Purification of a cortical complex containing two unconventional actins from Acanthamoeba by affinity chromatography on profilin-agarose

N-WASP plays a critical role in fibroblast adhesion and spreading

N-WASP (Neural Wiskott Aldrich Syndrome Protein) regulates actin polymerization by activating the Arp2/3 complex and promotes the formation of actin-rich structures such as filopodia. Such actin-rich structures play critical roles in cell adhesion and cell motility. Analysis of the adhesion properties of N-WASP+/+ and N-WASP-/- mouse embryonic fibroblasts to extracellular matrix proteins revealed that N-WASP is critical for cell adhesion to fibronectin. There was no significant difference in the localization of paxillin in the two cell lines, however the vinculin patches in WASP+/+ cells were thicker and more prominent than those in N-WASP-/- cells. The beta1 integrins in N-WASP+/+ cells were found in large clusters, while beta1 integrins were more dispersed in N-WASP-/- cells. The N-WASP-/- cells migrated more rapidly than N-WASP+/+ cells in a scratch migration assay. Thus, our data suggest that N-WASP deficiency leads to reduced adhesion to fibronectin and increased cell motility.

Mutants in the Dictyostelium Arp2/3 complex and chemoattractant-induced actin polymerization

We have investigated the role of the Arp2/3 complex in Dictyostelium cell chemotaxis towards cyclic-AMP and in the actin polymerization that is triggered by this chemoattractant. We confirm that the Arp2/3 complex is recruited to the cell perimeter, or into a pseudopod, after cyclic-AMP stimulation and that this is coincident with actin polymerization. This recruitment is inhibited when actin polymerization is blocked using latrunculin suggesting that the complex binds to pre-existing actin filaments, rather than to a membrane associated signaling complex. We show genetically that an intact Arp2/3 complex is essential in Dictyostelium and have produced partially active mutants in two of its subunits. In these mutants both phases of actin polymerization in response to cyclic-AMP are greatly reduced. One mutant projects pseudopodia more slowly than wild type and has impaired chemotaxis, together with slower movement. The second mutant chemotaxes poorly due to an adhesion defect, suggesting that the Arp2/3 complex plays a crucial part in adhering cells to the substratum as they move. We conclude that the Arp2/3 complex largely mediates the actin polymerization response to chemotactic stimulation and contributes to cell motility, pseudopod extension and adhesion in Dictyostelium chemotaxis.

Correlation between liver metastasis of the colocalization of actin-related protein 2 and 3 complex and WAVE2 in colorectal carcinoma

Directed movement of normal cells occurs when actin-related protein 2 and 3 complex (Arp2/3 complex) triggers the actin polymerization that forms lamellipodia immediately after binding to WAVE2. In order to determine whether the same mechanism correlates with liver metastasis from colorectal cancer, paired mirror sections of 154 cancer specimens (29 cases with liver metastasis and 125 cases without liver metastasis in which T factor, gender, primary tumor site, and age at operation were matched) were examined immunohistochemically for the localization of Arp2 and WAVE2. Expression of both Arp2 and WAVE2 was detected in the same cancer cells in 55 (35.7%) of the 154 cases, but not detected in the normal colonic epithelial cells. Univariate analysis showed that the colocalization was significantly predictive of liver metastasis (risk ratio [RR] 8.760. Likewise, histological grade (RR 2.46), lymphatic invasion (RR 9.95), and tumor budding (RR 4.00) were significant predictors. Among these, colocalization and lymphatic invasion were shown to be independent risk factors by multivariate analysis. Another 59 colorectal specimens were examined for mRNA expression of Arp2 by real time polymerase chain reaction. High mRNA levels of Arp2, that in situ hybridization revealed to be expressed by the cancer cells, were significantly associated with liver metastasis. However, its effect was absorbed by the influence of risk of the colocalization that is closely related to high expression of Arp2. These results indicate that the colocalization of Arp2 and WAVE2 is an independent risk factor for liver metastasis of colorectal carcinoma.

The Wiskott-Aldrich syndrome protein (WASP) is essential for myoblast fusion in Drosophila

In higher organisms, mononucleated myoblasts fuse to form multinucleated myotubes. During this process, myoblasts undergo specific changes in cell morphology and cytoarchitecture. Previously, we have shown that the actin regulator Kette (Hem-2/Nap-1) is essential for myoblast fusion. In this study, we describe the role of the evolutionary conserved Wiskott-Aldrich syndrome protein that serves as a regulator for the Arp2/3 complex for myoblast fusion. By screening an EMS mutagenesis collection, we discovered a new wasp allele that does not complete fusion during myogenesis. Interestingly, this new wasp3D3-035 allele is characterized by a disruption of fusion after precursor formation. The molecular lesion in this wasp allele leads to a stop codon preventing translation of the CA domain. Usually, the WASP protein exerts its function through the Arp2/3-interacting CA domain. Accordingly, a waspDeltaCA that is expressed in a wild-type background acts as dominant-negative during the fusion process. Furthermore, we show that the myoblast fusion phenotype of kette mutant embryos can be suppressed by reducing the gene dose of wasp3D3-035. Thus, Kette antagonizes WASP function during myoblast fusion.

The N-terminus of Dictyostelium Scar interacts with Abi and HSPC300 and is essential for proper regulation and function

Scar/WAVE proteins, members of the conserved Wiskott-Aldrich syndrome (WAS) family, promote actin polymerization by activating the Arp2/3 complex. A number of proteins, including a complex containing Nap1, PIR121, Abi1/2, and HSPC300, interact with Scar/WAVE, though the role of this complex in regulating Scar function remains unclear. Here we identify a short N-terminal region of Dictyostelium Scar that is necessary and sufficient for interaction with HSPC300 and Abi in vitro. Cells expressing Scar lacking this N-terminal region show abnormalities in F-actin distribution, cell morphology, movement, and cytokinesis. This is true even in the presence of wild-type Scar. The data suggest that the first 96 amino acids of Scar are necessary for participation in a large-molecular-weight protein complex, and that this Scar-containing complex is responsible for the proper localization and regulation of Scar. The presence of mis-regulated or unregulated Scar has significant deleterious effects on cells and may explain the need to keep Scar activity tightly controlled in vivo either by assembly in a complex or by rapid degradation.

