Nuclear actin dynamics and functions at a glance

Actin is well known for its cytoskeletal functions, where it helps to control and maintain cell shape and architecture, as well as regulating cell migration and intracellular cargo transport, among others. However, actin is also prevalent in the nucleus, where genome-regulating roles have been described, including it being part of chromatin-remodeling complexes. More recently, with the help of advances in microscopy techniques and specialized imaging probes, direct visualization of nuclear actin filament dynamics has helped elucidate new roles for nuclear actin, such as in cell cycle regulation, DNA replication and repair, chromatin organization and transcriptional condensate formation. In this Cell Science at a Glance article, we summarize the known signaling events driving the dynamic assembly of actin into filaments of various structures within the nuclear compartment for essential genome functions. Additionally, we highlight the physiological role of nuclear F-actin in meiosis and early embryonic development.

Actin filaments accumulated in the nucleus and remain in the vicinity of condensing chromosomes during prophase in zebrafish early embryo

In the cytoplasm, filamentous actin (F-actin) plays a critical role in cell regulation, including cell migration, stress fiber formation, and cytokinesis. Recent studies have shown that actin filaments that form in the nucleus are associated with diverse functions. Here, using live imaging of an F-actin-specific probe, superfolder GFP-tagged utrophin (UtrCH-sfGFP), we demonstrated the dynamics of nuclear actin in zebrafish (Danio rerio) embryos. In early zebrafish embryos up to around the high stage, UtrCH-sfGFP increasingly accumulated in nuclei during the interphase and reached a peak during the prophase. After nuclear envelope breakdown (NEBD), patches of UtrCH-sfGFP remained in the vicinity of condensing chromosomes during the prometaphase to metaphase. When zygotic transcription was inhibited by injecting α-amanitin, the nuclear accumulation of UtrCH-sfGFP was still observed at the sphere and dome stages, suggesting that zygotic transcription may induce a decrease in nuclear F-actin. The accumulation of F-actin in nuclei may contribute to proper mitotic progression of large cells with rapid cell cycles in zebrafish early embryos, by assisting in NEBD, chromosome congression, and/or spindle assembly.

β-actin mediated H3K27ac changes demonstrate the link between compartment switching and enhancer-dependent transcriptional regulation

BACKGROUND

Recent work has demonstrated that three-dimensional genome organization is directly affected by changes in the levels of nuclear cytoskeletal proteins such as β-actin. The mechanisms which translate changes in 3D genome structure into changes in transcription, however, are not fully understood. Here, we use a comprehensive genomic analysis of cells lacking nuclear β-actin to investigate the mechanistic links between compartment organization, enhancer activity, and gene expression.

RESULTS

Using HiC-Seq, ATAC-Seq, and RNA-Seq, we first demonstrate that transcriptional and chromatin accessibility changes observed upon β-actin loss are highly enriched in compartment-switching regions. Accessibility changes within compartment switching genes, however, are mainly observed in non-promoter regions which potentially represent distal regulatory elements. Our results also show that β-actin loss induces widespread accumulation of the enhancer-specific epigenetic mark H3K27ac. Using the ABC model of enhancer annotation, we then establish that these epigenetic changes have a direct impact on enhancer activity and underlie transcriptional changes observed upon compartment switching. A complementary analysis of fibroblasts undergoing reprogramming into pluripotent stem cells further confirms that this relationship between compartment switching and enhancer-dependent transcriptional change is not specific to β-actin knockout cells but represents a general mechanism linking compartment-level genome organization to gene expression.

CONCLUSIONS

We demonstrate that enhancer-dependent transcriptional regulation plays a crucial role in driving gene expression changes observed upon compartment-switching. Our results also reveal a novel function of nuclear β-actin in regulating enhancer function by influencing H3K27 acetylation levels.

Double-strand break repair and mis-repair in 3D

DNA double-strand breaks (DSBs) are lesions that arise frequently from exposure to damaging agents as well as from ongoing physiological DNA transactions. Mis-repair of DSBs leads to rearrangements and structural variations in chromosomes, including insertions, deletions, and translocations implicated in disease. The DNA damage response (DDR) limits pathologic mutations and large-scale chromosome rearrangements. DSB repair initiates in 2D at DNA lesions with the stepwise recruitment of repair proteins and local chromatin remodeling which facilitates break accessibility. More complex structures are then formed via protein assembly into nanodomains and via genome folding into chromatin loops. Subsequently, 3D reorganization of DSBs is guided by clustering forces which drive the assembly of repair domains harboring multiple lesions. These domains are further stabilized and insulated into condensates via liquid-liquid phase-separation. Here, we discuss the benefits and risks associated with this 3D reorganization of the broken genome.

