The p40/ARPC1 Subunit of Arp2/3 Complex Performs Multiple Essential Roles in WASp-regulated Actin Nucleation

The Arp2/3 complex is a conserved seven-subunit actin-nucleating machine activated by WASp (Wiskott Aldrich syndrome protein). Despite its central importance in a broad range of cellular processes, many critical aspects of the mechanism of the Arp2/3 complex have yet to be resolved. In particular, some of the individual subunits in the complex have not been assigned clear functional roles, including p40/ARPC1. Here, we dissected the structure and function of Saccharomyces cerevisiae p40/ARPC1, which is encoded by the essential ARC40 gene, by analyzing 39 integrated alleles that target its conserved surfaces. We identified three distinct sites on p40/ARPC1 required for function in vivo: one site contacts p19/ARPC4, one contacts p15/ARPC5, and one site resides in an extended structural “arm” of p40/ARPC1. Using a novel strategy, we purified the corresponding lethal mutant Arp2/3 complexes from yeast and compared their actin nucleation activities. Lethal mutations at the contact with p19/ARPC4 specifically impaired WASp-induced nucleation. In contrast, lethal mutations at the contact with p15/ARPC5 led to unregulated (“leaky”) nucleation in the absence of WASp. Lethal mutations in the extended arm drastically reduced nucleation, and the same mutations disrupted the ability of the purified p40/ARPC1 arm domain to bind the VCA domain of WASp. Together, these data indicate that p40/ARPC1 performs at least three distinct, essential functions in regulating Arp2/3 complex-mediated actin assembly: 1) suppression of spontaneous nucleation by the Arp2/3 complex, which requires proper contacts with p15/ARPC5; 2) propagation of WASp activation signals via contacts with p19/ARPC2; and 3) direct facilitation of actin nucleation through interactions of the extended arm with the VCA domain of WASp.     

Functional surfaces on the p35/ARPC2 subunit of Arp2/3 complex required for cell growth, actin nucleation, and endocytosis

The Arp2/3 complex is comprised of seven evolutionarily conserved subunits and upon activation by WASp or another nucleation promoting factor nucleates the formation of actin filaments. These events are critical for driving a wide range of cellular processes, including motility, endocytosis, and intracellular trafficking. However, an in depth understanding of the Arp2/3 complex activation and nucleation mechanism is still lacking. Here, we used a mutagenesis approach in Saccharomyces cerevisiae to dissect the structural and functional roles of the p35/ARPC2 subunit. Using integrated alleles that target conserved and solvent-exposed residues, we identified surfaces on p35/ARPC2 required for cell growth, actin organization, and endocytosis. In parallel, we purified the mutant Arp2/3 complexes and compared their actin assembly activities both in the presence and in the absence of WASp. The majority of alleles with defects mapped to one face of p35/ARPC2, where there was a close correlation between loss of actin nucleation and endocytosis. A second site required for nucleation and endocytosis was identified near the contact surface between p35/ARPC2 and p19/ARPC4. A third site was identified at a more distal conserved surface, which was critical for endocytosis but not nucleation. These findings pinpoint the key surfaces on p35/ARPC2 required for Arp2/3 complex-mediated actin assembly and cellular function and provide a higher resolution view of Arp2/3 structure and mechanism.     

Nucleotide- and Activator-Dependent Structural and Dynamic Changes of Arp2/3 Complex Monitored by Hydrogen/Deuterium Exchange and Mass Spectrometry

