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

Actin: its cumbersome pilgrimage through cellular compartments

In this article, we follow the history of one of the most abundant, most intensely studied proteins of the eukaryotic cells: actin. We report on hallmarks of its discovery, its structural and functional characterization and localization over time, and point to present days’ knowledge on its position as a member of a large family. We focus on the rather puzzling number of diverse functions as proposed for actin as a dual compartment protein. Finally, we venture on some speculations as to its origin.

Nuclear actin dynamics--from form to function

Cell biological functions of actin have recently expanded from cytoplasm to nucleus, with actin implicated in such diverse processes as gene expression, transcription factor regulation and intranuclear motility. Actin in the nucleus seems to behave differently than in the cytoplasm, raising new questions regarding the molecular mechanisms by which actin functions in cells. In this review, I will discuss dynamic properties of nuclear actin that are related to its polymerization cycle and nucleocytoplasmic shuttling. By comparing the behaviour of nuclear and cytoplasmic actin and their regulators, I try to dissect the underlying differences of these equally important cellular actin pools.

Nuclear actin is involved in the regulation of CSF1 gene transcription in a chromatin required, BRG1 independent manner

Actin is an important protein in nucleus and has been implicated in transcription, however, the mechanism of its function in transcription is still not clear. In this article, we studied the role of actin in the regulation of human CSF1 gene transcription. Our results showed that nuclear actin stimulates the activity of CSF1 promoter, and the role in augmenting CSF1 gene transcription requires the formation of chromatin and Z-DNA structure. The ATP binding motifs of nuclear actin are essential for its function in regulating CSF1 gene transcription, and upon actin overexpression, there is an increase in the ATPase activity of nuclear proteins. Further investigation revealed that nuclear actin regulates CSF1 gene transcription in a BRG1 independent manner. Together, these original results have provided evidence for further understanding the mechanism of nuclear actin in regulating gene transcription.

Role of nuclear lamina-cytoskeleton interactions in the maintenance of cellular strength

The response of individual cells to cellular stress is vital for cellular functioning. A large network of physically interconnected cellular components, starting from the structural components of the cells' nucleus, via cytoskeleton filaments to adhesion molecules and the extracellular matrix, constitutes an integrated matrix that functions as a scaffold allowing the cell to cope with mechanical stress. Next to a role in mechanical properties, this network also has a mechanotransductional function in the response to mechanical stress. This signaling route does not only regulate a rapid reorganization of structural components such as actin filaments, but also stimulates for example gene activation via NFkappaB and other transcription factors. The importance of an intact mechano-signaling network is illustrated by the physiological consequences of several genetic defects of cellular network components e.g. actin, dystrophin, desmin and lamins. These give rise to an impaired response of the affected cells to mechanical stress and often result in dystrophy of the affected tissue. Recently, the importance of the cell nucleus in cellular strength has been established. Several new interconnecting proteins, such as the nesprins that link the nuclear lamina to the cytoskeleton, have been identified. Furthermore, the function of nuclear lamins in determining cellular strength and nuclear stability was illustrated in lamin-knock-out cells. Absence of the A-type lamins or mutations in these structural components of the nuclear lamina lead to an impaired cellular response to mechanical stress and disturbances in cytoskeletal organization. In addition, laminopathies show clinical phenotypes comparable to those seen for diseases resulting from genetic defects in cytoskeletal components, further indicating that lamins play a central role in maintaining the mechanical properties of the cell.

Nuclear actin: to polymerize or not to polymerize

The form and function of actin in the nucleus have been enigmatic for over 30 years. Recently actin has been assigned numerous functional roles in the nucleus, but its form remains a mystery. The intricate relationship between actin form and function in the cytoplasm implies that understanding the structural properties of nuclear actin is elementary to fully understanding its function. In this issue, McDonald et al. (p. 541) use fluorescence recovery after photobleaching (FRAP) to tackle the question of whether nuclear actin exists as monomers or polymers.

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.

