Arp2/3 complex

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Atomic structure of bovine Arp2/3 complex [1] (PDB code: 1k8k). Color coding for subunits: Arp3, orange; Arp2, marine (subunits 1 & 2 not resolved and thus not shown); p40, green; p34, ice blue; p20, dark blue; p21, magenta; p16, yellow.

Arp2/3 complex is a seven-subunit protein 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[16]. Two of its subunits, the Actin-Related Proteins 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 by affinity chromatography from Acanthamoeba castellanii[14], though it had been previously isolated in 1989 in a search for proteins that bind to actin filaments in Drosophila melanogaster embryos [15]. It is found in most eukaryotic organisms, but absent from a number of Chromalveolates and plants[16].

Mechanisms of actin polymerization by Arp2/3[edit]

Side branching model of the Arp2/3 complex. Activated Arp2/3 complex binds to the side of a "mother" actin filament. Both Arp2 and Arp3 form the first two subunits in the new "daughter" filament.
Barbed end branching model of the Arp2/3 complex. Activated Arp2/3 competes with capping proteins to bind to the barbed end of an actin filament. Arp2 remains bound to the mother filament, while Arp3 is outside. The two Arp subunits form the first subunits of each branch and the two branches continue to grow by addition of G-actin to each Arp

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 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.[2] 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.[3]

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, and the balance of opinion in the field, 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[4][12][18], together with biochemical data on the filament nucleation and capping mechanisms of the Arp2/3 complex[13], 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[17]. This model, which is based on kinetic analysis and optical microscopy, is decreasingly favoured by the field.

Cellular uses of Arp2/3[edit]

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.[8] In mammals and the social amoeba Dictyostelium discoideum [9] [10] 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.[11] 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.[6] 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.[5][7]


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