Nuclear Actin Dynamics in Gene Expression, DNA Repair, and Cancer

Actin is a highly conserved protein in mammals. The actin dynamics is regulated by actin-binding proteins and actin-related proteins. Nuclear actin and these regulatory proteins participate in multiple nuclear processes, including chromosome architecture organization, chromatin remodeling, transcription machinery regulation, and DNA repair. It is well known that the dysfunctions of these processes contribute to the development of cancer. Moreover, emerging evidence has shown that the deregulated actin dynamics is also related to cancer. This chapter discusses how the deregulation of nuclear actin dynamics contributes to tumorigenesis via such various nuclear events.

The Role of Nuclear Actin in Genome Organization and Gene Expression Regulation During Differentiation

In the cell nucleus, actin participates in numerous essential processes. Actin is involved in chromatin as part of specific ATP-dependent chromatin remodeling complexes and associates with the RNA polymerase machinery to regulate transcription at multiple levels. Emerging evidence has also shown that the nuclear actin pool controls the architecture of the mammalian genome playing an important role in its hierarchical organization into transcriptionally active and repressed compartments, contributing to the clustering of RNA polymerase II into transcriptional hubs. Here, we review the most recent literature and discuss how actin involvement in genome organization impacts the regulation of gene programs that are activated or repressed during differentiation and development. As in the cytoplasm, we propose that nuclear actin is involved in key nuclear tasks in complex with different types of actin-binding proteins that regulate actin function and bridge interactions between actin and various nuclear components.

Impairment of nuclear F-actin formation and its relevance to cellular phenotypes in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder caused by a mutation of lamin A, which contributes to nuclear architecture and the spatial organization of chromatin in the nucleus. The expression of a lamin A mutant, named progerin, leads to functional and structural disruption of nuclear organization. Since progerin lacks a part of the actin-binding site of lamin A, we hypothesized that nuclear actin dynamics and function are altered in HGPS cells. Nuclear F-actin is required for the organization of nuclear shape, transcriptional regulation, DNA damage repair, and activation of Wnt/β-catenin signaling. Here we show that the expression of progerin decreases nuclear F-actin and impairs F-actin-regulated transcription. When nuclear F-actin levels are increased by overexpression of nuclear-targeted actin or by using jasplakinolide, a compound that stabilizes F-actin, the irregularity of nuclear shape and defects in gene expression can be reversed. These observations provide evidence for a novel relationship between nuclear actin and the etiology of HGPS.

Importin-9 regulates chromosome segregation and packaging in Drosophila germ cells

Germ cells undergo distinct nuclear processes as they differentiate into gametes. Although these events must be coordinated to ensure proper maturation, the stage-specific transport of proteins in and out of germ cell nuclei remains incompletely understood. Our efforts to genetically characterize Drosophila genes that exhibit enriched expression in germ cells led to the finding that loss of the highly conserved Importin β/karyopherin family member Importin-9 (Ipo9, herein referring to Ranbp9) results in female and male sterility. Immunofluorescence and fluorescent in situ hybridization revealed that Ipo9KO mutants display chromosome condensation and segregation defects during meiosis. In addition, Ipo9KO mutant males form abnormally structured sperm and fail to properly exchange histones for protamines. Ipo9 physically interacts with proteasome proteins, and Ipo9 mutant males exhibit disruption of the nuclear localization of several proteasome components. Thus, Ipo9 coordinates the nuclear import of functionally related factors necessary for the completion of gametogenesis. This article has an associated First Person interview with the first author of the paper.

Genome organization: Tag it, move it, place it

Chromosomes are selectively organized within the nuclei of interphase cells reflecting the current fate of each cell and are reorganized in response to various physiological cues to maintain homeostasis. Although substantial progress is being made to establish the various patterns of genome architecture, less is understood on how chromosome folding/positioning is achieved. Here, we discuss recent insights into the cellular mechanisms dictating chromatin movements including the use of epigenetic modifications and allosterically regulated transcription factors, as well as a nucleoskeleton system comprised of actin, myosin, and actin-binding proteins. Together, these nuclear factors help coordinate the positioning of both general and cell-specific genomic architectural features.