IQGAP1 regulates cell motility by linking growth factor signaling to actin assembly

IQGAP1 has been implicated as a regulator of cell motility because its overexpression or underexpression stimulates or inhibits cell migration, respectively, but the underlying mechanisms are not well understood. Here, we present evidence that IQGAP1 stimulates branched actin filament assembly, which provides the force for lamellipodial protrusion, and that this function of IQGAP1 is regulated by binding of type 2 fibroblast growth factor (FGF2) to a cognate receptor, FGFR1. Stimulation of serum-starved MDBK cells with FGF2 promoted IQGAP1-dependent lamellipodial protrusion and cell migration, and intracellular associations of IQGAP1 with FGFR1--and two other factors--the Arp2/3 complex and its activator N-WASP, that coordinately promote nucleation of branched actin filament networks. FGF2 also induced recruitment of IQGAP1, FGFR1, N-WASP and Arp2/3 complex to lamellipodia. N-WASP was also required for FGF2-stimulated migration of MDBK cells. In vitro, IQGAP1 bound directly to the cytoplasmic tail of FGFR1 and to N-WASP, and stimulated branched actin filament nucleation in the presence of N-WASP and the Arp2/3 complex. Based on these observations, we conclude that IQGAP1 links FGF2 signaling to Arp2/3 complex-dependent actin assembly by serving as a binding partner for FGFR1 and as an activator of N-WASP.

Visualizing Arp2/3 complex activation mediated by binding of ATP and WASp using structural mass spectrometry

Actin-related protein (Arp) 2/3 complex nucleates new branches in actin filaments playing a key role in controlling eukaryotic cell motility. This process is tightly regulated by activating factors: ATP and WASp-family proteins. However, the mechanism of activation remains largely hypothetical. We used radiolytic protein footprinting with mass spectrometry in solution to probe the effects of nucleotide- and WASp-binding on Arp2/3. These results represent two significant advances in such footprinting approaches. First, Arp2/3 is the most complex macromolecular assembly yet examined; second, only a few picomoles of Arp2/3 was required for individual experiments. In terms of structural biology of Arp 2/3, we find that ATP binding induces conformational changes within Arp2/3 complex in Arp3 (localized in peptide segments 5-18, 212-225, and 318-327) and Arp2 (within peptide segment 300-316). These data are consistent with nucleotide docking within the nucleotide clefts of the actin-related proteins promoting closure of the cleft of the Arp3 subunit. However, ATP binding does not induce conformational changes in the other Arp subunits. Arp2/3 complex binds to WASp within the C subdomain at residue Met 474 and within the A subdomain to Trp 500. Our data suggest a bivalent attachment of WASp to Arp3 (within peptides 162-191 and 318-329) and Arp2 (within peptides 66-80 and 87-97). WASp-dependent protections from oxidation within peptides 54-65 and 80-91 of Arp3 and in peptides 300-316 of Arp2 suggest domain rearrangements of Arp2 and Arp3 resulting in a closed conformational state consistent with an "actin-dimer" model for the active state.

Actin dynamics in Amoeba proteus motility

We studied the distribution of the endogenous Arp2/3 complex in Amoeba proteus and visualised the ratio of filamentous (F-actin) to total actin in living cells. The presented results show that in the highly motile Amoeba proteus, Arp2/3 complex-dependent actin polymerisation is involved in the formation of the branching network of the contractile layer, adhesive structures, and perinuclear cytoskeleton. The aggregation of the Arp2/3 complex in the cortical network, with the exception of the uroid and advancing fronts, and the spatial orientation of microfilaments at the leading edge suggest that actin polymerisation in this area is not sufficient to provide the driving force for membrane displacement. The examined proteins were enriched in the pinocytotic pseudopodia and the perinuclear cytoskeleton in pinocytotic amoebae. In migrating amoebae, the course of changes in F-actin concentration corresponded with the distribution of tension in the cell cortex. The maximum level of F-actin in migrating amoebae was observed in the middle-posterior region and in the front of retracting pseudopodia. Arp2/3 complex-dependent actin polymerisation did not seem to influence F-actin concentration. The strongly condensed state of the microfilament system could be attributed to strong isometric contraction of the cortical layer accompanied by its retraction from distal cell regions. Isotonic contraction was limited to the uroid.

N-WASP regulates extension of filopodia and processes by oligodendrocyte progenitors, oligodendrocytes, and Schwann cells—implications for axon ensheathment at myelination

The molecular mechanisms used by oligodendrocyte precursor cells (OPCs), oligodendrocytes (OLs), and Schwann cells (SCs) to advance processes for motility in the developing nervous system and to ensheath axons at myelination are currently not well defined. Here we demonstrate that OPCs, OLs, and SCs express the major proteins involved in actin polymerization-driven protrusion; these key proteins including F-actin, the Arp2/3 complex, neural-Wiskott Aldrich Syndrome protein (N-WASP) and WAVE proteins, and the RhoGTPases Rac and Cdc42 are present at the leading edges of processes being extended by OPCs, OLs, and SCs. We reveal by real-time PCR that OLs and SCs have different dominant WAVE isoforms. Inhibition of the WASP/WAVE protein, N-WASP, with wiskostatin that prevents activation of the Arp2/3 complex, blocks process extension by OPCs and SCs. Inhibition of N-WASP also causes OPC and SC process retraction, which is preceded by retraction of filopodia. This implicates filopodia in OPC and SC process stability and also of N-WASP in OPC and SC process dynamics. We also demonstrate that p34 (a component of the Arp2/3 complex), WASP/WAVE proteins, actin, alpha-tubulin, Rac, Cdc42, vinculin, and focal adhesion kinase are detected in water-shocked myelin purified from brain. Inhibition of N-WASP with wiskostatin decreases the number of axons undergoing initial ensheathment in intact optic nerve samples and reduces the Po content of dorsal root ganglia:SC co-cultures. Our findings indicate that OPCs, OLs, and SCs extend processes using actin polymerization-driven protrusion dependent on N-WASP. We hypothesize that inner mesaxons of OLs and SCs use the same mechanism to ensheath axons at myelination.

The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.

Actin dynamics: old friends with new stories

Actin dynamics, or the rapid turnover of actin filaments, play a central role in numerous cellular processes. A large and diverse cast of characters, accessory proteins known as actin-binding proteins, modulate actin dynamics. They do this by binding to the monomer pool, interacting with the side and ends of filaments, creating breaks along a filament, and generating new filaments de novo. Recent biochemical and single-filament imaging analyses of several conserved classes of plant actin-binding proteins reveal unusual and unexpected properties. Examples that are highlighted in this review include: an abundant monomer-binding protein that catalyzes nucleotide exchange; a barbed-end capping protein that is dissociated from filament ends by the signaling lipid, phosphatidic acid; a villin-like bundling protein that lacks all Ca(2+)-regulated activities; and a formin family member that is non-processive and is sufficient to generate actin filament bundles. These and other stories motivate a careful description of the properties of plant proteins in vitro as a prelude to greater insight into the molecular mechanism(s) underlying the regulation of actin dynamics in vivo.