Arp2/3 complex plays a central role in the de novo nucleation of filamentous actin as branches on existing filaments. The complex must bind ATP, protein activators [e.g., Wiskott-Aldrich syndrome protein (WASp)], and the side of an actin filament to form a new actin filament. Amide hydrogen/deuterium exchange coupled with mass spectrometry was used to examine the structural and dynamic properties of the mammalian Arp2/3 complex in the presence of both ATP and the activating peptide segment from WASp. Changes in the rate of hydrogen exchange indicate that ATP binding causes conformational rearrangements of Arp2 and Arp3 that are transmitted allosterically to the Arp complex (ARPC)1, ARPC2, ARPC4, and ARPC5 subunits. These data are consistent with the closure of nucleotide-binding cleft of Arp3 upon ATP binding, resulting in structural rearrangements that propagate throughout the complex. Binding of the VCA domain of WASp to ATP-Arp2/3 further modulates the rates of hydrogen exchange in these subunits, indicating that a global conformational reorganization is occurring. These effects may include the direct binding of activators to Arp3, Arp2, and ARPC1; alterations in the relative orientations of Arp2 and Arp3; and the long-range transmission of activator-dependent signals to segments proposed to be involved in binding the F-actin mother filament.     

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.     

Identification and characterisation of a novel human isoform of Arp2/3 complex subunit p16-ARC/ARPC5

The Arp2/3 complex is an actin filament nucleator that activates regulated actin assembly in response to extracellular signals. The mammalian complex is composed of seven subunits, the smallest of which is known as ARPC5 or p16-Arc. We have identified a human cDNA sequence with homology to ARPC5 and here provide evidence that this encodes a novel ARPC5 isoform. Specific antibodies were generated against the novel protein, which we have termed ARPC5B, as well as the previously characterised ARPC5 isoform, henceforth ARPC5A. The presence of both ARPC5 isoforms was detected in Arp2/3 complex affinity purified from human neutrophil extract. The tissue distribution of ARPC5A and B was analysed using the isoform-specific antibodies and it was found that the two isoforms exhibited significant differences; ARPC5A was found to be highly enriched in spleen and thymus, while ARPC5B exhibits a more regular expression, with levels in the brain being highest. Myc-tagged ARPC5A and B co-localised with the Arp2/3 complex when expressed in C2C12 cells and the cellular distribution of the two isoforms could not be distinguished. Our data show for the first time that mammalian cells contain multiple forms of the Arp2/3 complex.     

Interaction of cortactin and N-WASp with Arp2/3 complex

BACKGROUND: Dynamic actin assembly is required for diverse cellular processes and often involves activation of Arp2/3 complex. Cortactin and N-WASp activate Arp2/3 complex, alone or in concert. Both cortactin and N-WASp contain an acidic (A) domain that is required for Arp2/3 complex binding. RESULTS: We investigated how cortactin and the constitutively active VCA domain of N-WASp interact with Arp2/3 complex. Structural studies showed that cortactin is a thin, elongated monomer. Chemical crosslinking studies demonstrated selective interaction of the Arp2/3 binding NTA domain of cortactin (cortactin NTA) with the Arp3 subunit and VCA with Arp3, Arp2, and ARPC1/p40. Cortactin NTA and VCA crosslinking to the Arp3 subunit were mutually exclusive; however, cortactin NTA did not inhibit VCA crosslinking to Arp2 or ARPC1/p40, nor did it inhibit activation of Arp2/3 complex by VCA. We conducted an experiment in which a saturating concentration of cortactin NTA modestly lowered the binding affinity of VCA for Arp2/3; the results of this experiment provided further evidence for ternary complex formation. Consistent with a common binding site on Arp3, a saturating concentration of VCA abolished binding of cortactin to Arp2/3 complex. CONCLUSIONS: Under certain circumstances, cortactin and N-WASp can bind simultaneously to Arp2/3 complex, accounting for their synergy in activation of actin assembly. The interaction of cortactin NTA with Arp2/3 complex does not inhibit Arp2/3 activation by N-WASp, despite competition for a common binding site located on the Arp3 subunit. These results suggest a model in which cortactin may bridge Arp2/3 complex to actin filaments via Arp3 and N-WASp activates Arp2/3 complex by binding Arp2 and/or ARPC1/p40.     

