module: dmel_smn_complex
title: Survival motor neuron (SMN) complex in Drosophila melanogaster
label: SMN complex
module_type: PROTEIN_COMPLEX
description: >-
  The survival motor neuron (SMN) complex assembles the heptameric Sm protein
  ring onto the Sm site of spliceosomal snRNAs in the cytoplasm, the key step of
  Sm-class snRNP biogenesis. SMN oligomerizes and binds Gemin2, which holds a
  pentamer of Sm proteins; Gemin5 recognizes the snRNA, and Gemin3 is a DEAD-box
  helicase subunit. Drosophila has a reduced set of Gemin proteins compared with
  vertebrates and carries three Gemin4 paralogs (Glos/Gemin4a, Gem4b, Gem4c);
  FlyBase defines SMN complex variants that differ in which Gemin4 paralog they
  contain. Loss of Smn in flies causes defects in snRNP assembly, motor function
  and viability, modelling spinal muscular atrophy.
summary: >-
  SMN-Gemin complex that loads Sm rings onto snRNAs during snRNP assembly, with
  alternative Gemin4 paralogs.
concepts:
  - {id: GO:0032797, label: SMN complex}
parts:
  - id: smn_core
    label: SMN core subunits
    role: Sm ring loading onto snRNA
    description: SMN, Gemin2, the helicase Gemin3 and the snRNA-binding Gemin5 (rig).
    genes: [Smn, Gem2, Gem3, rig]
    complex_term: {id: GO:0032797, label: SMN complex}
    processes:
      - {id: GO:0000387, label: spliceosomal snRNP assembly}
  - id: gemin4_paralogs
    label: Gemin4 paralog subunit
    role: variable Gemin4 subunit
    variant_sets:
      - id: gemin4_variants
        label: Gemin4a, Gemin4b or Gemin4c
        selection: EXACTLY_ONE
        variants:
          - {id: gemin4a_variant, label: Gemin4a (Glos) SMN complex variant, genes: [Glos]}
          - {id: gemin4b_variant, label: Gemin4b SMN complex variant, genes: [Gem4b]}
          - {id: gemin4c_variant, label: Gemin4c SMN complex variant, genes: [Gem4c]}
notes: >-
  FlyBase groups the Gemins (GEM) and three SMN complex variants (SMN-C with
  Gem4a/Glos, SMN-C-4B with Gem4b, SMN-C-4C). The FlyBase SMN-C-4C group lists
  only Smn, Gem2, Gem3 and rig and omits Gem4c itself; Gem4c is assigned to the
  Gemin4c variant here on the basis of the group name and its GEM membership.
  Whether one complex holds exactly one Gemin4 paralog is inferred from the
  FlyBase variant definitions. Sm proteins, the substrate of the complex, are in
  dmel_spliceosome.
function_note: >-
  No molecular function is assigned: the SMN complex assembles Sm rings onto
  snRNAs, which is captured as a process, and the RNA helicase activity
  inferred for Gem3 (IBA) is not established for the fly protein.