Actin dynamics at sites of extracellular matrix degradation

The degradation of extracellular matrix (ECM) by proteases is crucial in physiological and pathological cell invasion alike. In vitro, degradation occurs at specific sites where invasive cells make contact with the ECM via specialized plasma membrane protrusions termed invadopodia. Here we present an extensive morpho-functional analysis of invadopodia actively engaged in ECM degradation and show that they are actin comet-based structures, not unlike the well-known bacteria-propelling actin tails. The relative mapping of the basic molecular components of invadopodia to actin tails is also provided. Finally, a live-imaging analysis of invadopodia highlights the intrinsic long-term stability of the structures coupled to a highly dynamic actin turnover. The results offer new insight into the tight coordination between signalling, actin remodelling and trafficking activities occurring at sites of focalized ECM degradation by invadopodia. In conclusion, invadopodia-associated actin comets are a striking example of consistently arising, spontaneous expression of actin-driven propulsion events that also represent a valuable experimental paradigm.

The ARP2/3 complex: an actin nucleator comes of age

The cellular functions of the actin cytoskeleton require precise regulation of both the initiation of actin polymerization and the organization of the resulting filaments. The actin-related protein-2/3 (ARP2/3) complex is a central player in this regulation. A decade of study has begun to shed light on the molecular mechanisms by which this powerful machine controls the polymerization, organization and recycling of actin-filament networks, both in vitro and in the living cell.

Sleeping beauty transposon-based phenotypic analysis of mice: lack of Arpc3 results in defective trophoblast outgrowth

The Sleeping Beauty (SB) transposon system has generated many transposon-insertional mutant mouse lines, some of which have resulted in embryonic lethality when bred to homozygosity. Here we report one such insertion mapped to the mouse actin-related protein complex subunit 3 gene (Arpc3). Arpc3 is a component of the Arp2/3 complex, which plays a major role in actin nucleation with Y-shaped branching from the mother actin filament in response to migration signaling. Arpc3 transposon-inserted mutants developed only to the blastocyst stage. In vitro blastocyst culture of Arpc3 mutants exhibited severe spreading impairment of trophoblasts. This phenotype was also observed in compound heterozygotes generated using conventional gene-targeted and transposon-inserted alleles. Arpc3-deficient mutants were shown to lack actin-rich structures in the spreading trophoblast. Electron microscopic analysis demonstrated the lack of mesh-like structures at the cell periphery, suggesting a role of Arpc3 in Y-shaped branching formation. These data indicate the importance of Arpc3 in the Arp2/3 complex for trophoblast outgrowth and suggest that Arpc3 may be indispensable for implantation.

Capping protein and the Arp2/3 complex regulate nonbundle actin filament assembly to indirectly control actin bundle positioning during Drosophila melanogaster bristle development

Drosophila melanogaster bristle development is dependent on actin assembly, and prominent actin bundles form against the elongating cell membrane, giving the adult bristle its characteristic grooved pattern. Previous work has demonstrated that several actin-regulating proteins are required to generate normal actin bundles. Here we have addressed how two actin regulators, capping protein, a barbed end binding protein, and the Arp2/3 complex, a potent actin assembly nucleator, function to generate properly organized bundles. As predicted from studies in motile cells, we find that capping protein and the Arp2/3 complex act antagonistically to one another during bristle development. However, these proteins do not primarily act directly on bundles, but rather on a dynamic population of actin filaments that are not part of the bundles. These nonbundle filaments, termed snarls, play an important role in determining the number and spacing of the actin bundles. Reduction of capping protein leads to an increase in snarls, which prevents actin bundles from properly attaching to the membrane. Conversely, loss of an Arp2/3 complex component leads to a loss of snarls and accumulation of excess membrane-attached bundles. These results indicate that in nonmotile cells dynamic actin filaments can function to regulate the positioning of stable actin structures. In addition, our results suggest that the Arpc1 subunit may have an additional function, independent of the rest of the Arp2/3 complex.

WASP family proteins act between cytoskeleton and cellular signaling pathways

This review considers the proteins of the WASP (Wiskott-Aldrich syndrome protein) family and their role in the regulation of actin-based motility. It contains detailed classification of the WASP family proteins and data on their subcellular localization. Impairments of expression of the WASP family proteins cause certain cell pathologies. The review also deals with domain organization of these proteins and proteins interacting with various domains of the WASP proteins. Special attention is given to analysis of the role of the WASP family proteins in initiating directed actin assembly in the leading edge of the migrating cell and on the surface of some bacteria. Putative pathways of regulation of WASP proteins by various protein ligands and their links with cell signaling systems are considered.

Reconstitution of the transition from lamellipodium to filopodium in a membrane-free system

The cellular cytoskeleton is a complex dynamical network that constantly remodels as cells divide and move. This reorganization process occurs not only at the cell membrane, but also in the cell interior (bulk). During locomotion, regulated actin assembly near the plasma membrane produces lamellipodia and filopodia. Therefore, most in vitro experiments explore phenomena taking place in the vicinity of a surface. To understand how the molecular machinery of a cell self-organizes in a more general way, we studied bulk polymerization of actin in the presence of actin-related protein 2/3 complex and a nucleation promoting factor as a model for actin assembly in the cell interior separate from membranes. Bulk polymerization of actin in the presence of the verprolin homology, cofilin homology, and acidic region, domain of Wiskott-Aldrich syndrome protein, and actin-related protein 2/3 complex results in spontaneous formation of diffuse aster-like structures. In the presence of fascin these asters transition into stars with bundles of actin filaments growing from the surface, similar to star-like structures recently observed in vivo. The transition from asters to stars depends on the ratio [fascin]/[G actin]. The polarity of the actin filaments during the transition is preserved, as in the transition from lamellipodia to filopodia. Capping protein inhibits star formation. Based on these experiments and kinetic Monte Carlo simulations, we propose a model for the spontaneous self-assembly of asters and their transition into stars. This mechanism may apply to the transition from lamellipodia to filopodia in vivo.