X-ray scattering study of activated Arp2/3 complex with bound actin-WCA

Previous structures of Arp2/3 complex, determined in the absence of a nucleation-promoting factor and actin, reveal its inactive conformation. The study of the activated structure has been hampered by uncontrollable polymerization. We have engineered a stable activated complex consisting of Arp2/3 complex, the WCA activator region of N-WASP, and one actin monomer, and studied its structure in solution by small angle X-ray scattering (SAXS). The scattering data support a model in which the first actin subunit binds at the barbed end of Arp2, and disqualify an alternative model that places the first actin subunit at the barbed end of Arp3. This location of the first actin and bound W motif constrains the binding site of the C motif to subunits Arp2 and ARPC1, from where the A motif can reach subunits Arp3 and ARPC3. The results support a model of activation that is consistent with most of the biochemical observations.     

Reconstitution of human Arp2/3 complex reveals critical roles of individual subunits in complex structure and activity.

The Arp2/3 complex is a seven-protein assembly that is critical for actin nucleation and branching in cells. Here we report the reconstitution of active human Arp2/3 complex after expression of all seven subunits in insect cells. Expression of partial complexes revealed that a heterodimer of the p34 and p20 subunits constitutes a critical structural core of the complex, whereas the remaining subunits are peripherally located. Arp3 is crucial for nucleation, consistent with it being a structural component of the nucleation site. p41, p21, and p16 contribute differently to nucleation and stimulation by ActA and WASP, whereas p34/p20 bind actin filaments and likely function in actin branching. This study reveals that the nucleating and organizing functions of Arp2/3 complex subunits are separable, indicating that these activities may be differentially regulated in cells.     

Molecular dynamics simulations of Arp2/3 complex activation

Actin-related protein 2 and 3 (Arp2/3) complex forms a dendritic network of actin filaments during endocytosis and cellular locomotion by nucleating branches on the sides of preexisting actin filaments. Reconstructions of electron tomograms of branch junctions show how Arp2/3 complex anchors the branch, with Arp2 and Arp3 serving as the first two subunits of the branch. Our aim was to characterize the massive conformational change that moves Arp2 ∼30 Å from its position in crystal structures of inactive Arp2/3 complex to its position in branch junctions. Starting with the inactive crystal structure, we used atomistic-scale molecular dynamics simulations to drive Arp2 toward the position observed in branch junctions. When we applied forces to Arp2 while restraining Arp3, one block of structure (Arp2, subunit ARPC1, the globular domain of ARPC4 and ARPC5) rotated counterclockwise by 30° around a pivot point in an α-helix of ARPC4 (Glu⁸¹-Asn¹⁰⁰) to align Arp2 next to Arp3 in a second block of structure including ARPC3 and the globular domains of ARPC2. This active structure buried more surface area than the inactive conformation. The complex was stable in all simulations. In most simulations, collisions of subdomain 2 of Arp2 with Arp3 impeded the movement of Arp2.     

Characterization of Arp2/3 complex in chicken tissues

Arp2/3 protein complex consists of seven subunits (Arp2, Arp3, p41-Arc, p34-Arc, p21-Arc, p20-Arc and p16-Arc) in apparent 1:1 stoichiometry. This complex has been shown to promote the formation of Y-branch structures of F-actin in cultured cells. We generated specific antibodies against chicken Arp2, Arp3, and p34-Arc to analyze the distribution of these subunits in chicken tissues.In whole samples of brain and gizzard, antibodies against each recombinant protein reacted with single bands of predicted molecular mass based on their cDNA sequences of the antigens. Anti-p34-Arc antibody detected at least two neighboring spots in 2D-PAGE, which might suggest the existence of isoforms or modified forms. Arp2/3 complex bound to an F-actin affinity column from gizzard extract. However, Arp2/3 complex did not tightly bind major actin cytoskeleton because the complex was extracted easily when gizzard smooth muscle was homogenized in PBS. Immunoblot analysis of various tissues revealed that the amounts of Arp2/3 subunits were lower in striated muscle than in non-muscle and smooth muscle tissues. Amounts and ratio of the three subunits varied in tissues, as estimated by quantitative immunoblotting. With immunofluorescence microscopy, we also observed localization of Arp3 and p34-Arc in frozen sections of gizzard with different staining patterns around blood vessels. These results suggest that the Arp2/3 complex exists also in places where rapid actin polymerization does not occur, and that a part of the subunits may exist in different forms from the complex containing the seven subunits in some tissues.     

