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Survival motor neuron protein



Drosophila melanogaster (Fruit fly)
Reviewed-Annotation score: Annotation score: 5 out of 5-Experimental evidence at protein leveli


The SMN complex plays an essential role in spliceosomal snRNP assembly in the cytoplasm, is required for pre-mRNA splicing in the nucleus and acts as a chaperone that discriminates target and non-target RNAs of Sm proteins. Required for normal expression of spliceosomal snRNAs and for U12 intron splicing. Required in cholinergic neurons, but not in motor neurons, to ensure correct splicing and proper levels of stas mRNA and normal neurotransmitter release by motor neurons (PubMed:23063130 and PubMed:23063131). However, Smn is required in motor neurons, but not in cholinergic neurons, for normal motor behavior but plays no role in synaptic transmission according to PubMed:23103409. In both muscle and neurons, required for the formation of a normal neuromuscular junction (NMJ) structure. Plays a neuron-specific role in long-term homeostatic compensation at the larval NMJ. In the thorax of adult flies, required for Act88F, an indirect flight muscle (IFM)-specific actin, expression and for proper IFM myofibril formation. In nurse cells, oocytes and follicle cells, required to maintain normal organization of nuclear compartments including chromosomes, nucleoli, Cajal bodies, histone locus bodies and heterochromatin. Required for the functional integrity of the cytoplasmic U snRNP body (U body) and P body. Required in dividing postembryonic neuroblasts (pNBs) for the correct basal localization of mira. The tight regulation of its expression is critical for stem cell division, proliferation and differentiation in male germline and developing central nervous system (CNS). Required for tracheal terminal cell lumen formation.12 Publications

GO - Molecular functioni

  • alpha-actinin binding Source: FlyBase
  • identical protein binding Source: IntAct
  • RNA binding Source: InterPro

GO - Biological processi

  • AMPA glutamate receptor clustering Source: UniProtKB
  • basal protein localization Source: UniProtKB
  • central nervous system development Source: UniProtKB
  • chromosome organization Source: UniProtKB
  • cytoplasmic mRNA processing body assembly Source: UniProtKB
  • cytoplasmic U snRNP body assembly Source: UniProtKB
  • embryo development Source: UniProtKB
  • excitatory postsynaptic potential Source: UniProtKB
  • larval development Source: UniProtKB
  • larval locomotory behavior Source: UniProtKB
  • mRNA splicing, via spliceosome Source: UniProtKB
  • neuromuscular junction development Source: UniProtKB
  • neuromuscular synaptic transmission Source: UniProtKB
  • oocyte morphogenesis Source: UniProtKB
  • positive regulation of synaptic transmission, cholinergic Source: UniProtKB
  • ribonucleoprotein complex assembly Source: FlyBase
  • skeletal muscle thin filament assembly Source: FlyBase
  • spliceosomal snRNP assembly Source: UniProtKB
  • stem cell differentiation Source: UniProtKB
  • stem cell division Source: UniProtKB
  • stem cell proliferation Source: UniProtKB
  • terminal button organization Source: UniProtKB
  • terminal cell fate specification, open tracheal system Source: FlyBase


Molecular functionDevelopmental protein
Biological processmRNA processing, mRNA splicing

Names & Taxonomyi

Protein namesi
Recommended name:
Survival motor neuron proteinImported
Gene namesi
ORF Names:CG16725
OrganismiDrosophila melanogaster (Fruit fly)
Taxonomic identifieri7227 [NCBI]
Taxonomic lineageiEukaryotaMetazoaEcdysozoaArthropodaHexapodaInsectaPterygotaNeopteraHolometabolaDipteraBrachyceraMuscomorphaEphydroideaDrosophilidaeDrosophilaSophophora
  • UP000000803 Componenti: Chromosome 3L

Organism-specific databases

FlyBaseiFBgn0036641. Smn.