In vitro reconstitution of cdc42-mediated actin assembly using purified components

In the accompanying chapter, we describe an in vitro system that uses Xenopus egg extracts to study actin assembly induced by phosphatidylinositol (4,5)bisphosphate (PIP2) and Cdc42. Biochemical fractionation and candidate screening experiments conducted in the extract system have identified the Arp2/3 complex, the N-WASP-WIP (or N-WASP-CR16) complex, and the Cdc42-binding protein Toca-1 as important mediators of PIP2- and Cdc42-actin signaling. Toward our ultimate goal of reconstituting an in vitro system that recapitulates the signaling properties observed in vivo, we then developed a purified actin assembly assay system consisting of the regulatory components that we discovered from extracts. In these assays, the stereotypical sigmoidal kinetics of actin polymerization are monitored by pyrene-actin fluorescence in the presence of defined recombinant or purified proteins, enabling the detailed study of mechanism and protein function. In this chapter, we describe the preparation of the components used in these purified actin assembly reactions, as well as the assay conditions under which we monitor actin polymerization kinetics in vitro.

Profilin: emerging concepts and lingering misconceptions

Conflicting data suggest that profilin might function to promote either actin polymerization or depolymerization in cells. There are theoretical reasons and supportive data to suggest that profilin might do both. Perhaps the most accurate description of profilin emphasizes its ability to augment actin-filament dynamics, both in polymerization and in depolymerization. The effect of profilin on the critical concentration of actin, its ability to depolymerize filaments at the barbed end and the formation of a ternary complex with thymosin beta(4) all need to be accurately represented in any attempt to determine a model for profilin function.

The proline-rich protein palladin is a binding partner for profilin

Palladin is an actin-associated protein that has been suggested to play critical roles in establishing cell morphology and maintaining cytoskeletal organization in a wide variety of cell types. Palladin has been shown previously to bind directly to three different actin-binding proteins vasodilator-stimulated phosphoprotein (VASP), alpha-actinin and ezrin, suggesting that it functions as an organizing unit that recruits actin-regulatory proteins to specific subcellular sites. Palladin contains sequences resembling a motif known to bind profilin. Here, we demonstrate that palladin is a binding partner for profilin, interacting with profilin via a poly proline-containing sequence in the amino-terminal half of palladin. Double-label immunofluorescence staining shows that palladin and profilin partially colocalize in actin-rich structures in cultured astrocytes. Our results suggest that palladin may play an important role in recruiting profilin to sites of actin dynamics.

Profilin and actin-related proteins regulate microfilament dynamics during early mammalian embryogenesis

BACKGROUND

Profilins are ubiquitous proteins widely distributed in animals, including humans. They regulate actin polymerization by sequestering actin monomers in association with other actin-related proteins (Arps). Actin remodelling is essential for oocyte maturation, fertilization and embryo development; yet the role of profilins in these events is not well understood. Here we investigate profilin distribution and function during bovine fertilization and early embryogenesis, and we examine profilin localization with respect to the co-distribution of other Arps.

METHODS AND RESULTS

Western blotting, confocal microscopy with immunofluorescence and protein inhibition studies with antibodies were implemented. Profilin distributes inside interphase nuclei, throughout the cytoplasm and near the cell cortex at different stages of bovine oocyte maturation, fertilization and embryo development. Expression is detected through the blastocyst stage, where profilin localizes to the inner cell mass as well as trophectoderm. Profilin co-distributes with actin monomers and Arps vasodilator-stimulated phospho protein, p140mDia, Arp 3 and p80 coilin in pronucleate-stage zygotes. Antiprofilin antibodies inhibit normal embryo development by disrupting microfilaments, but not microtubules, and result in a higher concentration of profilin and p140mDia mislocalized to the cortex.

CONCLUSIONS

These findings demonstrate that profilin regulates actin dynamics both within the cytoplasm and inside the nuclei of developing mammalian embryos and that its function is essential during fertilization to ensure successful development.

Arp2/3 and SCAR: plants move to the fore

The actin-nucleating Arp2/3 complex is essential for life in yeast and animals, but not in plants, in which mutants of Arp2/3 complex components show relatively minor developmental abnormalities. Animal cells control the activity of the Arp2/3 complex through the suppressor of cyclic AMP receptor (SCAR) complex to achieve cell motility. Amazingly, plants have also retained the SCAR cell-motility pathway, and now provide a unique model for the study of new aspects of SCAR function in the absence of cell motility.

Mechanism of filament nucleation and branch stability revealed by the structure of the Arp2/3 complex at actin branch junctions

Actin branch junctions are conserved cytoskeletal elements critical for the generation of protrusive force during actin polymerization-driven cellular motility. Assembly of actin branch junctions requires the Arp2/3 complex, upon activation, to initiate a new actin (daughter) filament branch from the side of an existing (mother) filament, leading to the formation of a dendritic actin network with the fast growing (barbed) ends facing the direction of movement. Using genetic labeling and electron microscopy, we have determined the structural organization of actin branch junctions assembled in vitro with 1-nm precision. We show here that the activators of the Arp2/3 complex, except cortactin, dissociate after branch formation. The Arp2/3 complex associates with the mother filament through a comprehensive network of interactions, with the long axis of the complex aligned nearly perpendicular to the mother filament. The actin-related proteins, Arp2 and Arp3, are positioned with their barbed ends facing the direction of daughter filament growth. This subunit map brings direct structural insights into the mechanism of assembly and mechanical stability of actin branch junctions.

Genetic labeling and electron microscopy were used to examine actin branch junctions assembled in vitro. The subunit map obtained offers insights into the assembly of these conserved cytoskeletal elements.

Invadopodia: a guided tour

The controlled degradation of extracellular matrix is crucial in physiological and pathological cell invasion alike. In cultured cells, degradation occurs at specific sites where invasive cells make contact with the extracellular matrix via specialized plasma membrane protrusions termed invadopodia. Considerable progress has been made in recent years towards understanding the basic molecular components and the ultrastructural features of invadopodia. This current knowledge will be reviewed here together with some of the most important open questions in invadopodia biology. Considering the substantial interest and momentum in the field, the need for an operational framework to correctly define and identify invadopodia will also be discussed.

