Nuclear actin dynamics – From form to function (original) (raw)
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New Insights into Cellular Functions of Nuclear Actin
Biology
Actin is one of the most abundant proteins in eukaryotic cells. There are different pools of nuclear actin often undetectable by conventional staining and commercial antibodies used to identify cytoplasmic actin. With the development of more sophisticated imaging and analytical techniques, it became clear that nuclear actin plays a crucial role in shaping the chromatin, genomic, and epigenetic landscape, transcriptional regulation, and DNA repair. This multifaceted role of nuclear actin is not only important for the function of the individual cell but also for the establishment of cell fate, and tissue and organ differentiation during development. Moreover, the changes in the nuclear, chromatin, and genomic architecture are preamble to various diseases. Here, we discuss some of the newly described functions of nuclear actin.
Tracking down the different forms of nuclear actin
Trends in Cell Biology
Actin is a rather uncommitted protein with a high degree of structural plasticity: it can adopt a variety of structural states, depending on the specific ionic conditions or the interaction with ligand proteins. These interactions lock actin into a distinct conformation, which specifies the oligomeric or polymeric form it can assume. The interplay between monomeric, oligomeric and polymeric forms is used by the cell to execute an enormous variety of motility processes, such as lamellipodium formation during locomotion or intracellular transport of vesicles. In these cytoplasmic events, monomeric G-actin and filamentous F-actin are the prevalent forms. However, there might be other structural states of actin in cells that have so far not received the attention they deserve. Here, we propose that specific, "unconventional" actin conformations might contribute especially to the multitude of functions executed by actin in the nucleus. We present evidence for the existence of d...
Persistent nuclear actin filaments inhibit transcription by RNA polymerase II
Journal of Cell Science, 2016
Actin is abundant in the nucleus and it is clear that nuclear actin has important functions. Yet, mystery surrounds the absence of classical actin filaments in the nucleus. To address this question, we investigated how polymerizing nuclear actin into persistent nuclear actin filaments affected transcription by RNA polymerase II. Nuclear filaments impaired nuclear actin dynamics by polymerizing and sequestering nuclear actin. Polymerizing actin into stable nuclear filaments disrupted the interaction of actin with RNA polymerase II and correlated with impaired RNA polymerase II localization, dynamics, gene recruitment, and reduced global transcription and cell proliferation. Polymerizing and crosslinking nuclear actin in vitro similarly disrupted the actin/RNA polymerase II interaction and inhibited transcription. These data rationalize the general absence of stable actin filaments in mammalian somatic nuclei. They also suggest a dynamic pool of nuclear actin is required for the prope...
Nuclear actin and protein 4.1: Essential interactions during nuclear assembly in vitro
Proceedings of the National Academy of Sciences, 2003
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, a...
Analysis of nuclear actin-interacting proteins and actin-regulated transcription factors
2019
Actin is best-known from its functions in the cytoplasm, where it is a key component of the cytoskeleton. Cytoskeleton is vital for cells as it enables cell movement and maintains cell shape. Nevertheless, functions of actin are not restricted to the cytoplasm, since actin is also present in the nucleus, where it has been linked to multiple functions from gene activation to chromatin remodeling. Live cell imaging with different nuclear actin probes have demonstrated the importance of actin dynamics inside the nucleus, but the molecular mechanisms by which actin operates in the nucleus are still poorly understood. This is mainly because of the lack of wellcharacterized binding partners for nuclear actin. Therefore, the aim of this thesis was to identify and characterize novel nuclear actin-binding partners and elucidate the molecular mechanisms behind actin regulated transcription factors. To identify nuclear actin-binding partners, we used two complementary mass spectrometry (MS) techniques, affinity purification combined with MS (AP-MS) and proximity dependent biotin identification with MS (BioID). AP-MS protocol was optimized to preserve complete nuclear complexes and BioID was geared towards identifying more transient interactions. We utilized different actin constructs to discriminate nuclear versus cytoplasmic interactions and to assess the requirement for actin polymerization for the putative nuclear interactions. Analysis of our interactome data revealed that actin can form stable complexes with proteins related to chromatin remodeling but seems to function in a dynamic fashion in other nuclear processes, such as transcription and DNA replication. In our experimental setup actin seemed to be monomeric when it associated with nuclear complexes. We also discovered a novel actin-containing complex, human Ada-Two-A-containing complex (hATAC). HATAC is a histone modifying complex and further studies showed that actin directly binds one of it subunits, lysine acetyltransferase 14 (KAT14). We showed that actin-binding modulates histone acetyl transferase (HAT) activity of KAT14 in vitro and in cells. We obtained numerous RNA splicing and mRNA processing factors with our BioID approach, which led us to investigate the role of actin in RNA splicing. Bimolecular fluorescent complementation (BiFC) assays demonstrated that actin associates with different splicing factors and we further showed, for the first time, that actin has a functional role in mRNA splicing, as alterations in nuclear actin levels disturbed survival motor neuron protein 2 (SMN2) alternative exon skipping. In addition, the nuclear actin interactome analysis provided new insights into nuclear processes already earlier linked to actin, such as chromatin remodeling, transcription and DNA replication, and hence this work provides a protein interaction platform for further mechanistic studies of nuclear actin-dependent functions. This thesis work has thereby broadened the knowledge of nuclear actin-binding partners as well as revealed novel regulatory properties of actin-regulated RPEL domain containing proteins.