Subcellular locationi

GO - Cellular componenti

  • Cajal body Source: FlyBase
  • cytoplasm Source: UniProtKB
  • cytoplasmic U snRNP body Source: UniProtKB
  • Gemini of coiled bodies Source: UniProtKB
  • I band Source: FlyBase
  • neuromuscular junction Source: FlyBase
  • nucleus Source: FlyBase
  • plasma membrane Source: GOC
  • postsynapse Source: GOC
  • SmD-containing SMN-Sm protein complex Source: UniProtKB
  • SMN complex Source: FlyBase
  • SMN-Gemin2 complex Source: UniProtKB
  • spliceosomal complex Source: UniProtKB-KW
  • Z disc Source: FlyBase

Keywords - Cellular componenti

Cytoplasm, Nucleus, Spliceosome

Pathology & Biotechi

Disruption phenotypei

Embryos lacking maternal and zygotic Smn die between 0 and 4 hours after egg laying. Zygotic mutants never initiate pupation but instead persist as third instar larvae, often surviving at this stage for several days. Mutant larvae exhibit reduced CNS, testes and muscle size, decreased locomotion and altered rhythmic motor activity. At the NMJ, mutant larvae show an overall decrease in the number of synaptic boutons, but an increase in enlarged ones, loss of large glutamate receptor clusters and an aberrant increase in evoked excitatory postsynaptic potential (eEPSP) amplitude and in miniature EPSP frequency. Mutant larvae also show defective mira subcellular localization in pNBs. Mutant larvae show a decrease of spliceosomal snRNA levels and splicing defects in U12 intron-containing genes (PubMed:23063131). But appreciable splicing defects in U12 intron-containing genes are not observed in mutant larvae, although a decrease in spliceosomal snRNA levels is detected, in PubMed:22813737.8 Publications


Feature keyPosition(s)DescriptionActionsGraphical viewLength
Mutagenesisi20D → V: Does not affect homodimer formation. 1 Publication1
Mutagenesisi70F → S: Does not affect homodimer formation. 1 Publication1
Mutagenesisi201S → F in allele Smn-B; homozygous lethal at late larval stages and abolishes homodimerization. 1 Publication1
Mutagenesisi202G → S in allele Smn-73Ao; homozygous lethal at late larval stages and abolishes homodimerization. 1 Publication1
Mutagenesisi203Y → C: Weakly inhibits homodimer formation. 1 Publication1
Mutagenesisi205T → I: Rescues larval viability and locomotion defects and only partially restores U5 and U12 snRNA levels in the null mutant. Weakly inhibits homodimer formation. Does not affect protein stability. 1 Publication1
Mutagenesisi206G → S: Inhibits homodimer formation. 1 Publication1

PTM / Processingi

Molecule processing

Feature keyPosition(s)DescriptionActionsGraphical viewLength
ChainiPRO_00004243741 – 226Survival motor neuron proteinAdd BLAST226

Proteomic databases



Tissue specificityi

In late first instar larvae, expressed in pNBs. Expression increases as the pNBs enlarge, with the highest accumulation observed in dividing pNBs of second and third instar larvae. Enriched in type ID (thoracic and brain lobe), type IA and all the mira-expressing NBs of the brain lobes. In larvae, also expressed in muscle fibers. In larval and adult testis, expressed in germline stem cells and gonialblast, expression decreases as cells differentiate into cysts and spermatocytes. In adult fly thorax, expressed in the IFMs. In adult ovary, expressed in germline stem cells, cystoblasts, follicle cells, nurse cells and oocyte (at protein level). Also expressed in larval salivary glands.6 Publications

Developmental stagei

Expressed both maternally and zygotically. Expressed ubiquitously throughout development. Expression is high during embryogenesis but decreases 30-fold in adult flies (at protein level).5 Publications

Gene expression databases

GenevisibleiQ9VV74. DM.


