Arp2 3 complex

Arp2/3 complex (Actin Related Protein 2/3 complex) is a seven-subunit protein complex that plays a major role in the regulation of the actin cytoskeleton. It is a major component of the actin cytoskeleton and is found in most actin cytoskeleton-containing eukaryotic cells.
Two of its subunits, the ''A''ctin-''R''elated ''P''roteins ARP2 and ARP3, closely resemble the structure of monomeric actin and serve as nucleation sites for new actin filaments. The complex binds to the sides of existing ("mother") filaments and initiates growth of a new ("daughter") filament at a distinctive 70-degree angle from the mother. Branched actin networks are created as a result of this nucleation of new filaments. The regulation of rearrangements of the actin cytoskeleton is important for processes like cell locomotion, phagocytosis, and intracellular motility of lipid vesicles.

The Arp2/3 complex was named after it was identified in 1994 by affinity chromatography from ''Acanthamoeba castellanii'', though it had been previously isolated in 1989 in a search for proteins that bind to actin filaments in ''Drosophila melanogaster'' embryos. It is found in most eukaryotic organisms, but absent from a number of Chromalveolates and plants.

Mechanisms of actin polymerization by Arp2/3

Many actin-related molecules create a free barbed end for polymerization by uncapping or severing pre-existing filaments and using these as actin nucleation cores. However, the Arp2/3 complex stimulates actin polymerization by creating a new nucleation core. Actin nucleation is an initial step in the formation of an actin filament. The nucleation core activity of Arp2/3 is activated by Nucleation Promoting Factors (NPFs) including members of the Wiskott-Aldrich syndrome family protein (WASP, N-WASP, WAVE, and WASH proteins). The V domain of a WASP protein interacts with actin monomers while the CA region associates with the Arp2/3 complex to create a nucleation core. However, de novo nucleation followed by polymerization is not sufficient to form integrated actin networks, since these newly synthesized polymers would not be associated with pre-existing filaments. Thus, the Arp2/3 complex binds to pre-existing filaments so that the new filaments can grow on the old ones and form a functional actin cytoskeleton. Capping proteins limit actin polymerization to the region activated by the Arp2/3 complex, and the elongated filament ends are recapped to prevent depolymerization and thus conserve the actin filament.

The Arp2/3 complex simultaneously controls nucleation of actin polymerization and branching of filaments. Moreover, autocatalysis is observed during Arp2/3-mediated actin polymerization. In this process, the newly formed filaments activate other Arp2/3 complexes, facilitating the formation of branched filaments.

The mechanism of actin filament initiation by Arp2/3 has been disputed. The question is where the complex binds the filament and nucleates a "daughter" filament. Historically two models have been proposed. Recent results favour the side branching model, in which the Arp2/3 complex binds to the side of a pre-existing ("mother") filament at a point different from the nucleation site. Although the field lacks a high-resolution crystal structure, data from electron microscopy, together with biochemical data on th
filament nucleation and capping mechanisms of the Arp2/3 complex
favour side branching. In the alternative barbed end branching model, Arp2/3 only associates at the barbed end of growing filaments, allowing for the elongation of the original filament and the formation of a branched filament., a model based on kinetic analysis and optical microscopy.

Recent computer docking, independently confirmed by EM data, favors a side-branching model. ARPC2 and ARPC4 together form an area that attach the base of the branch to the side of a mother filament. Large conformational changes occur on nucleotide and WASP binding.