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.     

NMR analyses of the activation of the Arp2/3 complex by neuronal Wiskott-Aldrich syndrome protein

The VCA domain of the neuronal Wiskott-Aldrich syndrome protein (N-WASP) is a potent activator of the Arp2/3 complex, a 240 kDa heteroheptameric actin-nucleating assembly. We used site-directed spin labeling of N-WASP peptides in conjunction with methyl-TROSY spectra of the intact, selectively labeled Arp2/3 complex to identify regions of the VCA that are proximal to the ARPC3 subunit of the assembly. We also cross-linked CA peptides to the Arp3, Arp2, ARPC1, and ARPC3 subunits. The combined data suggest that the extreme C-terminus of the A region and the C-terminus of the C region of N-WASP are proximal to ARPC3. These results have implications for the mechanism of Arp2/3 complex activation by VCA peptides. This study also demonstrates the utility of NMR spectroscopy for studying ligand binding events in large, asymmetric, macromolecular assemblies.     

The Arp2/3 complex is required for lamellipodia extension and directional fibroblast cell migration

The Arp2/3 complex nucleates the formation of the dendritic actin network at the leading edge of motile cells, but it is still unclear if the Arp2/3 complex plays a critical role in lamellipodia protrusion and cell motility. Here, we differentiated motile fibroblast cells from isogenic mouse embryonic stem cells with or without disruption of the ARPC3 gene, which encodes the p21 subunit of the Arp2/3 complex. ARPC3(-/-) fibroblasts were unable to extend lamellipodia but generated dynamic leading edges composed primarily of filopodia-like protrusions, with formin proteins (mDia1 and mDia2) concentrated near their tips. The speed of cell migration, as well as the rates of leading edge protrusion and retraction, were comparable between genotypes; however, ARPC3(-/-) cells exhibited a strong defect in persistent directional migration. This deficiency correlated with a lack of coordination of the protrusive activities at the leading edge of ARPC3(-/-) fibroblasts. These results provide insights into the Arp2/3 complex's critical role in lamellipodia extension and directional fibroblast migration.     

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.     

Interactions among subunits of human Arp2/3 complex: p20-Arc as the hub

The Arp2/3 complex is critical for nucleation and crosslinking of actin filaments. To gain insight into its subunit topology and assembly pathway, we systematically examined interactions among subunits of human Arp2/3 complex by yeast two-hybrid assays. It was shown that p20-Arc was able to interact with p21-Arc, p34-Arc, and p16-Arc, respectively. In contrast, p41-Arc only interacted with p20-Arc/p16-Arc heterodimer. In addition, we found that structural integrity was important for association between p20-Arc and p21-Arc, while the N-terminal half of p34-Arc was dispensable for its binding to p20-Arc. Our data suggest a key role of p20-Arc and a multistep pathway for the complex formation.     

Phosphorylation of the Arp2 subunit relieves auto-inhibitory interactions for Arp2/3 complex activation

