ACTIN-RELATED PROTEIN8 encodes an F-box protein localized to the nucleolus in Arabidopsis

Genomes of multicellular algal sisters to land plants illuminate signaling network evolution

Zygnematophyceae are the algal sisters of land plants. Here we sequenced four genomes of filamentous Zygnematophyceae, including chromosome-scale assemblies for three strains of Zygnema circumcarinatum. We inferred traits in the ancestor of Zygnematophyceae and land plants that might have ushered in the conquest of land by plants: expanded genes for signaling cascades, environmental response, and multicellular growth. Zygnematophyceae and land plants share all the major enzymes for cell wall synthesis and remodifications, and gene gains shaped this toolkit. Co-expression network analyses uncover gene cohorts that unite environmental signaling with multicellular developmental programs. Our data shed light on a molecular chassis that balances environmental response and growth modulation across more than 600 million years of streptophyte evolution.

Actin-Related Protein 4 Interacts with PIE1 and Regulates Gene Expression in Arabidopsis

As essential structural components of ATP-dependent chromatin-remodeling complex, the nucleolus-localized actin-related proteins (ARPs) play critical roles in many biological processes. Among them, ARP4 is identified as an integral subunit of chromatin remodeling complex SWR1, which is conserved in yeast, humans and plants. It was shown that RNAi mediated knock-down of Arabidopsisthaliana ARP4 (AtARP4) could affect plant development, specifically, leading to early flowering. However, so far, little is known about how ARP4 functions in the SWR1 complex in plant. Here, we identified a loss-of-function mutant of AtARP4 with a single nucleotide change from glycine to arginine, which had significantly smaller leaf size. The results from the split luciferase complementation imaging (LCI) and yeast two hybrid (Y2H) assays confirmed its physical interaction with the scaffold and catalytic subunit of SWR1 complex, photoperiod-independent early flowering 1 (PIE1). Furthermore, mutation of AtARP4 caused altered transcription response of hundreds of genes, in which the number of up-regulated differentially expressed genes (DEGs) was much larger than those down-regulated. Although most DEGs in atarp4 are related to plant defense and response to hormones such as salicylic acid, overall, it has less overlapping with other swr1 mutants and the hta9 hta11 double-mutant. In conclusion, our results reveal that AtARP4 is important for plant growth and such an effect is likely attributed to its repression on gene expression, typically at defense-related loci, thus providing some evidence for the coordination of plant growth and defense, while the regulatory patterns and mechanisms are distinctive from other SWR1 complex components.

Arabidopsis SWC4 Binds DNA and Recruits the SWR1 Complex to Modulate Histone H2A.Z Deposition at Key Regulatory Genes

Deposition of the H2A.Z histone variant by the SWR1 complex (SWR1-C) in regulatory regions of specific loci modulates transcription. Characterization of mutations in Arabidopsis thaliana homologs of yeast SWR1-C has revealed a role for H2A.Z exchange in a variety of developmental processes. Nevertheless, the exact composition of plant SWR1-C and how it is recruited to target genes remains to be established. Here we show that SWC4, the Arabidopsis homolog of yeast SANT domain protein Swc4/Eaf2, is a DNA-binding protein that interacts with SWR1-C subunits. We demonstrate that the swc4-1 knockout mutant is embryo-lethal, while SWC4 RNAi knockdown lines display pleiotropic phenotypic alterations in vegetative and reproductive traits, including acceleration of flowering time, indicating that SWC4 controls post-embryonic processes. Transcriptomic analyses and genome-wide profiling of H2A.Z indicate that SWC4 represses transcription of a number of genes, including the floral integrator FT and key transcription factors, mainly by modulating H2A.Z deposition. Interestingly, SWC4 silencing does not affect H2A.Z deposition at the FLC locus nor expression of this gene, a master regulator of flowering previously shown to be controlled by SWR1-C. Importantly, we find that SWC4 recognizes specific AT-rich DNA elements in the chromatin regions of target genes and that SWC4 silencing impairs SWR1-C binding at FT. Collectively, our data suggest that SWC4 regulates plant growth and development by aiding SWR1-C recruitment and modulating H2A.Z deposition.

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.

Molecular cloning and characterization of an F-box family gene CarF-box1 from chickpea (Cicer arietinum L.)

F-box protein family has been found to play important roles in plant development and abiotic stress responses via the ubiquitin pathway. In this study, an F-box gene CarF-box1 (for Cicer arietinum F-box gene 1, Genbank accession no. GU247510) was isolated based on a cDNA library constructed with chickpea seedling leaves treated by polyethylene glycol. CarF-box1 encoded a putative protein with 345 amino acids and contained no intron within genomic DNA sequence. CarF-box1 is a KFB-type F-box protein, having a conserved F-box domain in the N-terminus and a Kelch repeat domain in the C-terminus. CarF-box1 was localized in the nucleus. CarF-box1 exhibited organ-specific expression and showed different expression patterns during seed development and germination processes, especially strongly expressed in the blooming flowers. In the leaves, CarF-box1 could be significantly induced by drought stress and slightly induced by IAA treatment, while in the roots, CarF-box1 could be strongly induced by drought, salinity and methyl jasmonate stresses. Our results suggest that CarF-box1 encodes an F-box protein and may be involved in various plant developmental processes and abiotic stress responses.

