   #copyright

Arp2/3 complex

2007 Schools Wikipedia Selection. Related subjects: General Biology

   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.
   Enlarge
   Atomic structure of bovine Arp2/3 complex ( PDB code: 1k8k). Colour
   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. 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 first identified in Acanthamoeba castellanii and
   has since been found in every eukaryotic organism studied.

Mechanisms of Actin Polymerization by Arp2/3

   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.
   Enlarge
   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
   Enlarge
   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 nucleation cores. However, the Arp2/3 complex stimulates
   actin polymerization by creating a new nucleation core. The nucleation
   core activity of Arp2/3 is activated by members of the Wiskott-Aldrich
   syndrome family protein (WASP, N-WASP, and WAVE 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 mechanisms of actin polymerization by Arp2/3 has been the subject
   of dispute in the resent years. The question is where the complex binds
   the filament and how it nucleates a "daughter" filament. Historically
   two models have been proposed to describe the formation of branched
   filaments:

Side branching model

   In the side branching (or dendritic nucleation) model, the Arp2/3
   complex binds to the side of pre-existing ("mother") filaments at a
   point different from the nucleation site. Arp2/3 thus has two
   actin-binding sites — one to bind to the pre-existing actin filament
   and the other for the nucleation of a branched filament. Recent
   research provides strong support for this model.

Barbed end branching model

   In the barbed end branching model, Arp2/3 associates at the barbed end
   of growing filaments, allowing for the elongation of the original
   filament and the formation of a branched filament. This model is mainly
   based on kinetic analysis rather than structural data, suggesting that
   branching is induced with Arp2 and Arp3 being incorporated in two
   different actin filaments.

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 macropinocytotic cups, in the leading edges of
   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. 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 result in abnormal filament
   organization, which in turn affects the expansion of trichomes,
   pavement cells, hypocotyl cells, and root hair cells.
   Retrieved from " http://en.wikipedia.org/wiki/Arp2/3_complex"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