The septin-binding protein anillin is overexpressed in diverse human tumors

Anillin is an actin-binding protein that can bind septins and is a component of the cytokinetic ring. We assessed the anillin expression in 7,579 human tissue samples and cell lines by DNA microarray analysis. Anillin is expressed ubiquitously but with variable levels of expression, being highest in the central nervous system. The median level of anillin mRNA expression was higher in tumors than normal tissues (median fold increase 2.58; 95% confidence intervals, 2.19-5.68, P < 0.0001) except in the central nervous system where anillin mRNA levels were lower in tumors. We developed a sensitive reverse transcription-PCR strategy to show that anillin mRNA is expressed in cell lines and in cDNA panels derived from fetal and adult tissues, thus validating the microarray data. We compared anillin with Ki67 mRNA expression and found a significant linear relationship between anillin and Ki67 mRNA expression (Spearmann r approximately 0.6, P < 0.0001). Anillin mRNA expression was analyzed during tumor progression in breast, ovarian, kidney, colorectal, hepatic, lung, endometrial, and pancreatic tumors and in all tissues there was progressive increase in anillin mRNA expression from normal to benign to malignant to metastatic disease. Finally, we used anti-anillin sera and found nuclear anillin immunoreactivity to be widespread in normal tissues, often not correlating with proliferative compartments. These data provide insight into the existence of nonproliferation-associated activities of anillin and roles in interphase nuclei. Thus, anillin is overexpressed in diverse common human tumors, but not simply as a consequence of being a proliferation marker. Anillin may have potential as a novel biomarker.

The nuclear envelope: form and reformation

The membrane system that encloses genomic DNA is referred to as the nuclear envelope. However, with emerging roles in signaling and gene expression, these membranes clearly serve as more than just a physical barrier separating the nucleus and cytoplasm. Recent progress in our understanding of nuclear envelope architecture and composition has also revealed an intriguing connection between constituents of the nuclear envelope and human disease, providing further impetus to decipher this cellular structure and the dramatic remodeling process it undergoes with each cell division.

Nuclear actin extends, with no contraction in sight

Within the past two years, actin has been implicated in eukaryotic gene transcription by all three classes of RNA polymerase. Moreover, within just the past year, actin has been identified as a constituent of filaments attached to the nuclear pore complexes and extending into the nucleus. This review summarizes these and other very recent advances in the nuclear actin field and emphasizes the key present issues. On the one hand, we are confronted with a body of evidence for a role of actin in gene transcription but with no known structural basis; on the other hand, there is now evidence for polymeric actin--not likely in the classical F-actin conformation--in the nuclear periphery with no known function. In addition, numerous proteins that interact with either G- or F-actin are increasingly being detected in the nucleus, suggesting that both monomeric and oligomeric or polymeric forms of actin are at play and raising the possibility that the equilibrium between them, perhaps differentially regulated at various intranuclear sites, may be a major determinant of nuclear function.

Actin and myosin I in the nucleus: what next?

Several recent publications have demonstrated the importance of nuclear actin and nuclear myosin I in transcription. Here we review these publications and their implications. In addition, we discuss some important issues that should be addressed to gain a more comprehensive understanding of how these traditionally 'cytoplasmic' proteins are involved in transcription. We propose highly speculative models and mechanisms solely to stimulate thought and experimentation in this area.

Meiotic telomere clustering requires actin for its formation and cohesin for its resolution

In diploid organisms, meiosis reduces the chromosome number by half during the formation of haploid gametes. During meiotic prophase, telomeres transiently cluster at a limited sector of the nuclear envelope (bouquet stage) near the spindle pole body (SPB). Cohesin is a multisubunit complex that contributes to chromosome segregation in meiosis I and II divisions. In yeast meiosis, deficiency for Rec8 cohesin subunit induces telomere clustering to persist, whereas telomere cluster-SPB colocalization is defective. These defects are rescued by expressing the mitotic cohesin Scc1 in rec8delta meiosis, whereas bouquet-stage exit is independent of Cdc5 pololike kinase. An analysis of living Saccharomyces cerevisiae meiocytes revealed highly mobile telomeres from leptotene up to pachytene, with telomeres experiencing an actin- but not microtubule-dependent constraint of mobility during the bouquet stage. Our results suggest that cohesin is required for exit from actin polymerization-dependent telomere clustering and for linking the SPB to the telomere cluster in synaptic meiosis.

