Affinage

SMG9

Nonsense-mediated mRNA decay factor SMG9 · UniProt Q9H0W8

Length
520 aa
Mass
57.7 kDa
Annotated
2026-06-10
15 papers in source corpus 10 papers cited in narrative 10 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 8/8 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SMG9 is a regulatory subunit of the SMG1 kinase complex (SMG1C) that safeguards the efficiency of nonsense-mediated mRNA decay (NMD) (PMID:19417104, PMID:41830328). Together with SMG8, SMG9 was identified as a component of the SMG1 complex that suppresses SMG1 kinase activity and is required for NMD in mammals and nematodes (PMID:19417104). SMG9 consists of an N-terminal intrinsically disordered region and a folded C-terminal domain, both required for SMG1 binding and SMG1C integrity, while its C-terminus mediates interaction with SMG8 (PMID:20817927). The SMG8-SMG9 pair forms a G-domain heterodimer architecturally related to dynamin-like GTPases, with nucleotide binding occurring at the SMG9 G domain but not SMG8 (PMID:28389433). Within SMG1C, SMG8 carries a C-terminal kinase inhibitory domain (KID) that occludes the SMG1 catalytic pocket, and structural analysis indicates GTP hydrolysis by SMG9 repositions the KID to restore kinase activity (PMID:31729466). During substrate engagement, SMG8 and SMG9 promote high-affinity UPF1 binding while decelerating SMG1 and enhancing the stringency of phosphorylation-site selection (PMID:26130714). Genetic deletion of SMG9 causes only modest NMD impairment with moderately elevated UPF1 phosphorylation, establishing SMG8 and SMG9 as nonessential modulators that buffer NMD against perturbation, since SMG9-deficient cells are hypersensitive to partial SMG1 inhibition (PMID:41830328). Beyond NMD, SMG9 is tyrosine-phosphorylated at Tyr-41 to regulate IQGAP1 binding and actin-cytoskeleton association (PMID:21640080), directly binds GPX4 to promote its mitochondrial degradation and drive ferroptosis (PMID:34146907), and its loss confers ATR-inhibitor resistance through an SMG1-dependent effect on replication stress signaling (PMID:36273494).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2009 High

    Established SMG9 as a bona fide subunit of the SMG1 kinase complex and a functional NMD factor, defining its core biological context.

    Evidence Co-IP, RNAi knockdown in mammals and C. elegans, biochemical fractionation

    PMID:19417104

    Open questions at the time
    • Molecular basis of kinase suppression not resolved
    • Domain requirements for complex assembly undefined
  2. 2010 High

    Resolved SMG9's domain organization, showing both its IDR and folded C-terminus are required for SMG1C integrity while the C-terminus alone binds SMG8.

    Evidence Domain-deletion mapping, Co-IP, SEC, limited proteolysis, EM

    PMID:20817927

    Open questions at the time
    • Functional role of SMG9 homo-oligomers and SMG8:SMG9 sub-complexes unknown
    • No atomic structure of the interfaces
  3. 2015 High

    Defined how SMG8/SMG9 shape catalysis by showing they promote high-affinity UPF1 recruitment while decelerating SMG1 and sharpening phosphosite selection, with UPF2 triggering substrate release.

    Evidence Cryo-EM of SMG1-8-9-UPF1, binding and kinase assays

    PMID:26130714

    Open questions at the time
    • SMG9-specific contributions not separated from SMG8
    • Conformational dynamics not captured at high resolution
  4. 2017 High

    Revealed the SMG8-SMG9 core as a G-domain heterodimer resembling dynamin-like GTPases, localizing nucleotide binding to SMG9, providing a structural basis for a GTPase-driven regulatory switch.

    Evidence X-ray crystallography (2.5 Å) of C. elegans SMG8-SMG9, nucleotide-binding assays, EM fitting

    PMID:28389433

    Open questions at the time
    • GTP hydrolysis activity and turnover not measured
    • Coupling of nucleotide state to kinase regulation untested
  5. 2019 High

    Connected SMG9's GTPase domain to kinase control by showing SMG8's KID occludes the SMG1 catalytic pocket and that SMG9-driven conformational change would relieve this inhibition; an InsP6 cofactor supports optimal substrate phosphorylation.

    Evidence Cryo-EM (3.4–3.45 Å) of human SMG1-8-9, MS, kinase assays with InsP6-site mutants

    PMID:31729466 PMID:31792449

    Open questions at the time
    • Direct demonstration that SMG9 GTP hydrolysis triggers KID release in vivo absent
    • Timing of the switch during NMD undefined
  6. 2011 Medium

    Identified a non-NMD activity in which Tyr-41 phosphorylation of SMG9 governs binding to the actin modifier IQGAP1, implicating SMG9 in cytoskeletal signaling.

