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Actin is not only one of the most abundant proteins in eukaryotic cells, but also one of the most versatile. In addition to its familiar involvement in enabling contraction and establishing cellular motility and scaffolding in the cytosol, actin has well-documented roles in a variety of processes within the confines of the nucleus, such as transcriptional regulation and DNA repair. Interestingly, monomeric actin as well as actin-related proteins (Arps) are found as stoichiometric subunits of a variety of chromatin remodeling complexes and histone acetyltransferases, raising the question of precisely what roles they serve in these contexts. Actin and Arps are present in unique combinations in chromatin modifiers, helping to establish structural integrity of the complex and enabling a wide range of functions, such as recruiting the complex to nucleosomes to facilitate chromatin remodeling and promoting ATPase activity of the catalytic subunit. Actin and Arps are also thought to help modulate chromatin dynamics and maintain higher-order chromatin structure. Moreover, the presence of actin and Arps in several chromatin modifiers is necessary for promoting genomic integrity and an effective DNA damage response. In this review, we discuss the involvement of actin and Arps in these nuclear complexes that control chromatin remodeling and histone modifications, while also considering avenues for future study to further shed light on their functional importance. [ABSTRACT FROM AUTHOR]

Nuclear actin family proteins, comprising of actin and actin-related proteins (Arps), are essential functional components of the multiple chromatin remodeling complexes. The INO80 chromatin remodeling complex, which is evolutionarily conserved and has roles in transcription, DNA replication and repair, consists of actin and actin-related proteins Arp4, Arp5, and Arp8. We generated Arp5 knockout (KO) and Arp8 KO cells from the human Nalm-6 pre-B cell line and used these KO cells to examine the roles of Arp5 and Arp8 in the transcriptional regulation mediated by the INO80 complex. In both of Arp5 KO and Arp8 KO cells, the oxidative stress-induced expression of HMOX1 gene, encoding for heme oxygenase-1 (HO-1), was significantly impaired. Consistent with these observations, chromatin immunoprecipitation (ChIP) assay revealed that oxidative stress caused an increase in the binding of the INO80 complex to the regulatory sites of HMOX1 in wild-type cells. The binding of INO80 complex to chromatin was reduced in Arp8 KO cells compared to that in the wild-type cells. On the other hand, the binding of INO80 complex to chromatin in Arp5 KO cells was similar to that in the wild-type cells even under the oxidative stress condition. However, both remodeling of chromatin at the HMOX1 regulatory sites and binding of a transcriptional activator to these sites were impaired in Arp5 KO cells, indicating that Arp5 is required for the activation of the INO80 complex. Collectively, these results suggested that these nuclear Arps play indispensable roles in the function of the INO80 chromatin remodeling complex. [ABSTRACT FROM AUTHOR]

Highlights: [•] Chromatin-modifying complexes are a well-defined system for studying nuclear actin. [•] In contrast to cytosolic actin, nuclear actin in INO80 complex functions as a monomer. [•] Nuclear actin forms modules with actin-related proteins to interact with chromatin. [•] New insights into nuclear actin suggest a paradigm distinct from cytosolic actin. [Copyright &y& Elsevier]

The spatial organization of chromatin in the nucleus contributes to genome function and is altered during the differentiation of normal and tumorigenic cells. Although nuclear actin-related proteins (Arps) have roles in the local alteration of chromatin structure, it is unclear whether they are involved in the spatial positioning of chromatin. In the interphase nucleus of vertebrate cells, gene-dense and gene-poor chromosome territories (CTs) are located in the center and periphery, respectively. We analyzed chicken DT40 cells in which Arp6 had been knocked out conditionally, and showed that the radial distribution of CTs was impaired in these knockout cells. Arp6 is an essential component of the SRCAP chromatin remodeling complex, which deposits the histone variant H2A.Z into chromatin. The redistribution of CTs was also observed in H2A.Z-deficient cells for gene-rich microchromosomes, but to lesser extent for gene-poor macrochromosomes. These results indicate that Arp6 and H2A.Z contribute to the radial distribution of CTs through different mechanisms. Microarray analysis suggested that the localization of chromatin to the nuclear periphery per se is insufficient for the repression of most genes. [ABSTRACT FROM AUTHOR]

Actin-related proteins (ARPs) share moderate sequence homology and basal structure with conventional actins and are found in all eukaryotes. While the functions of most of the divergent ARPs are not clear, several of them are localized to the nucleus and have been identified as components of various chromatin-modifying complexes. Using an antibody toArabidopsisAtARP4, we found this conserved homolog of human BAF53 and yeast Arp4 is concentrated in the nucleoplasm ofArabidopsis,Brassica, and tobacco cells. To gain further insight into the role of ARP4, we have examinedArabidopsisplants that are defective inAtARP4expression. Phenotypic analysis of thearp4-1mutant allele, which has a T-DNA insertion in the promoter region and a moderate reduction in the level of AtARP4 protein expression, revealed partial sterility due to defects in anther development. Targeting the distinct, 3′ UTR ofAtARP4transcripts with RNA interference caused a drastic reduction in the level of AtARP4 protein expression in several independent transgenic lines, and resulted in strong pleiotropic phenotypes such as altered organization of plant organs, early flowering, delayed flower senescence and high levels of sterility. Western blot analysis and immunolabeling demonstrated a clear correlation between reductions in the level of AtARP4 expression and severity of the phenotypes. Based on our results and data on the orthologs ofAtARP4in yeast and other organisms, we suggest that AtARP4 is likely to exert its effects on plant development through the modulation of chromatin structure and subsequent changes in gene regulation. [ABSTRACT FROM AUTHOR]

Actin is a protein abundant in cells. Recently, it has been proved to be universally existent in the nuclei of many cell types. Actin and actin-binding proteins, as well as actin-related proteins, are necessary for the mediation of the conformation and function of nuclear actin, including the transformation of actin between unpolymerized and polymerized, chromatin remodeling, regulation of gene expression and RNA processing as well as RNA transportation. In this paper, we summarized the progresses in the research of nu- clear actin. [ABSTRACT FROM AUTHOR]

The catalytic subunits of SWI/SNF-family and INO80-family chromatin remodelers bind actin and actin-related proteins (Arps) through an N-terminal helicase/SANT-associated (HSA) domain. Between the HSA and ATPase domains lies a conserved post-HSA (pHSA) domain. The HSA domain of Sth1, the catalytic subunit of the yeast SWI/SNF-family remodeler RSC, recruits the Rtt102-Arp7/9 heterotrimer. Rtt102-Arp7/9 regulates RSC function, but the mechanism is unclear. We show that the pHSA domain interacts directly with another conserved region of the catalytic subunit, protrusion-1. Rtt102-Arp7/9 binding to the HSA domain weakens this interaction and promotes the formation of stable, monodisperse complexes with DNA and nucleosomes. A crystal structure of Rtt102-Arp7/9 shows that ATP binds to Arp7 but not Arp9. However, Arp7 does not hydrolyze ATP. Together, the results suggest that Rtt102 and ATP stabilize a conformation of Arp7/9 that potentiates binding to the HSA domain, which releases intramolecular interactions within Sth1 and controls DNA and nucleosome binding. Bengi Turegun et al. report an interaction of the highly-conserved pHSA and P1 domains of Sth1, the catalytic subunit of the SWI/SNF-family chromatin remodeler RSC. This interaction is released when ATP-bound Rtt102-Arp7/9 binds to the HSA domain, modulating DNA and nucleosome binding by Sth. [ABSTRACT FROM AUTHOR]