Nuclear actin in cancer biology

The presence of actin in the nucleus has been a matter of debate for many years. In recent years many important roles of actin in the nucleus (transcriptional regulation, chromatin remodeling, DNA repair, cell division, maintenance of nuclear architecture) have been identified, and the precise control of nuclear actin levels has been demonstrated. The vital importance of the actin driven processes in the cell make it highly likely that dysregulation of nuclear actin dynamics and structure can be linked to tumor induction and -progression. In this chapter I summarize our current knowledge about nuclear actin in the cancer context.

Nuclear actin: The new normal

The presence of actin in the nucleus has historically been a highly contentious issue. It is now, however, well accepted that actin has physiologically important roles in the nucleus. In this Review, we describe the evolution of our thinking about actin in the nucleus starting with evidence supporting its involvement in transcription, chromatin remodeling and intranuclear movements. We also review the growing literature on the mechanisms that regulate the import and export of actin and how post-translational modifications of actin could regulate nuclear actin. We end with an extended discussion of the role of nuclear actin in the repair of DNA double stranded breaks.

When Hippo meets actin in the nucleus

Nuclear actin is exported from the nucleus via the Exportin-6 (XPO6)/RAN GTPase complex. We recently identified that RASSF1A and the hippo pathway kinase Mammalian STE20-like protein kinase 2 (MST2) play a pivotal role in nucleocytoplasmic shuttling of actin by regulating the association of XPO6 with RAN GTPase. Importantly, loss of Ras association domain family 1A (RASSF1A) and hippo signaling in cancer cells highlights a key mechanism by which nuclear actin promotes tumorigenesis.

Transcriptional effects of actin-binding compounds: the cytoplasm sets the tone

Actin has emerged as a versatile regulator of gene transcription. Cytoplasmatic actin regulates mechanosensitive-signaling pathways such as MRTF-SRF and Hippo-YAP/TAZ. In the nucleus, both polymerized and monomeric actin directly interfere with transcription-associated molecular machineries. Natural actin-binding compounds are frequently used tools to study actin-related processes in cell biology. However, their influence on transcriptional regulation and intranuclear actin polymerization is poorly understood to date. Here, we analyze the effects of two representative actin-binding compounds, Miuraenamide A (polymerizing properties) and Latrunculin B (depolymerizing properties), on transcriptional regulation in primary cells. We find that actin stabilizing and destabilizing compounds inversely shift nuclear actin levels without a direct influence on polymerization state and intranuclear aspects of transcriptional regulation. Furthermore, we identify Miuraenamide A as a potent inducer of G-actin-dependent SRF target gene expression. In contrast, the F-actin-regulated Hippo-YAP/TAZ axis remains largely unaffected by compound-induced actin aggregation. This is due to the inability of AMOTp130 to bind to the amorphous actin aggregates resulting from treatment with miuraenamide. We conclude that actin-binding compounds predominantly regulate transcription via their influence on cytoplasmatic G-actin levels, while transcriptional processes relying on intranuclear actin polymerization or functional F-actin networks are not targeted by these compounds at tolerable doses.

Production and Assay of Recombinant Multisubunit Chromatin Remodeling Complexes

We have developed a novel system to facilitate the rapid and easy cloning of multiple genes (>10) in under a week. Using this system we have been able to successfully clone, overexpress, and purify a number of large multimeric proteins from insect cells, including the chromatin remodeling complexes SWR1 and INO80. Using Förster resonance energy transfer (FRET)-based assays we have demonstrated that our overexpressed enzymes have activities comparable to those purified from sources where the proteins are expressed under their endogenous promoters.

Nuclear α-catenin mediates the DNA damage response via β-catenin and nuclear actin

α-Catenin is an F-actin-binding protein widely recognized for its role in cell-cell adhesion. However, a growing body of literature indicates that α-catenin is also a nuclear protein. In this study, we show that α-catenin is able to modulate the sensitivity of cells to DNA damage and toxicity. Furthermore, nuclear α-catenin is actively recruited to sites of DNA damage. This recruitment occurs in a β-catenin-dependent manner and requires nuclear actin polymerization. These findings provide mechanistic insight into the WNT-mediated regulation of the DNA damage response and suggest a novel role for the α-catenin-β-catenin complex in the nucleus.