    Evidence Co-IP, Tyr-41 phospho-mutagenesis, immunofluorescence, EGF stimulation

    PMID:21640080

    Open questions at the time
    • Kinase responsible for Tyr-41 phosphorylation unknown
    • Functional consequence for actin dynamics not established
    • Single lab, limited orthogonal validation
  7. 2021 Medium

    Showed an NMD-independent role: SMG9 directly binds GPX4 and promotes its mitochondrial degradation, sensitizing cells to RSL3-induced ferroptosis.

    Evidence RNAi screen, Co-IP, siRNA knockdown, subcellular fractionation, xenografts

    PMID:34146907

    Open questions at the time
    • Mechanism of GPX4 degradation not defined
    • How SMG9 distinguishes mitochondrial GPX4 pools unclear
    • Single lab
  8. 2022 Medium

    Linked SMG9 to replication stress responses by showing its loss causes ATR-inhibitor resistance via an SMG1-mediated reduction in transcription-replication conflicts and DNA damage signaling.

    Evidence Genome-wide CRISPR resistance screen, genetic validation, DNA damage marker IF, TRC assays

    PMID:36273494

    Open questions at the time
    • Molecular target of SMG1 in this pathway unidentified
    • Whether the role requires NMD vs. a direct activity unresolved
    • Single lab
  9. 2026 High

    Reframed SMG9 (and SMG8) as nonessential NMD modulators by showing their deletion only modestly impairs NMD but sensitizes cells to partial SMG1 inhibition, and that the SMG8 KID is dispensable in vivo.

    Evidence CRISPR KO, pharmacological SMG1 inhibition, RNA-seq, UPF1 phospho-blot across cell lines

    PMID:41830328

    Open questions at the time
    • In vivo trigger and timing of the GTPase switch still undefined
    • Reconciliation of dispensable KID with structural inhibition model incomplete

Open questions

Synthesis pass · forward-looking unresolved questions
  • Whether and how SMG9 GTP hydrolysis is catalytically coupled to relief of SMG8-mediated kinase inhibition during a physiological NMD cycle remains unresolved.
  • No measurement of SMG9 GTPase turnover within active SMG1C in cells
  • Mechanistic basis of NMD-independent functions (ferroptosis, replication stress, cytoskeleton) not integrated

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 3 GO:0003924 GTPase activity 2
Localization
GO:0005739 mitochondrion 1
Pathway
R-HSA-8953854 Metabolism of RNA 3
Partners
GPX4IQGAP1SMG1SMG8UPF1
Complex memberships
SMG1C (SMG1-SMG8-SMG9)