Dissection of Arp2/3 complex actin nucleation mechanism and distinct roles for its nucleation-promoting factors in Saccharomyces cerevisiae

Actin nucleation by the Arp2/3 complex is under tight control, remaining inactive until stimulation by nucleation-promoting factors (NPFs). Although multiple NPFs are expressed in most cell types, little is known about how they are coordinated and whether they perform similar or distinct functions. We examined genetic relationships among the four S. cerevisiae NPFs. Combining las17delta with pan1-101 or myo3delta myo5delta was lethal at all temperatures, whereas combining pan1-101 with myo3delta myo5delta showed no genetic interaction and abp1delta partially suppressed las17delta. These data suggest that NPFs have distinct and overlapping functions in vivo. We also tested genetic interactions between each NPF mutant and seven different temperature-sensitive arp2 alleles and purified mutant Arp2/3 complexes to compare their activities. Two arp2 alleles with mutations at the barbed end were severely impaired in nucleation, providing the first experimental evidence that Arp2 nucleates actin at its barbed end in vitro and in vivo. Another arp2 allele caused partially unregulated ("leaky") nucleation in the absence of NPFs. Combining this mutant with a partially unregulated allele in a different subunit of Arp2/3 complex was lethal, suggesting that cells cannot tolerate high levels of unregulated activity. Genetic interactions between arp2 alleles and NPF mutants point to Abp1 having an antagonistic role with respect to other NPFs, possibly serving to attenuate their stronger activities. In support of this model, Abp1 binds strongly to Arp2/3 complex, yet has notably weak nucleation-promoting activity and inhibits Las17 activity on Arp2/3 complex in a dose-responsive manner.

Actin nucleation: spire - actin nucleator in a class of its own

The rate limiting step for actin filament polymerisation is nucleation, and two types of nucleator have been described: the Arp2/3 complex and the formins. A recent study has now identified in Spire a third class of actin nucleator. The four short WH2 repeats within Spire bind four consecutive actin monomers to form a novel single strand nucleus for 'barbed end' actin filament elongation.

Actin-related protein2/3 complex component ARPC1 is required for proper cell morphogenesis and polarized cell growth in Physcomitrella patens

The actin-related protein2/3 (Arp2/3) complex functions as a regulator of actin filament dynamics in a wide array of eukaryotic cells. Here, we focus on the role of the Arp2/3 complex subunit ARPC1 in elongating tip cells of protonemal filaments of the moss Physcomitrella patens. Using RNA interference (RNAi) to generate loss-of-function mutants, we show dramatic defects in cell morphology manifested as short, irregularly shaped cells with abnormal division patterns. The arpc1 RNAi plants lack the rapidly elongating caulonemal cell type found in wild-type protonemal tissue. The absence of this cell type prevents normal bud formation even in response to cytokinin treatment and results in filamentous colonies lacking leafy gametophores. In addition, arpc1 protoplasts show an increased sensitivity to osmotic shock and are defective in their ability to properly establish a polarized outgrowth during regeneration from a single cell. This failure of arpc1 protoplasts to undergo proper tip growth is rescued by ARPC1 overexpression and is phenocopied in wild-type protoplasts treated with Latrunculin B, a potent inhibitor of actin polymerization. We show in moss that ARPC1, and by inference the Arp2/3 complex, plays a critical role in controlling polarized growth and cell division patterning through its regulation of actin dynamics at the cell apex.

IRREGULAR TRICHOME BRANCH1 in Arabidopsis encodes a plant homolog of the actin-related protein2/3 complex activator Scar/WAVE that regulates actin and microtubule organization

The dynamic actin cytoskeleton is important for a myriad of cellular functions, including intracellular transport, cell division, and cell shape. An important regulator of actin polymerization is the actin-related protein2/3 (Arp2/3) complex, which nucleates the polymerization of new actin filaments. In animals, Scar/WAVE family members activate Arp2/3 complex-dependent actin nucleation through interactions with Abi1, Nap1, PIR121, and HSCP300. Mutations in the Arabidopsis thaliana genes encoding homologs of Arp2/3 complex subunits PIR121 and NAP1 all show distorted trichomes as well as additional epidermal cell expansion defects, suggesting that a Scar/WAVE homolog functions in association with PIR121 and NAP1 to activate the Arp2/3 complex in Arabidopsis. In a screen for trichome branching defects, we isolated a mutant that showed irregularities in trichome branch positioning and expansion. We named this gene IRREGULAR TRICHOME BRANCH1 (ITB1). Positional cloning of the ITB1 gene showed that it encodes SCAR2, an Arabidopsis protein related to Scar/WAVE. Here, we show that itb1 mutants display cell expansion defects similar to those reported for the distorted class of trichome mutants, including disruption of actin and microtubule organization. In addition, we show that the scar homology domain (SHD) of ITB1/SCAR2 is necessary and sufficient for in vitro binding to Arabidopsis BRK1, the plant homolog of HSPC300. Overexpression of the SHD in transgenic plants causes a dominant negative phenotype. Our results extend the evidence that the Scar/WAVE pathway of Arp2/3 complex regulation exists in plants and plays an important role in regulating cell expansion.

The formin homology 1 domain modulates the actin nucleation and bundling activity of Arabidopsis FORMIN1

The organization of actin filaments into large ordered structures is a tightly controlled feature of many cellular processes. However, the mechanisms by which actin filament polymerization is initiated from the available pool of profilin-bound actin monomers remain unknown in plants. Because the spontaneous polymerization of actin monomers bound to profilin is inhibited, the intervention of an actin promoting factor is required for efficient actin polymerization. Two such factors have been characterized from yeasts and metazoans: the Arp2/3 complex, a complex of seven highly conserved subunits including two actin-related proteins (ARP2 and ARP3), and the FORMIN family of proteins. The recent finding that Arabidopsis thaliana plants lacking a functional Arp2/3 complex exhibit rather modest morphological defects leads us to consider whether the large FORMIN family plays a central role in the regulation of actin polymerization. Here, we have characterized the mechanism of action of Arabidopsis FORMIN1 (AFH1). Overexpression of AFH1 in pollen tubes has been shown previously to induce abnormal actin cable formation. We demonstrate that AFH1 has a unique behavior when compared with nonplant formins. The activity of the formin homology domain 2 (FH2), containing the actin binding activity, is modulated by the formin homology domain 1 (FH1). Indeed, the presence of the FH1 domain switches the FH2 domain from a tight capper (Kd approximately 3.7 nM) able to nucleate actin filaments that grow only in the pointed-end direction to a leaky capper that allows barbed-end elongation and efficient nucleation of actin filaments from actin monomers bound to profilin. Another exciting feature of AFH1 is its ability to bind to the side and bundle actin filaments. We have identified an actin nucleator that is able to organize actin filaments directly into unbranched actin filament bundles. We suggest that AFH1 plays a central role in the initiation and organization of actin cables from the pool of actin monomers bound to profilin.