Actin filament assembly by the actin-related protein (Arp) 2/3 complex is necessary to build many cellular structures, including lamellipodia at the leading edge of motile cells and phagocytic cups, and to move endosomes and intracellular pathogens. The crucial role of the Arp2/3 complex in cellular processes requires precise spatiotemporal regulation of its activity. While binding of nucleation-promoting factors (NPFs) has long been considered essential to Arp2/3 complex activity, we recently showed that phosphorylation of the Arp2 subunit is also necessary for Arp2/3 complex activation. Using molecular dynamics simulations and biochemical assays with recombinant Arp2/3 complex, we now show how phosphorylation of Arp2 induces conformational changes permitting activation. The simulations suggest that phosphorylation causes reorientation of Arp2 relative to Arp3 by destabilizing a network of salt-bridge interactions at the interface of the Arp2, Arp3, and ARPC4 subunits. Simulations also suggest a gain-of-function ARPC4 mutant that we show experimentally to have substantial activity in the absence of NPFs. We propose a model in which a network of auto-inhibitory salt-bridge interactions holds the Arp2 subunit in an inactive orientation. These auto-inhibitory interactions are destabilized upon phosphorylation of Arp2, allowing Arp2 to reorient to an activation-competent state.     

Arp2/3- and Cofilin-coordinated Actin Dynamics Is Required for Insulin-mediated GLUT4 Translocation to the Surface of Muscle Cells

GLUT4 vesicles are actively recruited to the muscle cell surface upon insulin stimulation. Key to this process is Rac-dependent reorganization of filamentous actin beneath the plasma membrane, but the underlying molecular mechanisms have yet to be elucidated. Using L6 rat skeletal myoblasts stably expressing myc-tagged GLUT4, we found that Arp2/3, acting downstream of Rac GTPase, is responsible for the cortical actin polymerization evoked by insulin. siRNA-mediated silencing of either Arp3 or p34 subunits of the Arp2/3 complex abrogated actin remodeling and impaired GLUT4 translocation. Insulin also led to dephosphorylation of the actin-severing protein cofilin on Ser-3, mediated by the phosphatase slingshot. Cofilin dephosphorylation was prevented by strategies depolymerizing remodeled actin (latrunculin B or p34 silencing), suggesting that accumulation of polymerized actin drives severing to enact a dynamic actin cycling. Cofilin knockdown via siRNA caused overwhelming actin polymerization that subsequently inhibited GLUT4 translocation. This inhibition was relieved by reexpressing Xenopus wild-type cofilin-GFP but not the S3E-cofilin-GFP mutant that emulates permanent phosphorylation. Transferrin recycling was not affected by depleting Arp2/3 or cofilin. These results suggest that cofilin dephosphorylation is required for GLUT4 translocation. We propose that Arp2/3 and cofilin coordinate a dynamic cycle of actin branching and severing at the cell cortex, essential for insulin-mediated GLUT4 translocation in muscle cells.     

The putative Arabidopsis arp2/3 complex controls leaf cell morphogenesis

The evolutionarily conserved Arp2/3 complex has been shown to activate actin nucleation and branching in several eukaryotes, but its biological functions are not well understood in multicellular organisms. The model plant Arabidopsis provides many advantages for genetic dissection of the function of this conserved actin-nucleating machinery, yet the existence of this complex in plants has not been determined. We have identified Arabidopsis genes encoding homologs of all of the seven Arp2/3 subunits. The function of the putative Arabidopsis Arp2/3 complex has been studied using four homozygous T-DNA insertion mutants for ARP2, ARP3, and ARPC5/p16. All four mutants display identical defects in the development of jigsaw-shaped epidermal pavement cells and branched trichomes in the leaf. These loss-of-function mutations cause mislocalization of diffuse cortical F-actin to the neck region and inhibit lobe extension in pavement cells. The mutant trichomes resemble those treated with the actin-depolymerizing drug cytochalasin D, exhibiting stunted branches but dramatically enlarged stalks due to depolarized growth suggesting defects in the formation of a fine actin network. Our data demonstrate that the putative Arabidopsis Arp2/3 complex controls cell morphogenesis through its roles in cell polarity establishment and polar cell expansion. Furthermore, our data suggest a novel function for the putative Arp2/3 complex in the modulation of the spatial distribution of cortical F-actin and provide evidence that the putative Arp2/3 complex may activate the polymerization of some types of actin filaments in specific cell types.