Spectrin repeat proteins in the nucleus

Spectrin repeat sequences are among the more common repeat elements identified in proteins, typically occurring in large structural proteins. Examples of spectrin repeat-containing proteins include dystrophin, alpha-actinin and spectrin itself--all proteins with well-demonstrated roles of establishing and maintaining cell structure. Over the past decade, it has become clear that, although these proteins display a cytoplasmic and plasma membrane distribution, several are also found both at the nuclear envelope, and within the intranuclear space. In this review, we provide an overview of recent work regarding various spectrin repeat-containing structural proteins in the nucleus. As well, we hypothesize about the regulation of their nuclear localization and possible nuclear functions based on domain architecture, known interacting proteins and evolutionary relationships. Given their large size, and their potential for interacting with multiple proteins and with chromatin, spectrin repeat-containing proteins represent strong candidates for important organizational proteins within the nucleus. Supplementary material for this article can be found on the BioEssays website (http://www.interscience.wiley.com/jpages/0265-9247/suppmat/index.html).

Nuclear actin is partially associated with Cajal bodies in human cells in culture and relocates to the nuclear periphery after infection of cells by adenovirus 5

Cajal bodies are intra-nuclear structures enriched in proteins involved in transcription and mRNA processing. In this study, immunofluorescence microscopy experiments using a highly specific antibody to actin revealed nuclear actin spots that colocalized in part with p80 coilin-positive Cajal bodies. Actin remained associated with Cajal bodies in cells extracted to reveal the nuclear matrix. Adenovirus infection, which is known to disassemble Cajal bodies, resulted in loss of actin from these structures late in infection. In infected cells, nuclear actin was observed to relocate to structures at the periphery of the nucleus, inside the nuclear envelope. Based on these findings, it is suggested that actin may play an important role in the organization or function of the Cajal body.

The nuclear lamina comes of age

Many nuclear proteins form lamin-dependent complexes, including LEM-domain proteins, nesprins and SUN-domain proteins. These complexes have roles in chromatin organization, gene regulation and signal transduction. Some link the nucleoskeleton to cytoskeletal structures, ensuring that the nucleus and centrosome assume appropriate intracellular positions. These complexes provide new insights into cell architecture, as well as a foundation for the understanding of the molecular mechanisms that underlie the human laminopathies - clinical disorders that range from Emery-Dreifuss muscular dystrophy to the accelerated ageing seen in Hutchinson-Gilford progeria syndrome.

Laminopathies: Involvement of structural nuclear proteins in the pathogenesis of an increasing number of human diseases

Just at the beginning of the millennium the neologism laminopathies has been introduced in the scientific vocabulary. An exponential increase of interest on the subject started concomitantly, so that a formerly quite neglected group of rare human diseases is now widely investigated. This review will cover the history of the identification of the molecular basis for fourteen (since now) hereditary diseases arising from defects in genes that encode nuclear envelope and nuclear lamina-associated proteins and will also consider the hypotheses that can account for the role of structural nuclear proteins in the pathogenesis of diseases affecting a wide spectrum of tissues.

Breaking and making of the nuclear envelope

During mitosis, a single nucleus gives rise to two nuclei that are identical to the parent nucleus. Mitosis consists of a continuous sequence of events that must be carried out once and only once. Two such important events are the disassembly of the nuclear envelope (NE) during the first stages of mitosis, and its accurate reassembly during the last stages of mitosis. NE breakdown (NEBD) is initiated when maturation-promoting factor (MPF) enters the nucleus and starts phosphorylating nuclear pore complexes (NPCs) and nuclear lamina proteins, followed by NPC and lamina breakdown. Nuclear reassembly starts when nuclear membranes assemble onto the chromatin. This article focuses on the different models of NEBD and reassembly with emphasis on recent data.

Economy, speed and size matter: evolutionary forces driving nuclear genome miniaturization and expansion

BACKGROUND

Nuclear genome size varies 300 000-fold, whereas transcriptome size varies merely 17-fold. In the largest genomes nearly all DNA is non-genic secondary DNA, mostly intergenic but also within introns. There is now compelling evidence that secondary DNA is functional, i.e. positively selected by organismal selection, not the purely neutral or 'selfish' outcome of mutation pressure. The skeletal DNA theory argued that nuclear volumes are genetically determined primarily by nuclear DNA amounts, modulated somewhat by genes affecting the degree of DNA packing or unfolding; the huge spread of nuclear genome sizes is the necessary consequence of the origin of the nuclear envelope and the nucleation of its assembly by DNA, plus the adaptively significant 300 000-fold range of cell volumes and selection for balanced growth by optimizing karyoplasmic volume ratios (essentially invariant with cell volume in growing/multiplying cells). This simple explanation of the C-value paradox is refined here in the light of new insights into the nature of heterochromatin and the nuclear lamina, the genetic control of cell volume, and large-scale eukaryote phylogeny, placing special emphasis on protist test cases of the basic principles of nuclear genome size evolution.