Synergistic interaction between the Arp2/3 complex and cofilin drives stimulated lamellipod extension

Both the Arp2/3 complex and cofilin are believed to be important for the generation of protrusive force at the leading edge; however, their relative contributions have not been explored in vivo. Our results with living cells show that cofilin enters the leading edge immediately before the start of lamellipod extension, slightly earlier than Arp2/3, which begins to be recruited slightly later as the lamellipod is extended. Blocking either the Arp2/3 complex or cofilin function in cells results in failure to extend broad lamellipods and inhibits free barbed ends, suggesting that neither factor on its own can support actin polymerization-mediated protrusion in response to growth factor stimulation. High-resolution analysis of the actin network at the leading edge supports the idea that both the severing activity of cofilin and the specific branching activity of the Arp2/3 complex are essential for lamellipod protrusion. These results are the first to document the relative contributions of cofilin and Arp2/3 complex in vivo and indicate that cofilin begins to initiate the generation of free barbed ends that act in synergy with the Arp2/3 complex to create a large burst in nucleation activity.

The Arp2/3 complex nucleates actin arrays during zygote polarity establishment and growth

Previous work has demonstrated that dynamic actin arrays are important for axis establishment and polar growth in the fucoid zygote, Silvetia compressa. Transitions between these arrays are mediated by depolymerization of an existing array and polymerization of a new array. To begin to understand how polymerization of new arrays might be regulated, we investigated the role of the highly conserved, actin-nucleating, Actin-related protein 2/3 (Arp2/3) complex. Arp2, a subunit of the complex, was cloned and peptide antibodies were raised to the C-terminal domain. In immunolocalization studies of polarizing zygotes, actin and Arp2 colocalized around the nucleus and in a patch at the rhizoid pole. In germinated zygotes, a cone of Arp2 and actin extended from the nucleus to the subapex. Within the rhizoid tip, three structural zones were observed in the majority of zygotes: the extreme apex was devoid of label, the subapex was enriched for Arp2, and further back both actin and Arp2 were present. This zonation suggests that actin nucleation occurs at the leading edge of the cone, in the Arp2-enriched region. In two sets of experiments, we showed that tip zonation is important for growth. First, pharmacological treatments that disrupted Arp2/actin zonation arrested tip growth. Second, changes in the direction of tip growth during negative phototropism were preceded by a reorientation of the zonation in accordance with the new growth direction. This work represents the first investigation of Arp2/3 complex localization in tip-growing algal cells.

The actin cytoskeleton in normal and pathological cell motility

Cell motility is crucial for tissue formation and for development of organisms. Later on cell migration remains essential throughout the lifetime of the organism for wound healing and immune responses. The actin cytoskeleton is the cellular engine that drives cell motility downstream of a complex signal transduction cascade. The basic molecular machinery underlying the assembly and disassembly of actin filaments consists of a variety of actin binding proteins that regulate the dynamic behavior of the cytoskeleton in response to different signals. The multitude of proteins and regulatory mechanisms partaking in this system makes it vulnerable to mutations and alterations in expression levels that ultimately may cause diseases. The most familiar one is cancer that in later stages is characterized by active aberrant cell migration. Indeed tumor invasion and metastasis are increasingly being associated with deregulation of the actin system.

DISTORTED3/SCAR2 is a putative arabidopsis WAVE complex subunit that activates the Arp2/3 complex and is required for epidermal morphogenesis

In a plant cell, a subset of actin filaments function as a scaffold that positions the endomembrane system and acts as a substrate on which organelle motility occurs. Other actin filament arrays appear to be more dynamic and reorganize in response to growth signals and external cues. The distorted group of trichome morphology mutants provides powerful genetic tools to study the control of actin filament nucleation in the context of morphogenesis. In this article, we report that DISTORTED3 (DIS3) encodes a plant-specific SCAR/WAVE homolog. Null alleles of DIS3, like those of other Arabidopsis thaliana WAVE and Actin-Related Protein (ARP) 2/3 subunit genes, cause trichome distortion, defects in cell-cell adhesion, and reduced hypocotyl growth in etiolated seedlings. DIS3 efficiently activates the actin filament nucleation and branching activity of vertebrate Arp2/3 and functions within a WAVE-ARP2/3 pathway in vivo. DIS3 may assemble into a WAVE complex via a physical interaction with a highly diverged Arabidopsis Abi-1-like bridging protein. These results demonstrate the utility of the Arabidopsis trichome system to understand how the WAVE and ARP2/3 complexes translate signaling inputs into a coordinated morphogenetic response.

Breaking the WAVE complex: the point of Arabidopsis trichomes

Actin filaments comprise an essential cytoskeletal array that organizes the cytoplasm during growth and cell division. In growing cells, actin filaments carry out many functions. Actin filaments position the endomembrane system and act as a substrate on which organelle motility occurs. Other actin-filament arrays appear to be more dynamic and to reorganize in response to growth signals and external cues. The diverse cellular functions of the actin cytoskeleton are mediated by actin-binding proteins that nucleate, destabilize, and bundle actin filaments. The distorted trichome morphology mutants provide a simple genetic system in which to study mechanisms of actin-dependent morphogenesis. Recent results from several groups indicate that 'distorted group' genes encode subunits of the actin-related protein (Arp)2/3 and WAVE complexes, and function in a cell morphogenesis pathway.

Actin' like actin?

The most biologically significant property of actin is its ability to self-associate and form two-stranded polymeric microfilaments. In living cells, these micro filaments form the actin cytoskeleton, essential for maintenance of the shape, passive mechanical properties and active motility of eukaryotic cells. Recently discovered actin-related proteins (ARPs) appear to share a common ancestor with conventional actin. At present, six classes of ARPs have been discovered, three of which have representatives in diverse species across eukaryotic phyla and may share functional characteristics with conventional actin. The three most ubiquitous ARPs are predicted to share a common core structure with actin and contain all the residues required for ATP binding. Surface residues involved in protein protein interactions, however, have diverged. Models of these proteins based on the atomic structure of actin provide some clues about how ARPs interact with each other, with conventional actin and with conventional actin-binding proteins.

The ARP2/3 complex: giving plant cells a leading edge

The seven-subunit ARP2/3 complex is an efficient modulator of the actin cytoskeleton with well-recognized roles in amoeboid locomotion and subcellular motility of organelles and microbes. The recent identification of different subunit homologs suggests the existence of a functional ARP2/3 complex in higher plants. Mutations in some of the subunits have revealed a pivotal role for the complex in determining the shape of walled cells and focused attention on the interlinked processes of cortical-actin organization, growth-site selection, organelle motility and actin-microtubule interactions during plant cell morphogenesis. The findings supporting a global conservation of molecular mechanisms for membrane protrusion have been further strengthened by the identification of plant homologs of upstream regulators of the complex such as PIR121, NAP125 and HSPC300. As discussed here, the recent studies suggest that there might be hitherto unappreciated molecular and cell-biological commonalities between protrusion mediated motility of animal cells and polarized, expansion-mediated growth of plant cells.