Cellular uses of Arp2/3

The Arp2/3 complex appears to be important in a variety of specialized cell functions that involve the actin cytoskeleton.
The complex is found in cellular regions characterized by dynamic actin filament activity: in macropinocytic cups, in the leading edge of motile cells (lamellipodia), and in motile actin patches in yeast. In mammals and the social amoeba ''Dictyostelium discoideum'' it is required for phagocytosis. The complex has also been shown to be involved in the establishment of cell polarity and the migration of fibroblast monolayers in a wound-healing model. In mammalian oocytes, the Arp2/3 complex is involved in oocyte asymmetric division and polar body emission, which result from the failure of spindle migration (a unique feature of oocyte division) and cytokinesis. Moreover, enteropathogenic organisms like ''Listeria monocytogenes'' and ''Shigella'' use the Arp2/3 complex for actin-polymerization- dependent rocketing movements. The Arp2/3 complex also regulates the intracellular motility of endosomes, lysosomes, pinocytic vesicles, and mitochondria. Moreover, recent studies show that the Arp2/3 complex is essential for proper polar cell expansion in plants. Arp2/3 mutations in ''Arabidopsis thaliana'' result in abnormal filament organization, which in turn affects the expansion of trichomes, pavement cells, hypocotyl cells, and root hair cells. Chemical inhibition or genetic mutation of the '' Chlamydomonas reinhardtii'' Arp2/3 complex decreases the length of flagella.

Subunits

The Arp2/3 complex is composed of seven subunits: Arp2/ ACTR2, Arp3/ ACTR3, p41/ ARPC1A& B/Arc40/Sop2/p40, p34/ ARPC2/ARC35/p35, p21/ARPC3/ARC18/p19, p20/ ARPC4/ARC19/p18, p16/ ARPC5/ARC15/p14. The subunits Arp2 and Arp3 closely resemble monomeric actin allowing for a thermodynamically stable actin-like dimer. p41 has been proposed to interact with nucleation promoting factors (NPFs) because it is only known to have minor contacts with the mother filament and there is a major loss of nucleation efficiency in the absence of p41. p34 and p20 dimerize to form a structural backbone that mediates the interaction with the mother filament. p21 forms a bridge between Arp3 and the mother filament, increasing nucleation efficiency. p16 tethers Arp2 to the rest of the complex.

Several subunits of the Arp2/3 complex exist in multiple isoforms, which can modulate its function, localization, and interactions in a context-dependent manner. For example, the ARPC1 subunit has two isoforms in mammals: ARPC1A and ARPC1B. Though they share considerable sequence similarity, they exhibit different expression patterns and functional roles. ARPC1B is predominantly expressed in hematopoietic cells, and its deficiency in humans has been associated with primary immunodeficiency characterized by thrombocytopenia, eczema, and increased susceptibility to infections, resembling Wiskott–Aldrich syndrome-like symptoms.

Similarly, ARPC5 exists in two isoforms: ARPC5 and APRC5L (ARPC5-like). Recent studies suggest that these isoforms are not functionally redundant. ARPC5L has been implicated in driving increased actin nucleation activity, particularly in dynamic cellular regions such as lamellipodia, whereas ARPC5 appears to support more stable actin networks in less motile cells. These findings suggest that the specific isoform composition of the Arp2/3 complex determines its behavior in different cellular environments, including during cell migration, immune responses, and tissue morphogenesis.

Moreover, the dynamic incorporation of isoforms during development or in response to external stimuli suggests that the Arp2/3 complex operates as a modular machine, capable of tuning actin dynamics to suit specific cellular needs. Ongoing studies continue to investigate additional isoform-specific interactions and regulatory mechanisms that may further enhance our understanding of how actin networks are customized at the molecular level.

Structural Insights

The Arp2/3 complex is composed of seven subunits: two actin-related proteins (Arp2 and Arp3) and five additional subunits (ARPC1–ARPC5). In its inactive state, the complex adopts a compact conformation where Arp2 and Arp3 are spatially separated and not aligned in a way that mimics an actin dimer. This conformation prevents spontaneous actin nucleation.

Upon activation by nucleation-promoting factors (NPFs)—such as WASP, WAVE, and WASH—the Arp2/3 complex undergoes a dramatic conformational rearrangement. Cryo-electron microscopy (cryo-EM) and molecular dynamics simulations have revealed that the binding of NPFs and ATP induces repositioning of Arp2 and Arp3 into a short-pitch actin-like dimer, the geometry required for nucleating a new actin filament branch.This transition forms a structural "template" that resembles a barbed-end actin nucleus, which then elongates by recruiting actin monomers.

