the absence of ATP, but inhibited the ATP-induced detachment of mature mRNA. RNA release was also achieved with a monoclonal antibody against actin but not with monoclonal antibodies against tubulin and intermediate filaments. These results suggest that actin-containing filaments are involved in the restriction of immature messengers to the cell nucleus.

Expression of cell-cycle-dependent genes in serum stimulated cells whose entry into S phase is blocked by cytochalasin D

A low concentration (0.6 micrograms/ml) of cytochalasin D inhibits the initiation of DNA synthesis after serum stimulation of growth-arrested GC-7 cells. Since actin-containing structures are suggested to be involved in the transfer of the growth signal to nuclei and in the synthesis and transport of nascent RNA, the effect of cytochalasin D on the expression of cell-cycle-regulated genes after serum stimulation was studied by Northern blot analysis. Cytoplasmic accumulation of such mRNAs as or c-fos, c-myc, beta-actin an ornithine decarboxylase occurred in serum-stimulated cells regardless of the presence of cytochalasin D, whereas that of thymidine kinase and histone H3 was blocked by the drug.

The major nucleoside triphosphatase of nuclear scaffold is distinct from actin

The major nucleoside triphosphatase of rat liver nuclear scaffold, a 46 kD protein thought to participate in nucleocytoplasmic RNA translocation, is distinct from immunologically-identified scaffold actin on Western blots, has a substantially different amino acid composition, and its enzymatic activity is not affected by anti-actin antibodies. Thus, although the contractile protein actin is found in nuclear scaffold and appears to interact with RNA, it is not associated with the nucleoside triphosphatase activity in such preparations.

Preferential association of acidic actin with nuclei and nuclear matrix from mouse leukemia L5178Y cells

Nuclear matrix prepared from mouse leukemia L5178Y cells contained not only the two common actin isomers, beta and gamma actins, but also two additional acidic species of actin (pI 5.1 and 5.3). An anti-actin antibody recognized these acidic species as well as beta and gamma actins on a nitrocellulose filter following western blotting of two-dimensional electrophoresis. These acidic species were co-purified with beta and gamma actins using DNase I-Sepharose affinity chromatography on the nuclear matrix. Limited digestion of the acidic actin with protease V8 or trypsin gave very similar peptide fragments as did digestion of beta and gamma actins. These acidic actins were found to be distributed in the nuclear fraction, but were scarcely detectable in the cytoplasmic fraction. One of the acidic actins (pI 5.3) was found in all subnuclear fractions (DNase extract, high-salt extract and nuclear matrix), while the other species, the most acidic actin (pI 5.1), was localized predominantly in the nuclear matrix.