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HIGH IMPACT

High Impact

University of Illinois Urbana, IL

	THIS MONTH'S SELECTION

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This month's selection for High Impact is "The putative Arabidopsis Arp2/3 complex controls leaf cell morphogenesis" by Li et al. (2003), which appeared in our August 2003 issue and as of November 2005 had been cited 44 times (Thompson ISI Web of Science, http://www.isinet.com). The research presented in the article has furthered the understanding of the dynamic regulation of the actin cytoskeleton and its involvement in cell shape determination.

	BACKGROUND

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The actin cytoskeleton is involved many cellular processes, including signal transduction, organelle positioning, and determination of cell shape. Actin filaments have also been shown to accumulate in the leading edge of tip-growing cells, such as root hairs and pollen tubes, as well as diffuse growing cells like epidermal pavement cells. Actin has no enzymatic activity; instead, it provides a dynamic structure for interaction between other proteins. The actin cytoskeleton needs to be rapidly assembled and disassembled in order for it to provide this scaffolding where and when it is needed. One way this process is controlled and the dynamic nature of actin is maintained is by actin-nucleating proteins such as the actin-related proteins (ARPs) like Arp2/3.

Arp2/3 is a complex of seven subunits and is found in all eukaryotic kingdoms. The Arp2/3 complex's ability to initiate actin polymerization is achieved by binding an actin filament and nucleating a "daughter" filament from the side. Arp2/3 was originally discovered in amoeba cells (Machesky et al., 1994) and has been intensively studied in a variety of diverse systems, including yeast (Saccharomyces cerevisiae), Caenorhabditis elegans, Drosophila, and mammals (for review, see Deeks and Hussey, 2003; Mathur, 2005). The first plant homolog/subunit (ARP2) was cloned in 1999 by Klahre and Chua (1999), and the remaining six putative genes were genetically identified and described in the article by Li et al. (2003). Concurrently, the labs of Dan Szymanski (Le et al., 2003) and Martin Hulskamp (Mathur et al., 2003a, 2003b) independently identified subunits of the Arp2/3 complex and confirmed the importance of this complex in cell morphogenesis.

	WHAT WAS SHOWN

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Li et al. (2003) identified subunits of Arabidopsis (Arabidopsis thaliana) Arp2/3 from the Arabidopsis genome database based on similarity to yeast Arp2/3 subunits. T-DNA mutants were identified for three of the subunit genes, arp2, arpc5, and arp3, allowing functional characterization. Although no obvious changes to the whole-plant morphology or to tip-growing cells were observed in these mutants, inactivation of any of the three genes produced identical defects in the development of epidermal pavement cells and leaf trichomes. Pavement cells, nonspecialized epidermal cells, were observed to have less wavy margins in the mutants than wild type, and closer investigation of these cells demonstrated reduced height of the lobes with no change in the neck width.

The aberrant trichomes of the Arp2/3 subunit mutants were similar to what is observed in the "distorted" class of mutants in which the trichomes have reduced branch length and are stunted. The application of drugs disrupting actin cytoskeleton phenocopy these mutants, leading to the hypothesis that the mutant phenotypes could be defects in the formation of a fine actin network.

Investigation of actin cytoskeleton structure in leaf epidermal cells with transiently expressed green fluorescent protein-tagged actin-binding domain of mouse talin revealed no obvious defects in the actin structure until the pavement cells started expanding. At that stage, a change in the localization of actin patches in the lobes occurred. Patches of diffuse F-actin, found at the expanding lobes in wild-type cells, were shown to be more evenly distributed throughout the cell edges rather than in the lobes of the mutant cells.

This work demonstrates the role of the Arp2/3 complex in cell morphogenesis in the formation of actin patches involved in diffuse polar growth in plants. The authors also suggest a novel function for Arp2/3, the possibility of cell-specific actin polymerization, and the regulation of actin spatial distribution.

	THE IMPACT

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The identification of the Arp2/3 complex in plants lead to the next question: How is it regulated? By itself the complex is inactive; nucleation-promoting factors are required for its activation. In animal systems, the regulation of the Arp2/3 complex has been studied and was determined to involve WASP (Wiskott-Aldrich syndrome protein)/Scar (suppressor of cAMP receptor defects)/WAVE (WASP family verprolin homologous protein) family members. Utilizing a variety of approaches, Arp2/3 regulation was explored in numerous subsequent papers, including those by Frank et al. (2004) and Basu et al. (2005), leading to the identification of plant homologs of the WASP/Scar/WAVE regulatory complex.

In Arabidopsis and maize (Zea mays), a family of Scar/WAVE-related proteins, AtSCAR1 to 4 and ZmSCAR1, were identified by Frank et al. (2004) and were shown to interact with BRICK1, a WAVE-binding protein believed to be involved in Scar/WAVE complex regulation. AtSCAR was found to activate the bovine Arp2/3 complex in vitro, supporting the idea that they have a similar function of activating Arp2/3 in plants.