Actin visualization at a glance

Actin functions in a multitude of cellular processes owing to its ability to polymerize into filaments, which can be further organized into higher-order structures by an array of actin-binding and regulatory proteins. Therefore, research on actin and actin-related functions relies on the visualization of actin structures without interfering with the cycles of actin polymerization and depolymerization that underlie cellular actin dynamics. In this Cell Science at a Glance and the accompanying poster, we briefly evaluate the different techniques and approaches currently applied to analyze and visualize cellular actin structures, including in the nuclear compartment. Referring to the gold standard F-actin marker phalloidin to stain actin in fixed samples and tissues, we highlight methods for visualization of actin in living cells, which mostly apply the principle of genetically fusing fluorescent proteins to different actin-binding domains, such as LifeAct, utrophin and F-tractin, as well as anti-actin-nanobody technology. In addition, the compound SiR-actin and the expression of GFP-actin are also applicable for various types of live-cell analyses. Overall, the visualization of actin within a physiological context requires a careful choice of method, as well as a tight control of the amount or the expression level of a given detection probe in order to minimize its influence on endogenous actin dynamics.

From Cytoskeleton to Gene Expression: Actin in the Nucleus

Although most people still associate actin mainly with the cytoskeleton, several lines of evidence, with the earliest studies dating back to decades ago, have emphasized the importance of actin also inside the cell nucleus. Actin has been linked to many gene expression processes from gene activation to chromatin remodeling, but also to maintenance of genomic integrity and intranuclear movement of chromosomes and chromosomal loci. Recent advances in visualizing different forms and dynamic properties of nuclear actin have clearly advanced our understanding of the basic concepts by which actin operates in the nucleus. In this chapter we address the different breakthroughs in nuclear actin studies, as well as discuss the regulation nuclear actin and the importance of nuclear actin dynamics in relation to its different nuclear functions. Our aim is to highlight the fact that actin should be considered as an essential component of the cell nucleus, and its nuclear actions should be taken into account also in experiments on cytoplasmic actin networks.

Nucleus-associated actin in Amoeba proteus

The presence, spatial distribution and forms of intranuclear and nucleus-associated cytoplasmic actin were studied in Amoeba proteus with immunocytochemical approaches. Labeling with different anti-actin antibodies and staining with TRITC-phalloidin and fluorescent deoxyribonuclease I were used. We showed that actin is abundant within the nucleus as well as in the cytoplasm of A. proteus cells. According to DNase I experiments, the predominant form of intranuclear actin is G-actin which is associated with chromatin strands. Besides, unpolymerized actin was shown to participate in organization of a prominent actin layer adjacent to the outer surface of nuclear envelope. No significant amount of F-actin was found in the nucleus. At the same time, the amoeba nucleus is enclosed in a basket-like structure formed by circumnuclear actin filaments and bundles connected with global cytoplasmic actin cytoskeleton. A supposed architectural function of actin filaments was studied by treatment with actin-depolymerizing agent latrunculin A. It disassembled the circumnuclear actin system, but did not affect the intranuclear chromatin structure. The results obtained for amoeba cells support the modern concept that actin is involved in fundamental nuclear processes that have evolved in the cells of multicellular organisms.

Actin, actin-binding proteins, and actin-related proteins in the nucleus

Extensive research in the past decade has significantly broadened our view about the role actin plays in the life of the cell and added novel aspects to actin research. One of these new aspects is the discovery of the existence of nuclear actin which became evident only recently. Nuclear activities including transcriptional activation in the case of all three RNA polymerases, editing and nuclear export of mRNAs, and chromatin remodeling all depend on actin. It also became clear that there is a fine-tuned equilibrium between cytoplasmic and nuclear actin pools and that this balance is ensured by an export-import system dedicated to actin. After over half a century of research on conventional actin and its organizing partners in the cytoplasm, it was also an unexpected finding that the nucleus contains more than 30 actin-binding proteins and new classes of actin-related proteins which are not able to form filaments but had evolved nuclear-specific functions. The actin-binding and actin-related proteins in the nucleus have been linked to RNA transcription and processing, nuclear transport, and chromatin remodeling. In this paper, we attempt to provide an overview of the wide range of information that is now available about actin, actin-binding, and actin-related proteins in the nucleus.