Differential sublocalization of actin variants within the nucleus

Conventional actin has been implicated in various nuclear processes including chromatin remodeling, transcription, nuclear transport, and overall nuclear structure. Moreover, actin has been identified as a component of several chromatin remodeling complexes present in the nucleus. In animal cells, nuclear actin exists as a dynamic equilibrium of monomers and polymers. Actin-binding proteins (ABPs) such as ADF/cofilin and profilin play a role in actin import and export, respectively. However, very little is known about the localization and roles of nuclear actin in plants. In multicellular plants and animals, actin is comprised of an ancient and divergent family of protein variants. Here, we have investigated the presence and differential localization of two ancient subclasses of actin in isolated Arabidopsis nuclei. Although the subclass 1 variants ACT2 and ACT8 and subclass 2 variant ACT7 were found distributed throughout the nucleoplasm, ACT7 was often found more concentrated in nuclear speckles than subclass 1 variants. The nuclei from the act2-1/act8-2 double null mutant and the act7-5 null mutant lacked their corresponding actin variants. In addition, serial sectioning of several independent nuclei revealed that ACT7 was notably more abundant in the nucleolus than the subclass 1 actins. Profilin and ADF proteins were also found in significant levels in plant nuclei. The possible functions of differentially localized nuclear actin variants are discussed.

Actin-related proteins in the nucleus: life beyond chromatin remodelers

Since their discovery in the mid-1990s, nuclear actin-related proteins (ARPs) have gained attention for their roles as structural components of ATP-dependent chromatin-remodeling complexes. These remodelers can move nucleosomes along the DNA, evict them from chromatin, and exchange histone variants to alter chromatin states locally. Chromatin-remodeling facilitates DNA-templated processes such as transcription regulation, DNA replication, and repair. Consistent with a role for ARPs in shaping chromatin structure, recent genetic studies show that they affect developmental and cell-type specific transcriptional programming. Here, we focus on recent results that suggest a specific contribution of ARPs to long-range interactions in the nucleus, and review evidence indicating that some ARPs may act independently of chromatin-remodeling machines.

Arabidopsis actin-related protein ARP5 in multicellular development and DNA repair

Actin-related protein 5 (ARP5) is a conserved subunit of the INO80 chromatin-remodeling complex in yeast and mammals. We have characterized the expression and subcellular distribution of Arabidopsis thaliana ARP5 and explored its role in the epigenetic control of multicellular development and DNA repair. ARP5-specific monoclonal antibodies localized ARP5 protein to the nucleoplasm of interphase cells in Arabidopsis and Nicotiana tabacum. ARP5 promoter-reporter fusions and the ARP5 protein are ubiquitously expressed. A null mutant and a severe knockdown allele produced moderately dwarfed plants with all organs smaller than the wild type. The small and slightly deformed organs such as leaves and hypocotyls were composed of small-sized cells. The ratio of leaf stomata to epidermal cells was high in the mutant, which also exhibited a delayed stomatal development compared with the wild type. Mutant plants were hypersensitive to DNA-damaging reagents including hydroxyurea, methylmethane sulfonate, and bleocin, demonstrating a role for ARP5 in DNA repair. Interestingly, the hypersensitivity phenotype of ARP5 null allele arp5-1 is stronger than the severe knockdown allele arp5-2. Moreover, a wild-type transgene fully complemented all developmental and DNA repair mutant phenotypes. Despite the common participation of both ARP4 and ARP5 in the INO80 complex, ARP4- and ARP5-deficient plants displayed only a small subset of common phenotypes and each displayed novel phenotypes, suggesting that in Arabidopsis they have both shared and unique functions.

Chapter 5. Nuclear actin-related proteins in epigenetic control

The nuclear actin-related proteins (ARPs) share overall structure and low-level sequence homology with conventional actin. They are indispensable subunits of macromolecular machines that control chromatin remodeling and modification leading to dynamic changes in DNA structure, transcription, and DNA repair. Cellular, genetic, and biochemical studies suggest that the nuclear ARPs are essential to the epigenetic control of the cell cycle and cell proliferation in all eukaryotes, while in plants and animals they also exert epigenetic controls over most stages of multicellular development including organ initiation, the switch to reproductive development, and senescence and programmed cell death. A theme emerging from plants and animals is that in addition to their role in controlling the general compaction of DNA and gene silencing, isoforms of nuclear ARP-containing chromatin complexes have evolved to exert dynamic epigenetic control over gene expression and different phases of multicellular development. Herein, we explore this theme by examining nuclear ARP phylogeny, activities of ARP-containing chromatin remodeling complexes that lead to epigenetic control, expanding developmental roles assigned to several animal and plant ARP-containing complexes, the evidence that thousands of ARP complex isoforms may have evolved in concert with multicellular development, and ARPs in human disease.