Subunit structurei

Homodimer (via C-terminal region) (PubMed:11113446, PubMed:12783845, PubMed:22813737). Part of the core SMN complex, which seems to be composed of Smn and Gem2 only (PubMed:18621711, PubMed:23333303). The SMN complex associates with the entire set of spliceosomal snRNP Sm proteins, SmB, SmD1, SmD2, SmD3, SmE, SmF and SmG, and with the snRNP-specific proteins snRNP-U1-70K, U2A, snf/U1A and U5-116KD (PubMed:18621711, PubMed:23333303). Associates weakly with Gem3 (PubMed:18621711). Interacts with SmB and SmD1; the interaction is favored by methylation of the Sm proteins (PubMed:16753561, PubMed:18369183). Interacts with Actn; the interaction occurs in thoracic tissues and in adult flies (PubMed:17353360). Interacts with Rpp20 (PubMed:14715275). Interacts with msk and Snup; these interactions are RNA-dependent (PubMed:23885126).10 Publications

Binary interactionsi

Show more details

GO - Molecular functioni

  • alpha-actinin binding Source: FlyBase
  • identical protein binding Source: IntAct

Protein-protein interaction databases

BioGridi65147. 19 interactors.
IntActiQ9VV74. 7 interactors.


Secondary structure

Legend: HelixTurnBeta strandPDB Structure known for this area
Show more details
Feature keyPosition(s)DescriptionActionsGraphical viewLength
Helixi15 – 23Combined sources9

3D structure databases

Select the link destinations:
Links Updated
PDB entryMethodResolution (Å)ChainPositionsPDBsum

Family & Domainsi

Domains and Repeats

Feature keyPosition(s)DescriptionActionsGraphical viewLength
Domaini69 – 128TudorSequence analysisAdd BLAST60


Feature keyPosition(s)DescriptionActionsGraphical viewLength
Regioni159 – 226Required for homodimerization1 PublicationAdd BLAST68

Sequence similaritiesi

Belongs to the SMN family.Sequence analysis

Phylogenomic databases

eggNOGiKOG4327. Eukaryota.

Family and domain databases

InterProiView protein in InterPro
IPR010304. Survival_motor_neuron.
IPR002999. Tudor.
PfamiView protein in Pfam
PF06003. SMN. 2 hits.
SMARTiView protein in SMART
SM00333. TUDOR. 1 hit.


Sequence statusi: Complete.

Q9VV74-1 [UniParc]FASTAAdd to basket

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Mass (Da):24,622
Last modified:May 1, 2000 - v1

Sequence databases

Select the link destinations:
Links Updated
AF296281 mRNA. Translation: AAG17893.1.
AE014296 Genomic DNA. Translation: AAF49446.1.
AE014296 Genomic DNA. Translation: AGB94647.1.
AY058529 mRNA. Translation: AAL13758.1.
RefSeqiNP_001261954.1. NM_001275025.1.
NP_524112.1. NM_079388.4.

Genome annotation databases

EnsemblMetazoaiFBtr0075395; FBpp0075153; FBgn0036641.
FBtr0329921; FBpp0302954; FBgn0036641.
UCSCiCG16725-RA. d. melanogaster.

Similar proteinsi

Links to similar proteins from the UniProt Reference Clusters (UniRef) at 100%, 90% and 50% sequence identity:
100%UniRef100 combines identical sequences and sub-fragments with 11 or more residues from any organism into one UniRef entry.
90%UniRef90 is built by clustering UniRef100 sequences that have at least 90% sequence identity to, and 80% overlap with, the longest sequence (a.k.a seed sequence).
50%UniRef50 is built by clustering UniRef90 seed sequences that have at least 50% sequence identity to, and 80% overlap with, the longest sequence in the cluster.

Entry informationi

Entry nameiSMN_DROME
AccessioniPrimary (citable) accession number: Q9VV74
Entry historyiIntegrated into UniProtKB/Swiss-Prot: November 13, 2013
Last sequence update: May 1, 2000
Last modified: June 7, 2017
This is version 150 of the entry and version 1 of the sequence. See complete history.
Entry statusiReviewed (UniProtKB/Swiss-Prot)
Annotation programDrosophila annotation project


Keywords - Technical termi

3D-structure, Complete proteome, Reference proteome


  1. Drosophila
    Drosophila: entries, gene names and cross-references to FlyBase
  2. PDB cross-references
    Index of Protein Data Bank (PDB) cross-references
  3. SIMILARITY comments
    Index of protein domains and families