Crystal structures of actin-related protein 2/3 complex with bound ATP or ADP

Actin-related protein (Arp) 2/3 complex stimulates formation of actin filaments at the leading edge of motile cells. Nucleation of filaments depends on hydrolysis of ATP bound to Arp2. Here we report crystal structures of Arp2/3 complex with bound ATP or ADP. The nucleotides are immobilized on the face of subdomains 3 and 4 of Arp2, whereas subdomains 1 and 2 are flexible and absent from the electron density maps. This flexibility may explain why Arp2 does not hydrolyze ATP until the complex is activated. ATP stabilizes a relatively closed conformation of Arp3 with the gamma-phosphate bridging loops from opposite sides of the cleft. ADP binds Arp3 in a unique conformation that favors an open cleft, revealing a conformational change that may occur in actin and Arps when ATP is hydrolyzed and phosphate dissociates. These structures provide the an opportunity to compare all nucleotide-binding states in an actin-related protein and give insights into the function of both the Arp2/3 complex and actin.

Signalling to actin assembly via the WASP (Wiskott-Aldrich syndrome protein)-family proteins and the Arp2/3 complex

The assembly of a branched network of actin filaments provides the mechanical propulsion that drives a range of dynamic cellular processes, including cell motility. The Arp2/3 complex is a crucial component of such filament networks. Arp2/3 nucleates new actin filaments while bound to existing filaments, thus creating a branched network. In recent years, a number of proteins that activate the filament nucleation activity of Arp2/3 have been identified, most notably the WASP (Wiskott-Aldrich syndrome protein) family. WASP-family proteins activate the Arp2/3 complex, and consequently stimulate actin assembly, in response to extracellular signals. Structural studies have provided a significant refinement in our understanding of the molecular detail of how the Arp2/3 complex nucleates actin filaments. There has also been much progress towards an understanding of the complicated signalling processes that regulate WASP-family proteins. In addition, the use of gene disruption in a number of organisms has led to new insights into the specific functions of individual WASP-family members. The present review will discuss the Arp2/3 complex and its regulators, in particular the WASP-family proteins. Emphasis will be placed on recent developments in the field that have furthered our understanding of actin dynamics and cell motility.

ARPC1/Arc40 mediates the interaction of the actin-related protein 2 and 3 complex with Wiskott-Aldrich syndrome protein family activators

The actin-related protein 2 and 3 (Arp2/3) complex is a seven-subunit protein complex that nucleates actin filaments at the cell cortex. Despite extensive cross-linking, crystallography, genetic and biochemical studies, the contribution of each subunit to the activity of the complex remains largely unclear. In this study we characterized the function of the 40-kDa subunit, ARPC1/Arc40, of the yeast Arp2/3 complex. We showed that this subunit is indeed a stable component of the Arp2/3 complex, but its highly unusual electrophoretic mobility eluded detection in previous studies. Recombinant Arc40 bound the VCA domain of Wiskott-Aldrich syndrome protein family activators at a K(d) of 0.45 mum, close to that of the full complex with VCA (0.30 microm), and this interaction was dependent on the conserved tryptophan at the COOH terminus of VCA. Using a newly constructed Delta arc40 yeast strain, we showed that loss of Arc40 severely reduced the binding affinity of the Arp2/3 complex with VCA as well as the nucleation activity of the complex, suggesting that Arc40 contains an important contact site of the Arp2/3 complex with VCA. The Delta arc40 cells exhibited reduced growth rate, loss of actin patches, and accumulation of cables like actin aggregates, phenotypes typical of other subunit nulls, suggesting that Arc40 functions exclusively within the Arp2/3 complex.

NAPP and PIRP encode subunits of a putative wave regulatory protein complex involved in plant cell morphogenesis

The ARP2/3 complex is an important regulator of actin nucleation and branching in eukaryotic organisms. All seven subunits of the ARP2/3 complex have been identified in Arabidopsis thaliana, and mutation of at least three of the subunits results in defects in epidermal cell expansion, including distorted trichomes. However, the mechanisms regulating the activity of the ARP2/3 complex in plants are largely unknown. In mammalian cells, WAVE and WASP proteins are involved in activation of the ARP2/3 complex. WAVE1 activity is regulated by a protein complex containing NAP1/HEM/KETTE/GEX-3 and PIR121/Sra-1/CYFIP/GEX-2. Here, we show that the WAVE1 regulatory protein complex is partly conserved in plants. We have identified Arabidopsis genes encoding homologs of NAP1 (NAPP), PIR121 (PIRP), and HSPC300 (BRK1). T-DNA inactivation of NAPP and PIRP results in distorted trichomes, similar to ARP2/3 complex mutants. The napp-1 mutant is allelic to the distorted mutant gnarled. The actin cytoskeleton in napp-1 and pirp-1 mutants shows orientation defects and increased bundling compared with wild-type plants. The results presented show that activity of the ARP2/3 complex in plants is regulated through an evolutionarily conserved mechanism.

Sra-1 interacts with Kette and Wasp and is required for neuronal and bristle development in Drosophila

Regulation of growth cone and cell motility involves the coordinated control of F-actin dynamics. An important regulator of F-actin formation is the Arp2/3 complex, which in turn is activated by Wasp and Wave. A complex comprising Kette/Nap1, Sra-1/Pir121/CYFIP, Abi and HSPC300 modulates the activity of Wave and Wasp. We present the characterization of Drosophila Sra-1 (specifically Rac1-associated protein 1). sra-1 and kette are spatially and temporally co-expressed,and both encoded proteins interact in vivo. During late embryonic and larval development, the Sra-1 protein is found in the neuropile. Outgrowing photoreceptor neurons express high levels of Sra-1 also in growth cones. Expression of double stranded sra-1 RNA in photoreceptor neurons leads to a stalling of axonal growth. Following knockdown of sra-1function in motoneurons, we noted abnormal neuromuscular junctions similar to what we determined for hypomorphic kette mutations. Similar mutant phenotypes were induced after expression of membrane-bound Sra-1 that lacks the Kette-binding domain, suggesting that sra-1 function is mediated through kette. Furthermore, we could show that both proteins stabilize each other and directly control the regulation of the F-actin cytoskeleton in a Wasp-dependent manner.

