module: dmel_arp2_3_complex
title: Arp2/3 complex in Drosophila melanogaster
label: Arp2/3 complex
module_type: PROTEIN_COMPLEX
description: >-
  The Arp2/3 complex is a seven-subunit actin nucleator that binds the side of an
  existing actin filament and starts a new filament at a 70 degree angle, producing
  branched actin networks. Two actin-related proteins, Arp2 and Arp3, form the
  template for the daughter filament, while the five ARPC subunits (ARPC1-ARPC5)
  hold them in place and contact the mother filament. The complex is intrinsically
  inactive and is switched on by nucleation-promoting factors such as SCAR/WAVE and
  WASp. In Drosophila, Arp2/3-driven branched actin is needed for lamellipodia,
  ring canal growth, myoblast fusion and many other processes. Drosophila has two
  ARPC3 paralogs, Arpc3A and Arpc3B, so two forms of the complex are possible.
summary: >-
  Seven-subunit nucleator of branched actin networks built around Arp2 and Arp3,
  with alternative Arpc3A or Arpc3B subunits.
concepts:
  - {id: GO:0005885, label: Arp2/3 protein complex}
parts:
  - id: arp2_arp3
    label: Actin-related subunits Arp2 and Arp3
    role: daughter-filament template
    description: >-
      Arp2 and Arp3 adopt an actin-like dimer on activation and serve as the first
      two subunits of the new filament.
    genes: [Arp2, Arp3]
    complex_label: Arp2-Arp3 heterodimer
    processes:
      - {id: GO:0034314, label: Arp2/3 complex-mediated actin nucleation}
  - id: arpc_core_subunits
    label: ARPC1, ARPC2, ARPC4 and ARPC5 subunits
    role: structural subunits
    description: >-
      Non-actin-related subunits that hold Arp2 and Arp3 together and contact the
      mother filament; ARPC2 and ARPC4 form the structural core of the complex.
    genes: [Arpc1, Arpc2, Arpc4, Arpc5]
    complex_label: ARPC1/2/4/5 subunits
    processes:
      - {id: GO:0034314, label: Arp2/3 complex-mediated actin nucleation}
  - id: arpc3_subunit
    label: ARPC3 subunit
    role: ARPC3 position (paralog choice)
    variant_sets:
      - id: arpc3_variants
        label: Arpc3A or Arpc3B
        selection: EXACTLY_ONE
        variants:
          - {id: arpc3a_variant, label: Arpc3A-containing complex, genes: [Arpc3A]}
          - {id: arpc3b_variant, label: Arpc3B-containing complex, genes: [Arpc3B]}
notes: >-
  FlyBase defines two complexes, ARPC3A and ARPC3B variants, that differ only in
  the ARPC3 paralog; they are modelled as one complex with an EXACTLY_ONE choice at
  the ARPC3 position. Whether the two variants differ in activity or expression is
  not established here. Nucleation-promoting factors (SCAR/WAVE regulatory complex,
  WASp) are upstream regulators and are not parts (see dmel_wave_regulatory_complex).
group: ARPC-C
function_note: >-
  No molecular function is assigned. GO has no molecular function term for
  Arp2/3-mediated actin nucleation, which is captured as a process; the fly
  subunits carry only structural constituent of cytoskeleton (ISS) and actin
  filament binding (IBA).