GENOME MINIATURIZATION

and Expansion Intracellular parasites (e.g. Plasmodium, microsporidia) dwarfed their genomes by gene loss and eliminating virtually all secondary DNA. The primary driving forces for genome reduction are metabolic and spatial economy and cell multiplication speed. Most extreme nuclear shrinkage yielded genomes as tiny as 0.38 Mb (making the nuclear genome size range effectively 1.8 million-fold!) in some minute enslaved nuclei (nucleomorphs) of cryptomonads and chlorarachneans, chimaeric cells that also retain a separate normal large nucleus. The latter shows typical correlation between genome size and cell volume, but nucleomorphs do not despite co-existing in the same cell for >500 My. Thus mutation pressure does not inexorably increase genome size; selection can eliminate essentially all non-coding DNA if need be. Nucleomorphs and microsporidia even reduced gene size. Expansion of secondary DNA in the main nucleus, and in large-celled eukaryotes generally, must be positively selected for function. Ciliate nuclear dimorphism provides a key test that refutes the selfish DNA and strongly supports the skeletal DNA/karyoplasmic ratio interpretation of genome size evolution.

GENETIC CONTROL OF CELL VOLUME IS MULTIGENIC

The quantitatively proportional correlation between genome size and cell size cannot be explained by purely mutational theories, as eukaryote cell volumes are causally determined by cell cycle control genes, not by DNA amounts.

Proteome Analysis of NIH3T3 Cells Transformed by Activated Gα12: Regulation of Leukemia-Associated Protein SET

Galpha(12), the alpha-subunit of the G12 family of heterotrimeric G proteins is involved in the regulation of cell proliferation and neoplastic transformation. GTPase-deficient, constitutively activated mutant of Galpha(12) (Galpha(12)Q229L or Galpha(12)QL) has been previously shown to induce oncogenic transformation of NIH3T3 cells promoting serum- and anchorage-independent growth. Reduced growth-factor dependent, autonomous cell growth forms a critical defining point at which a normal cell turns into an oncogenic one. To identify the underlying mechanism involved in such growth-factor/serum independent growth of Galpha(12)QL-transformed NIH3T3, we carried out a two-dimensional differential proteome analysis of Galpha(12)QL-transformed NIH3T3 cells and cells expressing vector control. This analysis revealed a total of 22 protein-spots whose expression was altered by more than 3-folds. Two of these spots were identified by MALDI-MS analysis as proliferating cell nuclear antigen (PCNA) and myeloid-leukemia-associated SET protein. The increased expressions of these proteins in Galpha(12)QL cells were validated by immunoblot analysis. Furthermore, transient transfection studies with NIH3T3 cells indicated that the expression of activated Galpha(12) readily increased the expression of SET protein by 24 h. As SET has been previously reported to be an inhibitor of phosphatase PP2A, the nuclear phosphatase activity was monitored in cells expressing activated Galpha(12). Our results indicate that the nuclear phosphatase activity is inhibited by greater than 50% in Galpha(12)QL cells compared to vector control cells. Thus, our results from differential proteome analysis presented here report for the first time a role for SET in Galpha(12)-mediated signaling pathways and a role for Galpha(12) in the regulation of the leukemia-associated SET-protein expression.

Mitotic regulation of protein 4.1R involves phosphorylation by cdc2 kinase

The nonerythrocyte isoform of the cytoskeletal protein 4.1R (4.1R) is associated with morphologically dynamic structures during cell division and has been implicated in mitotic spindle function. In this study, we define important 4.1R isoforms expressed in interphase and mitotic cells by RT-PCR and mini-cDNA library construction. Moreover, we show that 4.1R is phosphorylated by p34cdc2 kinase on residues Thr60 and Ser679 in a mitosis-specific manner. Phosphorylated 4.1R135 isoform(s) associate with tubulin and Nuclear Mitotic Apparatus protein (NuMA) in intact HeLa cells in vivo as well as with the microtubule-associated proteins in mitotic asters assembled in vitro. Recombinant 4.1R135 is readily phosphorylated in mitotic extracts and reconstitutes mitotic aster assemblies in 4.1R-immunodepleted extracts in vitro. Furthermore, phosphorylation of these residues appears to be essential for the targeting of 4.1R to the spindle poles and for mitotic microtubule aster assembly in vitro. Phosphorylation of 4.1R also enhances its association with NuMA and tubulin. Finally, we used siRNA inhibition to deplete 4.1R from HeLa cells and provide the first direct genetic evidence that 4.1R is required to efficiently focus mitotic spindle poles. Thus, we suggest that 4.1R is a member of the suite of direct cdc2 substrates that are required for the establishment of a bipolar spindle.