Erk/Src phosphorylation of cortactin acts as a switch on-switch off mechanism that controls its ability to activate N-WASP

The Arp2/3 complex can be independently activated to initiate actin polymerization by the VCA domain of WASP family members and by the acidic N-terminal and F-actin-binding repeat region of cortactin, which possesses a C-terminal SH3 domain. Cortactin is a target for phosphorylation by Src tyrosine kinases and by serine/threonine kinases that include Erk. Here we demonstrate that cortactin binds N-WASP and WASP via its SH3 domain, induces in vitro N-WASP-mediated actin polymerization, and colocalizes with N-WASP and WASP at sites of active actin polymerization. Erk phosphorylation and a mimicking S405,418D double mutation enhanced cortactin binding and activation of N-WASP. In contrast, Src phosphorylation inhibited the ability of cortactin previously phosphorylated by Erk, and that of S405,418D double mutant cortactin, to bind and activate N-WASP. Furthermore, Y-->D mutation of three tyrosine residues targeted by Src (Y421, Y466, and Y482) inhibited the ability of S405,418D cortactin to activate N-WASP. We propose that Erk phosphorylation liberates the SH3 domain of cortactin from intramolecular interactions with proline-rich regions, causing it to synergize with WASP and N-WASP in activating the Arp2/3 complex, and that Src phosphorylation terminates cortactin activation of N-WASP and WASP.

DISTORTED2 encodes an ARPC2 subunit of the putative Arabidopsis ARP2/3 complex

Arabidopsis trichomes are unicellular, branched structures that have highly constrained requirements for the cytoskeleton. The 'distorted group' genes function downstream from microtubule-based branch initiation, and are required during the actin-dependent phase of polarized stalk and branch expansion. Of the eight known 'distorted group' genes, a subset encode homologs of ARP2/3 complex subunits. In eukaryotic cells, the seven-protein ARP2/3 complex nucleates actin filament networks that push on the plasma membrane and organelles. In plants cells, the existence and function of an ARP2/3 complex is unclear. In this paper, we report that DISTORTED2 (DIS2) encodes a paralogue of the ARP2/3 complex subunit ARPC2. DIS2 has ARPC2 activity, based on its ability to rescue the growth defects of arpc2 (arc35Delta) null yeast cells. Like known ARPC2s, DIS2 physically interacts with ARPC4. Mutations in DIS2 cause a distorted trichome phenotype, defects in cell-cell adhesion, and a modest reduction in shoot FW. The actin cytoskeleton in dis2 trichomes is extensive, but developing branches fail to generate and maintain highly organized cytoplasmic actin bundles.

Involvement of Arp2/3 complex in the process of colorectal carcinogenesis

Increased motility is one of the characteristics of cancer cells, and actin polymerization and disassembly are essential for cellular motility. Since actin-related protein (Arp) 2/3 complex acts as a nucleus for actin polymerization, in this study, we immunohistochemically investigated the expression of Arp2 and Arp3 in 175 colorectal tumors in various stages of neoplastic progression. Arp2 and Arp3 showed identical expression patterns, and both were expressed in the stromal cells around neoplastic tubules or glands and in the tumor cells themselves. The frequency of expression of Arp2 and Arp3 (Arp2 and 3) by the stromal cells increased with the atypia of the colorectal neoplasms, from 5.5% (3/55) in adenoma with mild or moderate atypia, to 11.8% (2/17) in adenoma with severe atypia, 53.3% (16/30) in intramucosal carcinoma, and 91.8% (67/73) in invasive carcinoma (P<0.0001). The frequency of expression of Arp2 and 3 in the tumor cells was similar and was 1.8% (1/55) in adenoma with mild or moderate atypia, 23.5% (4/17) in adenoma with severe atypia, 23.5% (7/30) in intramucosal carcinoma, and 32.9% (24/73) in invasive carcinoma. Expression of Arp2 and 3 by the stromal cells was significantly correlated with nuclear accumulation of p53 in the tumor cells and stromal expression of CD10. These results suggest that formation of Arp2/3 complex by both neoplastic and stromal cells contributes to the increased motility of both cell types and thus provides suitable conditions for invasion.

The WASP–Arp2/3 pathway: genetic insights

Arp2/3 complex nucleates the formation of dendritic actin filament arrays, which are especially prominent at the leading edges of motile cells. Recent genetic and other loss-of-function studies have highlighted the importance of the Arp2/3 complex for normal cell functions, and especially for cell motility. WASP/Scar family proteins regulate the activity of the Arp2/3 complex, and also link it to several signaling pathways. Recent studies suggest that Scar is a more important regulator of Arp2/3 activity in actin-dependent morphological processes than WASP, which may have a more restricted role in specialized cellular events. It has also become clear that precise regulation of both Scar and WASP activity is of the utmost importance for their physiological functions.

Plant actin-related proteins

Actin-related proteins (ARPs) constitute a family of divergent and evolutionarily ancient eukaryotic proteins whose primary sequences display homology to conventional actins. Whereas actins play well-characterized cytoskeletal roles, the ARPs are implicated in various cellular functions in both the cytoplasm and in the nucleus. Cytoplasmic ARPs, for example, are known to participate in the assembly of branched actin filaments and dynein-mediated movement of vesicles in many eukaryotes. Nuclear ARPs, by contrast, are enigmatic components of various chromatin-modifying complexes involved in transcriptional regulation. Here, we review homologs to several known classes of ARPs and two distinct ARP classes in plants, and summarize recent work elucidating the biological functions of ARPs in eukaryotes.

Identification of functionally important residues of Arp2/3 complex by analysis of homology models from diverse species

We constructed homology models from the crystal structure of bovine Arp2/3 complex and sequences from six phylogenetically diverse species (Arabidopsis thaliana, Caenorhabditis elegans, Dictyostelium discoideum, Drosophila melanogaster, Saccharomyces cerevisiae, Schizosaccharomyces pombe) representing over 800 million years of evolution and used conserved surface residues to search for functionally important structural elements. The folds of the seven subunits and their core residues are well conserved, as well as residues at subunit interfaces. Only 45% of solvent-exposed surface residues are conserved and only 15% are identical across the seven species. Arp residues expected to interact with nucleotide and with the first and second actin subunits in a daughter filament are conserved and similar to actin. Arp residues required to form an Arp dimer differ from actin and may contribute to the dissociated state of the Arps in the unactivated complex. Conserved patches of surface residues guided us to candidate sites for nucleation promoting factors to interact with Arp3, Arp2, and ARPC3. Other conserved residues were used with experimental constraints to propose how residues on the subunits ARPC1, ARPC2, ARPC4 and ARPC5 might interact with the mother filament at branch junctions.