A dynamic actin-dependent nucleoskeleton and cell identity
The Journal of Biochemistry
Actin is an essential regulator of cellular functions. In the eukaryotic cell nucleus, actin regulates chromatin as a bona fide component of chromatin remodelling complexes, it associates with nuclear RNA polymerases to regulate transcription and is involved in co-transcriptional assembly of nascent RNAs into ribonucleoprotein complexes. Actin dynamics are, therefore, emerging as a major regulatory factor affecting diverse cellular processes. Importantly, the involvement of actin dynamics in nuclear functions is redefining the concept of nucleoskeleton from a rigid scaffold to a dynamic entity that is likely linked to the three-dimensional organization of the nuclear genome. In this review, we discuss how nuclear actin, by regulating chromatin structure through phase separation may contribute to the architecture of the nuclear genome during cell differentiation and facilitate the expression of specific gene programs. We focus specifically on mitochondrial genes and how their dysregu...
The Journal of Cell Biology, 2006
541-552 http://www.jcb.org/cgi/doi/10.1083/jcb. 200507101 JCB 541 < d o i > 1 0 . 1 0 8 3 / j c b . 2 0 0 5 0 7 1 0 1 < / d o i > < a i d > 2 0 0 5 0 7 1 0 1 < / a i d > Nucleoplasmic β-actin exists in a dynamic equilibrium between low-mobility polymeric species and rapidly diffusing populations β -Actin, once thought to be an exclusively cytoplasmic protein, is now known to have important functions within the nucleus. Nuclear β-actin associates with and functions in chromatin remodeling complexes, ribonucleic acid polymerase complexes, and at least some ribonucleoproteins. Proteins involved in regulating actin polymerization are also found in the interphase nucleus. We defi ne the dynamic properties of nuclear actin molecules using fl uorescence recovery after photobleaching.
Actin complexes in the cell nucleus: new stones in an old field
Histochemistry and Cell Biology, 2010
Actin is a well-known protein that has shown a myriad of activities in the cytoplasm. However, recent findings of actin involvement in nuclear processes are overwhelming. Actin complexes in the nucleus range from very dynamic chromatin-remodeling complexes to structural elements of the matrix with single partners known as actin-binding proteins (ABPs). This review summarizes the recent findings of actin-containing complexes in the nucleus. Particular attention is given to key processes like chromatin remodeling, transcription, DNA replication, nucleocytoplasmic transport and to actin roles in nuclear architecture. Understanding the mechanisms involving ABPs will definitely lead us to the principles of the regulation of gene expression performed via concerting nuclear and cytoplasmic processes.
REVIEW Actin complexes in the cell nucleus: new stones in an old field
2010
Actin is a well-known protein that has shown a myriad of activities in the cytoplasm. However, recent findings of actin involvement in nuclear processes are overwhelming. Actin complexes in the nucleus range from very dynamic chromatin-remodeling complexes to struc-tural elements of the matrix with single partners known as actin-binding proteins (ABPs). This review summarizes the recent findings of actin-containing complexes in the nucleus. Particular attention is given to key processes like chromatin remodeling, transcription, DNA replication, nucleocytoplasmic transport and to actin roles in nuclear architecture. Understanding the mechanisms involving ABPs will definitely lead us to the principles of the regu-lation of gene expression performed via concerting nuclear and cytoplasmic processes.