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.

Nuclear Envelope Breakdown Requires Overcoming the Mechanical Integrity of the Nuclear Lamina

In prophase cells, lamin B1 is the major component of the nuclear lamina, a filamentous network underlying the nucleoplasmic side of the nuclear membrane, whereas lamin A/C is dissociated from the scaffold. In vivo fluorescence microscopy studies have shown that, during the G2/M transition, the first gap in the nuclear envelope (NE) appears before lamin B1 disassembly and is caused by early spindle microtubules impinging on the NE. This result suggests that the mechanical tearing of the NE by microtubules plays a central role to the progression of mitosis. To investigate whether this microtubule-induced NE deformation is sufficient for NE breakdown, we assess the mechanical resilience of a reconstituted lamin B1 network. Quantitative rheological methods demonstrate that human lamin B1 filaments form stiff networks that can resist much greater deformations than those caused by microtubules impinging on the NE. Moreover, lamin B1 networks possess an elastic stiffness, which increases under tension, and an exceptional resilience against shear deformations. These results demonstrate that both mechanical tearing of the lamina and biochemical modification of lamin B1 filaments are required for NE breakdown.

Phorbol ester-induced translocation of PKC epsilon to the nucleus in fibroblasts: identification of nuclear PKC epsilon-associating proteins

We show that phorbol ester treatment of NIH 3T3 fibroblasts induces rapid translocation of PKC from a perinuclear site to the nucleus, extending findings in PC12 and NG108-15 cells and in myocytes. We have immunoprecipitated the PKC from nuclei isolated from phorbol ester-treated fibroblasts and identified six proteins which associate with nuclear PKC. These have been characterised as matrin 3, transferrin, Rac GTPase activating protein 1, vimentin, beta-actin and annexin II by MALDI-TOF-MS. We have confirmed that these proteins associate with PKC by gel overlay and/or dot blotting assays. The role of these PKC-associating proteins in the nucleus and their interaction with PKC are considered.

Actin and ARPs: action in the nucleus

Actin is an ancient and abundant protein with well-established roles in fundamental processes ranging from cell migration to membrane transport. Most eukaryotic cells also contain at least eight actin-related proteins (ARPs) that are, themselves, conserved between organisms as divergent as yeast and mammals. Although many ARPs are cytoskeletal, recent biochemical and genetic work has demonstrated that some ARPs function largely or entirely in the nucleus. Evidence for the participation of both actin and ARPs in chromatin remodeling is becoming conclusive, and support for the still controversial involvement of actin in processes ranging from transcription to nuclear assembly is growing. The existence of conserved nuclear ARPs, together with accumulating biochemical, genetic and cell biology data, points to ancient and fundamental roles of actin in the nucleus, but the nature of these roles is just beginning to be revealed.

snRNPs are present in the karyosome capsule in the weevil germinal vesicle

Within the oocyte nucleus of the apple blossom weevil, Anthonomus pomorum (Insecta, Coleoptera) highly condensed and transcriptionaly inactive chromosomes form the karyosome. During its formation, within the nucleoplasm numerous, variably sized spherical inclusions termed nuclear bodies occur. As oogenesis progresses, the karyosome is gradually surrounded by a prominent sheath, the karyosome capsule. The function and molecular composition of both the nuclear bodies and the karyosome capsule are largely unknown. Using cytochemical methods we demonstrate that DNA is confined to the karyosome and there is no extrachromosomal DNA accumulations within the nucleoplasm. In addition, none of the oocyte nucleus subdomains contain argyrophilic proteins. Our immunoEM study revealed that in contrast to similar structures in germinal vesicles in other insect species, the nuclear bodies of A. pomorum do not cross-react with antibodies recognising small nuclear ribonucleoproteins, coilin or the splicing factor SC-35. Unexpectedly, we found that as the karyosome capsule develops, mature small nuclear RNAs and proteins containing the Sm epitope associate with the capsule material. We suggest that the karyosome capsule is a storage site for small nuclear ribonucleoprotein particles, which may be used during early embryonic development.