Another piece of the regulation puzzle was uncovered by Basu et al. (2005) when the gene responsible for the Arabidopsis mutant DISTORTED3 (DIS3) was identified as SCAR2, a member of the Scar/WAVE complex. DIS3/SCAR2 was shown to function within a WAVE-Apr2/3 complex in vivo and in a polymerization assay with bovine Arp2/3 to activate the complex, inducing actin filament nucleation and branching activity.

The identification of the Arabidopsis Arp2/3 complex also aided in the further understanding of the establishment of zygote polarity in a paper by Hable and Kropf (2005). Default polarity in fertilized zygotes of fucoid brown algae is established at the site of sperm entry, where an F-actin patch forms. However, depending upon spatial cues from the local environment, this axis can be re-established by actin patch disassembly at the initial site and reassembly at the new axis, altering the polarity. To understand the polymerization of the new actin arrays in the zygotes, the relationship between actin and Arp2/3 complex during actin-dependent processes was observed using antibodies to the C-terminal peptide of brown algae ARP2 ortholog. Both actin and Arp2/3 were observed colocalized in a patch at the rhizoid pole as well as around the nucleus, indicating that they are associated together, suggesting that Arp2/3 is involved in nucleating dynamic actin arrays that function in polarity establishment, as was shown by Li et al. (2003).

	SUMMARY

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The 2003 article by Li et al. identified subunits of the plant Arp2/3 complex and functionally characterized three of these subunits. The foundation laid by this article aided in the identification of Arp2/3 regulatory proteins, which suggests that plant Arp2/3 may be regulated in a similar manner to animal systems. In addition, this article furthered the understanding of polarity establishment in fucoid zygotes.

	FOOTNOTES

 
www.plantphysiol.org/cgi/doi/10.1104/pp.104.900179.

	LITERATURE CITED

TOP THIS MONTH'S SELECTION BACKGROUND WHAT WAS SHOWN THE IMPACT SUMMARY LITERATURE CITED

 
Basu D, Le J, El-Essal SE, Huang S, Zhang C, Mallery EL, Koliantz G, Staiger CJ, Szymanski DB (2005) DISTORTED3/SCAR2 is a putative Arabidopsis WAVE complex subunit that activated the Arp2/3 complex and is required for epidermal morphogenesis. Plant Cell 17: 502–524[Abstract/Free Full Text]

Deeks MJ, Hussey PJ (2003) Arp2/3 and ‘the shape of things to come’. Curr Opin Plant Biol 6: 561–567[CrossRef][Web of Science][Medline]

Frank M, Egile C, Dyachok J, Djakovic S, Nolasco M, Li R, Smith LC (2004) Activation of Arp2/3 complex-dependent actin polymerization by plant proteins distantly related to Scar/WAVE. Proc Natl Acad Sci USA 101: 16379–16384[Abstract/Free Full Text]

Hable WE, Kropf DL (2005) The Arp2/3 complex nucleates actin arrays during zygote polarity establishment and growth. Cell Motil Cytoskeleton 61: 9–20[CrossRef][Web of Science][Medline]

Klahre U, Chua NH (1999) The Arabidopsis actin-related protein 2 (AtARP2) promoter directs expression in xylem precursor cells and pollen. Plant Mol Biol 41: 65–73[CrossRef][Web of Science][Medline]

Le J, El-Assal SE, Basu D, Saad ME, Szymanski DB (2003) Requirements for Arabidopsis ATARP2 and ATARP3 during epidermal development. Curr Biol 13: 1341–1347[CrossRef][Web of Science][Medline]

Li S, Blanchoin L, Yang Z, Lord EM (2003) The putative Arabidopsis Arp2/3 complex controls lead cell morphogenesis. Plant Physiol 132: 2034–2044[Abstract/Free Full Text]

Machesky LM, Atkinson SJ, Ampe C, Vandekerckhove J, Pollard TD (1994) Purification of a cortical complex containing two unconventional actins from Acanthamoeba by affinity chromatography on profilin-agarose. J Cell Biol 127: 107–115[Abstract/Free Full Text]

Mathur J (2005) The ARP2/3 complex: giving plant cells a leading edge. Bioessays 27: 377–387[CrossRef][Web of Science][Medline]

Mathur J, Mathur N, Kernebeck B, Hulskamp M (2003a) Mutations in actin-related proteins 2 and 3 affect cell shape development in Arabidopsis. Plant Cell 15: 1632–1645[Abstract/Free Full Text]

Mathur J, Mathur N, Kirik V, Kernebeck B, Srinivas BP, Hulskamp M (2003b) Arabidopsis CROOKED encodes for the smallest subunit of the ARP2/3 complex and controls cell shape by region specific fine F-actin formation. Development 130: 3137–3146[Abstract/Free Full Text]

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