A Novel Role of the Actin-nucleating Arp2/3 Complex in the Regulation of RNA Polymerase II-dependent Transcription

Department of Molecular Medicine, Cornell University, Ithaca, New York 14853, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 04/2007; 282(10):7616-23. DOI: 10.1074/jbc.M607596200
Source: PubMed


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.

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    • "From the data available, it seems that the state of nuclear actin engaged in chromatin remodeling complexes and in complex with hnRNPs (Kapoor et al. 2013; Obrdlik et al. 2008; Percipalle et al. 2002) is rather monomeric, whereas in transcription both forms seem to be involved (Miyamoto et al. 2011; Obrdlik and Percipalle 2011; Qi et al. 2011; Wu et al. 2006; Ye et al. 2008; Yoo et al. 2007). Similarly, actin in its polymeric form is essential for the movement of genomic loci throughout the nucleus during transcriptional activation (Dundr et al. 2007; Hu et al. 2008). "
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    ABSTRACT: Although actin monomers polymerize into filaments in the cytoplasm, the form of actin in the nucleus remains elusive. We searched for the form and function of β-actin fused to nuclear localization signal and to enhanced yellow fluorescent protein (EN-actin). Our results reveal that EN-actin is either dispersed in the nucleoplasm (homogenous EN-actin) or forms bundled filaments in the nucleus (EN-actin filaments). Formation of such filaments was not connected with increased EN-actin levels. Among numerous actin-binding proteins tested, only cofilin is recruited to the EN-actin filaments. Overexpression of EN-actin causes increase in the nuclear levels of actin-related protein 3 (Arp3). Although Arp3, a member of actin nucleation complex Arp2/3, is responsible for EN-actin filament nucleation and bundling, the way cofilin affects nuclear EN-actin filaments dynamics is not clear. While cells with homogenous EN-actin maintained unaffected mitosis during which EN-actin re-localizes to the plasma membrane, generation of nuclear EN-actin filaments severely decreases cell proliferation and interferes with mitotic progress. The introduction of EN-actin manifests in two mitotic-inborn defects-formation of binucleic cells and generation of micronuclei-suggesting that cells suffer aberrant cytokinesis and/or impaired chromosomal segregation. In interphase, nuclear EN-actin filaments passed through chromatin region, but do not co-localize with either chromatin remodeling complexes or RNA polymerases I and II. Surprisingly presence of EN-actin filaments was connected with increase in the overall transcription levels in the S-phase by yet unknown mechanism. Taken together, EN-actin can form filaments in the nucleus which affect important cellular processes such as transcription and mitosis.
    Full-text · Article · Jul 2014 · Histochemie
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    • "Although reports of endogenous nuclear ARP2/3 are rare, our results are consistent with previous reports that other plant ARP2/3 pathway components and actin have been detected in the nucleus (Cruz and Moreno Diaz de la Espina, 2008; Zhang et al., 2010; Dyachok et al., 2011). Historically in the actin field, the topic of F-actin in the nucleus in general has been controversial, and the precise function of nuclear ARP2/3 is not known (Goley et al., 2006; Yoo et al., 2007). However, there is an increasing recognition that nuclear pools of nucleation promoting factors and ARP2/3 are likely to have a functional importance (Weston et al., 2012). "
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    ABSTRACT: In plant cells the actin cytoskeleton adopts many configurations, but is best understood as an unstable, interconnected track that rearranges to define the patterns of long distance transport of organelles during growth. Actin filaments do not form spontaneously; instead filament nucleators, such as the evolutionarily conserved actin-related protein (ARP) 2/3 complex, can efficiently generate new actin filament networks when in a fully activated state. A growing number of genetic experiments have shown that ARP2/3 is necessary for morphogenesis in processes that range from tip growth during root nodule formation to the diffuse polarized growth of leaf trichomes and pavement cells. Although progress has been rapid in the identification of proteins that function in series to positively regulate ARP2/3, less has been learned about the actual function of ARP2/3 in cells. In this paper, we analyze the localization of ARP2/3 in Arabidopsis leaf pavement cells. We detect a pool of ARP2/3 in the nucleus, and also find that ARP2/3 is efficiently and specifically clustered on multiple organelle surfaces and associates with both the actin filament and microtubule cytoskeletons. Our mutant analyses and ARP2/3 and actin double labeling experiments indicate that the clustering of ARP2/3 on organelle surfaces and an association with actin bundles does not necessarily reflect an active pool of ARP2/3, and instead most of the complex appears to exist as a latent organelle-associated pool.
    Full-text · Article · Jul 2013 · Frontiers in Plant Science
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    • "Collectively, WASP plays an important role in transcriptional activation of genes required for T cell differentiation by regulating active histone marks and possibly transcription. Moreover, the actin nucleating Arp2/3 complex is found in nuclei and is required for a full Pol II activity [96]. Arp2/3 and F-actin bind to the gene regions to which WASP is bound [17]. "
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    ABSTRACT: Proper regulation of transcription is essential for cells to acquire and maintain cell identity. Transcriptional activation plays a central role in gene regulation and can be modulated by introducing transcriptional activators such as transcription factors. Activators act on their specific target genes to induce transcription. Reprogramming experiments have revealed that as cells become differentiated, some genes are highly silenced and even introduction of activators that target these silenced genes does not induce transcription. This can be explained by chromatin-based repression that restricts access of transcriptional activators to silenced genes. Transcriptional activation from these genes can be accomplished by opening chromatin, in addition to providing activators. Once a de novo transcription network is established, cells are differentiated or reprogrammed to a new cell type. Emerging evidence suggests that actin in the nucleus (nuclear actin) and nuclear actin-binding proteins are implicated in these transcriptional regulatory processes. This review summarizes roles of nuclear actin and actin-binding proteins in transcriptional regulation. We also discuss possible functions of nuclear actin during reprogramming in the context of transcription and chromatin remodeling.
    Full-text · Article · Dec 2012 · Cellular and Molecular Life Sciences CMLS
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