Eukaryotic cells and their cell bodies: Cell Theory revised

BACKGROUND

Cell Theory, also known as cell doctrine, states that all eukaryotic organisms are composed of cells, and that cells are the smallest independent units of life. This Cell Theory has been influential in shaping the biological sciences ever since, in 1838/1839, the botanist Matthias Schleiden and the zoologist Theodore Schwann stated the principle that cells represent the elements from which all plant and animal tissues are constructed. Some 20 years later, in a famous aphorism Omnis cellula e cellula, Rudolf Virchow annunciated that all cells arise only from pre-existing cells. General acceptance of Cell Theory was finally possible only when the cellular nature of brain tissues was confirmed at the end of the 20th century. Cell Theory then rapidly turned into a more dogmatic cell doctrine, and in this form survives up to the present day. In its current version, however, the generalized Cell Theory developed for both animals and plants is unable to accommodate the supracellular nature of higher plants, which is founded upon a super-symplasm of interconnected cells into which is woven apoplasm, symplasm and super-apoplasm. Furthermore, there are numerous examples of multinucleate coenocytes and syncytia found throughout the eukaryote superkingdom posing serious problems for the current version of Cell Theory.

SCOPE

To cope with these problems, we here review data which conform to the original proposal of Daniel Mazia that the eukaryotic cell is composed of an elemental Cell Body whose structure is smaller than the cell and which is endowed with all the basic attributes of a living entity. A complement to the Cell Body is the Cell Periphery Apparatus, which consists of the plasma membrane associated with other periphery structures. Importantly, boundary structures of the Cell Periphery Apparatus, although capable of some self-assembly, are largely produced and maintained by Cell Body activities and can be produced from it de novo. These boundary structures serve not only as mechanical support for the Cell Bodies but they also protect them from the hostile external environment and from inappropriate interactions with adjacent Cell Bodies within the organism.

CONCLUSIONS

From the evolutionary perspective, Cell Bodies of eukaryotes are proposed to represent vestiges of hypothetical, tubulin-based 'guest' proto-cells. After penetrating the equally hypothetical actin-based 'host' proto-cells, tubulin-based 'guests' became specialized for transcribing, storing and partitioning DNA molecules via the organization of microtubules. The Cell Periphery Apparatus, on the other hand, represents vestiges of the actin-based 'host' proto-cells which have become specialized for Cell Body protection, shape control, motility and for actin-mediated signalling across the plasma membrane.

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.

Two Protein 4.1 Domains Essential for Mitotic Spindle and Aster Microtubule Dynamics and Organization in Vitro

Multifunctional structural proteins belonging to the 4.1 family are components of nuclei, spindles, and centrosomes in vertebrate cells. Here we report that 4.1 is critical for spindle assembly and the formation of centrosome-nucleated and motor-dependent self-organized microtubule asters in metaphase-arrested Xenopus egg extracts. Immunodepletion of 4.1 disrupted microtubule arrays and mislocalized the spindle pole protein NuMA. Remarkably, assembly was completely rescued by supplementation with a recombinant 4.1R isoform. We identified two 4.1 domains critical for its function in microtubule polymerization and organization utilizing dominant negative peptides. The 4.1 spectrin-actin binding domain or NuMA binding C-terminal domain peptides caused morphologically disorganized structures. Control peptides with low homology or variant spectrin-actin binding domain peptides that were incapable of binding actin had no deleterious effects. Unexpectedly, the addition of C-terminal domain peptides with reduced NuMA binding caused severe microtubule destabilization in extracts, dramatically inhibiting aster and spindle assembly and also depolymerizing preformed structures. However, the mutant C-terminal peptides did not directly inhibit or destabilize microtubule polymerization from pure tubulin in a microtubule pelleting assay. Our data showing that 4.1 is a crucial factor for assembly and maintenance of mitotic spindles and self-organized and centrosome-nucleated microtubule asters indicates that 4.1 is involved in regulating both microtubule dynamics and organization. These investigations underscore an important functional context for protein 4.1 in microtubule morphogenesis and highlight a previously unappreciated role for 4.1 in cell division.