The core subunits, particularly ARPC2 and ARPC4, form a structural scaffold that binds the side of an existing ("mother") actin filament at a 70-degree angle. This binding provides the platform from which a new ("daughter") filament can branch. Structural data show that ATP binding is essential for this transition. Arp2 and Arp3 bind ATP, and their hydrolysis states may regulate both activation and deactivation of the complex, influencing filament stability over time.

Recent high-resolution cryo-EM studies have further refined our understanding by capturing the Arp2/3 complex at different stages of the activation cycle. These structures demonstrate a multi-step activation mechanism, starting from NPF binding, through ATP-dependent conformational change, to the final stabilization of the branch junction. Importantly, these studies also highlight the flexibility and heterogeneity of Arp2/3 complexes in different cellular contexts, suggesting that isoform composition and regulatory protein interactions can subtly alter the conformational landscape and activity profile of the complex.

Additionally, structural studies have shown that phosphoinositides, such as PIP2, can modulate the activity of the Arp2/3 complex by altering its interaction with upstream activators. This indicates that lipid signaling is also structurally integrated into Arp2/3 regulation.

Cellular and Physiological Roles

The Arp2/3 complex is essential for the spatial and temporal regulation of actin cytoskeleton dynamics, playing a central role in numerous fundamental cellular processes. Its ability to nucleate branched actin filaments allows cells to generate protrusive forces, organize membrane architecture, and support a range of activities critical for survival, communication, and development.

One of the most well-characterized roles of the Arp2/3 complex is in cell motility, particularly through the formation of lamellipodia, sheet-like protrusions at the leading edge of migrating cells. This is vital in processes such as embryonic development, immune cell trafficking, and wound healing. The complex, activated by nucleation-promoting factors like WASP and WAVE, catalyzes the branching of actin filaments to push the plasma membrane forward.

In neuronal development, the Arp2/3 complex plays a pivotal role in axon guidance, neurite outgrowth, and dendritic spine formation. Disruption of Arp2/3 function in mouse models leads to severe abnormalities in the cortical layering of neurons, loss of neuronal polarity, and impaired synaptic plasticity, indicating that actin nucleation is critical for establishing and maintaining neural networks.

The complex is also integral to endocytosis and intracellular trafficking. It facilitates the deformation of membranes during clathrin-mediated endocytosis by generating branched actin networks that drive membrane invagination and vesicle scission. In yeast and mammalian cells, Arp2/3-dependent actin polymerization is required for vesicle propulsion and the recycling of membrane components, making it essential for nutrient uptake and receptor turnover.

In epithelial tissues, the Arp2/3 complex contributes to cell-cell adhesion, apical-basal polarity, and tight junction maintenance. Its activity supports the formation of microvilli and filopodia, ensuring proper barrier function and nutrient absorption. Genetic knockdown studies in intestinal epithelial cells have shown that loss of Arp2/3 activity leads to compromised epithelial integrity and altered morphogenesis.

In the immune system, Arp2/3-mediated actin remodeling is necessary for phagocytosis, immune synapse formation, and migration of leukocytes. During antigen presentation, dendritic cells utilize Arp2/3 to form dynamic actin-rich protrusions that increase surface area and facilitate T-cell interactions. Moreover, ARPC1B deficiency in humans has been linked to severe combined immunodeficiency, further underscoring the critical role of Arp2/3 in immune homeostasis.

Beyond normal physiology, the Arp2/3 complex is implicated in pathological conditions, such as cancer. Many tumors exhibit upregulated Arp2/3 expression, correlating with increased invasiveness and metastatic potential. The complex promotes invadopodia formation, which are actin-rich protrusions that degrade extracellular matrix and facilitate tumor cell dissemination.

References

External links

MBInfo - Arp2/3 Mediated Nucleation
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Arp2/3 filament nucleation model, illustrated with 2D animations and 3D structure-based movies

{{DEFAULTSORT:Arp2 3 complex
Cytoskeleton proteins
Protein complexes