β- and γ-Actins in the nucleus of human melanoma A375 cells

Actin is a highly conserved protein that is expressed in all eukaryotic cells and has essential functions in the cytoplasm and the nucleus. Nuclear actin is involved in transcription by all three RNA polymerases, chromatin remodelling, RNA processing, intranuclear transport, nuclear export and in maintenance of the nuclear architecture. The nuclear actin level and polymerization state are important factors regulating nuclear processes such as transcription. Our study shows that, in contrast to the cytoplasm, the majority of endogenous nuclear actin is unpolymerized in human melanoma A375 cells. Most mammalian cells express the two non-muscle β- and γ-actin isoforms that differ in only four amino acids. Despite their sequence similarity, studies analysing the cytoplasmic functions of these isoforms demonstrated that β- and γ-actins show differences in localization and function. However, little is known about the involvement of the individual actin isoforms in nuclear processes. Here, we used the human melanoma A375 cell line to analyse actin isoforms in regard to their nuclear localization. We show that both β- and γ-non-muscle actin isoforms are present in nuclei of these cells. Immunolocalization studies demonstrate that both isoforms co-localize with RNA polymerase II and hnRNP U. However, we observe differences in the ratio of cytoplasmic to nuclear actin distribution between the isoforms. We show that β-actin has a significantly higher nucleus-to-cytoplasm ratio than γ-actin.

Nuclear actin and myosins in adenovirus infection

Adenovirus serotypes have been shown to cause drastic changes in nuclear organization, including the transcription machinery, during infection. This ability of adenovirus to subvert transcription in the host cell facilitates viral replication. Because nuclear actin and nuclear myosin I, myosin V and myosin VI have been implicated as direct regulators of transcription and important factors in the replication of other viruses, we sought to determine how nuclear actin and myosins are involved in adenovirus infection. We first confirmed reorganization of the host's transcription machinery to viral replication centers. We found that nuclear actin also reorganizes to sites of transcription through the intermediate but not the advanced late phase of viral infection. Furthermore, nuclear myosin I localized with nuclear actin and sites of transcription in viral replication centers. Intriguingly, nuclear myosins V and VI, which also reorganized to viral replication centers, exhibited different localization patterns, suggesting specialized roles for these nuclear myosins. Finally, we assessed the role of actin in adenovirus infection and found both cytoplasmic and nuclear actin likely play roles in adenovirus infection and replication. Together our data suggest the involvement of actin and multiple myosins in the nuclear replication and late viral gene expression of adenovirus.

Sumoylated α-skeletal muscle actin in the skeletal muscle of adult rats

Skeletal muscles are composed of two major muscle fiber types: slow-twitch oxidative fibers and fast-twitch glycolytic fibers. The proteins in these muscle fibers are known to differ in their expression, relative abundance, and post-translational modifications. In this study, we report a previously unreported post-translational modification of α-skeletal muscle actin in the skeletal muscles of adult male F344 rats in vivo. Using two-dimensional electrophoresis (2D-PAGE), we first examined the differences in the protein expression profiles between the soleus and plantaris muscles. We found higher intensity protein spots at approximately 60 kDa and pH 9 on 2D-PAGE for the soleus muscle compared with the plantaris muscle. These spots were identified as α-skeletal muscle actin by liquid chromatography-nanoelectrospray ionization-tandem mass spectrometry and western blot analyses. In addition, we found that the 60 kDa α-skeletal muscle actin is modified by small ubiquitin-like modifier (SUMO) 1, using 2D-PAGE and western blot analyses. Furthermore, we found that α-skeletal muscle actin with larger molecular weight was localized in the nuclear and cytosol of the skeletal muscle, but not in the myofibrillar fraction by the combination of subcellular fractionation and western blot analyses. These results suggest that α-skeletal muscle actin is modified by SUMO-1 in the skeletal muscles, localized in nuclear and cytosolic fractions, and the extent of this modification is much higher in the slow muscles than in the fast muscles. This is the first study to show the presence of SUMOylated actin in animal tissues.

Extracellular signaling cues for nuclear actin polymerization

Contrary to cytoplasmic actin structures, the biological functions of nuclear actin filaments remain largely enigmatic. Recent progress in the field, however, has determined nuclear actin structures in somatic cells either under steady state conditions or in response to extracellular signaling cues. These actin structures differ in size and shape as well as in their temporal appearance and dynamics. Thus, a picture emerges that suggests that mammalian cells may have different pathways and mechanisms to assemble nuclear actin filaments. Apart from serum- or LPA-triggered nuclear actin polymerization, integrin activation by extracellular matrix interaction was recently implicated in nuclear actin polymerization through the linker of nucleoskeleton and cytoskeleton (LINC) complex. Some of these extracellular cues known so far appear to converge at the level of nuclear formin activity and subsequent regulation of myocardin-related transcription factors. Nevertheless, as the precise signaling events are as yet unknown, the regulation of nuclear actin polymerization may be of significant importance for different cellular functions as well as disease conditions caused by altered nuclear dynamics and architecture.

Proneural proteins Achaete and Scute associate with nuclear actin to promote formation of external sensory organs

Basic helix-loop-helix (bHLH) proneural proteins promote neurogenesis through transcriptional regulation. Although much is known about the tissue-specific regulation of proneural gene expression, how proneural proteins interact with transcriptional machinery to activate downstream target genes is less clear. Drosophila proneural proteins Achaete (Ac) and Scute (Sc) induce external sensory organ formation by activating neural precursor gene expression. Through co-immunoprecipitation and mass spectrometric analyses, we found that nuclear but not cytoplasmic actin associated with the Ac and Sc proteins in Drosophila S2 cells. Daughterless (Da), the common heterodimeric partner of Drosophila bHLH proteins, was observed to associate with nuclear actin through proneural proteins. A yeast two-hybrid assay revealed that the binding specificity between actin and Ac or Sc was conserved in yeast nuclei without the presence of additional Drosophila factors. We further show that actin is required in external sensory organ formation. Reduction in actin gene activity impaired proneural-protein-dependent expression of the neural precursor genes, as well as formation of neural precursors. Furthermore, increased nuclear actin levels, obtained by expression of nucleus-localized actin, elevated Ac-Da-dependent gene transcription as well as Ac-mediated external sensory organ formation. Taken together, our in vivo and in vitro observations suggest a novel link for actin in proneural-protein-mediated transcriptional activation and neural precursor differentiation.

Actin is required for cellular death

Actin is one of the most abundant cytoskeletal proteins, which takes part in many cellular processes. This review provides information on the history, forms and localization of actin and its role, in particular in cellular death processes. We discuss the relationships between reorganization of actin filaments and apoptosis, mitotic catastrophe and differentiation. Finally, we discuss the translocation and accumulation of actin in the nuclear area. Moreover, owing to the difficulties of F-actin localization by transmission electron microscopy (TEM), the phalloidin-based method of its detection using streptavidin-coated quantum dots is presented in this review.

Stimulation of in vivo nuclear transport dynamics of actin and its co-factors IQGAP1 and Rac1 in response to DNA replication stress

Actin, a constituent of the cytoskeleton, is now recognized to function in the nucleus in gene transcription, chromatin remodeling and DNA replication/repair. Actin shuttles in and out of the nucleus through the action of transport receptors importin-9 and exportin-6. Here we have addressed the impact of cell cycle progression and DNA replication stress on actin nuclear localization, through study of actin dynamics in living cells. First, we showed that thymidine-induced G1/S phase cell cycle arrest increased the nuclear levels of actin and of two factors that stimulate actin polymerization: IQGAP1 and Rac1 GTPase. When cells were exposed to hydroxyurea to induce DNA replication stress, the nuclear localization of actin and its regulators was further enhanced. We employed live cell photobleaching assays and discovered that in response to DNA replication stress, GFP-actin nuclear import and export rates increased by up to 250%. The rate of import was twice as fast as export, accounting for actin nuclear accumulation. The faster shuttling dynamics correlated with reduced cellular retention of actin, and our data implicate actin polymerization in the stress-dependent uptake of nuclear actin. Furthermore, DNA replication stress induced a nuclear shift in IQGAP1 and Rac1 with enhanced import dynamics. Proximity ligation assays revealed that IQGAP1 associates in the nucleus with actin and Rac1, and formation of these complexes increased after hydroxyurea treatment. We propose that the replication stress checkpoint triggers co-ordinated nuclear entry and trafficking of actin, and of factors that regulate actin polymerization.

Co-transcriptional nuclear actin dynamics

Actin is a key player for nuclear structure and function regulating both chromosome organization and gene activity. In the cell nucleus actin interacts with many different proteins. Among these proteins several studies have identified classical nuclear factors involved in chromatin structure and function, transcription and RNA processing as well as proteins that are normally involved in controlling the actin cytoskeleton. These discoveries have raised the possibility that nuclear actin performs its multi task activities through tight interactions with different sets of proteins. This high degree of promiscuity in the spectrum of protein-to-protein interactions correlates well with the conformational plasticity of actin and the ability to undergo regulated changes in its polymerization states. Several of the factors involved in controlling head-to-tail actin polymerization have been shown to be in the nucleus where they seem to regulate gene activity. By focusing on the multiple tasks performed by actin and actin-binding proteins, possible models of how actin dynamics controls the different phases of the RNA polymerase II transcription cycle are being identified.