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2009 SMG-9 (together with SMG-8) was identified as a novel subunit of the SMG-1 kinase complex; SMG-8 and SMG-9 suppress SMG-1 kinase activity in the isolated SMG-1 complex and are involved in NMD in both mammals and nematodes. SMG-8 recruits SMG-1 to the mRNA surveillance complex. Co-immunoprecipitation, functional RNAi knockdown in mammals and C. elegans, biochemical fractionation Genes & development High 19417104
2010 SMG-9 comprises an N-terminal intrinsically disordered region (IDR, ~180 residues) followed by a well-folded C-terminal domain; both domains are required for SMG-1 binding and SMG1C complex integrity, whereas the C-terminus alone is sufficient to interact with SMG-8. SMG-9 also forms SMG-9:SMG-9 homo-oligomers and SMG-8:SMG-9 complexes that are distinct from SMG1C. Biochemical domain-deletion mapping, co-immunoprecipitation, biophysical characterization (SEC, limited proteolysis), electron microscopy Nucleic acids research High 20817927
2015 Cryo-EM of the SMG-1-8-9-UPF1 complex revealed that UPF1 is recruited to both the SMG-1 kinase domain and C-terminal insertion domain, inducing opening of the head domain to expose the active site. SMG-8 and SMG-9 interact with the SMG-1 C-insertion domain, promoting high-affinity UPF1 binding while decelerating SMG-1 kinase activity and enhancing stringency of phosphorylation site selection. UPF2 binding destabilizes the SMG-1-8-9-UPF1 complex, promoting substrate release. Electron cryo-microscopy (cryo-EM) of SMG-1-8-9-UPF1 complex, biochemical binding assays, kinase activity assays Nucleic acids research High 26130714
2017 Crystal structure (2.5 Å) of the SMG8-SMG9 core complex from C. elegans revealed a G-domain heterodimer with architectural similarity to dynamin-like GTPases (Atlastin, GBP1). Nucleotide binding occurs at the G domain of SMG9 but not of SMG8. The heterodimer forms in the absence of nucleotides, with interactions conserved from worms to humans. X-ray crystallography (2.5 Å), nucleotide-binding assays, fitting into EM densities of human SMG1-SMG8-SMG9 RNA (New York, N.Y.) High 28389433
2019 Cryo-EM structure (3.4 Å) of the human SMG1-SMG8-SMG9 complex showed that SMG8 contains a C-terminal kinase inhibitory domain (KID) that covers the catalytic pocket of SMG1. Structural analysis suggested GTP hydrolysis by SMG9 would cause a conformational change moving the KID away from the inhibitory position to restore SMG1 kinase activity. Cryo-EM (3.4 Å and 3.6 Å resolution), biochemical kinase inhibition assays Cell research High 31729466
2019 Cryo-EM structure (3.45 Å) of human SMG1-SMG8-SMG9 combined with MS analysis revealed the presence of inositol hexaphosphate (InsP6) bound in the SMG1 kinase; the InsP6-binding site is required for optimal in vitro phosphorylation of SMG1 substrates. Cryo-EM (3.45 Å), mass spectrometry, in vitro kinase activity assays with InsP6-binding site mutants Nature structural & molecular biology High 31792449
2011 SMG-9 is tyrosine-phosphorylated at Tyr-41; phosphorylation at this site regulates binding of SMG-9 to IQGAP1, an actin cytoskeleton modifier. SMG-9 co-localizes with IQGAP1 at sites of actin enrichment in non-stimulated cells but not in EGF-stimulated cells. EGF stimulation increases the ability of SMG-9 to bind SMG-8. Co-immunoprecipitation, phospho-site mutagenesis (Tyr-41), immunofluorescence co-localization, EGF stimulation assays Biochemical and biophysical research communications Medium 21640080
2021 SMG9 directly binds GPX4 and promotes its degradation in response to the GPX4 inhibitor RSL3 (but not erastin). Genetic inhibition of SMG9 increases GPX4 accumulation specifically in mitochondria, preventing mitochondrial oxidative damage and conferring ferroptosis resistance. This function is independent of SMG9's role in NMD. RNAi screen, co-immunoprecipitation (SMG9-GPX4 direct binding), siRNA knockdown, subcellular fractionation (mitochondrial GPX4 accumulation), xenograft mouse models Biochemical and biophysical research communications Medium 34146907
2022 Loss-of-function mutations in SMG9 (or SMG8) cause ATR inhibitor resistance through an SMG1-mediated mechanism. SMG8/9-deficient cells showed reduced ATRi-induced transcription/replication conflicts (TRCs) and lacked characteristic ATRi-induced DNA damage signaling changes (ATM/CHK2, γH2AX, phospho-RPA, 53BP1), establishing SMG8/SMG9/SMG1 pathway involvement in the cellular response to replication stress. Genome-wide CRISPR-Cas9 resistance screen, loss-of-function genetic validation, cell cycle analysis, DNA damage marker immunofluorescence, TRC measurement assays Cancer research Medium 36273494
2026 Complete deletion of SMG9 (or SMG8) in human cells caused only modest NMD impairment with moderately increased UPF1 phosphorylation. Deletion of the SMG8 kinase inhibitory domain (KID) alone did not affect UPF1 phosphorylation or NMD efficiency, demonstrating the KID is dispensable in vivo. However, SMG9-deficient (and SMG8-deficient) cells showed pronounced hypersensitivity to partial pharmacological SMG1 inhibition, establishing SMG8 and SMG9 as nonessential modulators that safeguard NMD efficiency and perturbation tolerance. CRISPR-Cas9 gene deletion, pharmacological SMG1 inhibition, RNA-seq transcriptome-wide NMD target analysis, UPF1 phosphorylation western blot, multiple human cell lines Nucleic acids research High 41830328

Source papers

Stage 0 corpus · 15 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2009 SMG-8 and SMG-9, two novel subunits of the SMG-1 complex, regulate remodeling of the mRNA surveillance complex during nonsense-mediated mRNA decay. Genes & development 199 19417104
2010 Characterization of SMG-9, an essential component of the nonsense-mediated mRNA decay SMG1C complex. Nucleic acids research 73 20817927
2015 A network of SMG-8, SMG-9 and SMG-1 C-terminal insertion domain regulates UPF1 substrate recruitment and phosphorylation. Nucleic acids research 52 26130714
2021 SMG9 drives ferroptosis by directly inhibiting GPX4 degradation. Biochemical and biophysical research communications 38 34146907
2019 InsP6 binding to PIKK kinases revealed by the cryo-EM structure of an SMG1-SMG8-SMG9 complex. Nature structural & molecular biology 31 31792449
2019 Cryo-EM structure of SMG1-SMG8-SMG9 complex. Cell research 27 31729466
2022 SMG8/SMG9 Heterodimer Loss Modulates SMG1 Kinase to Drive ATR Inhibitor Resistance. Cancer research 14 36273494
2022 A novel variant in SMG9 causes intellectual disability, confirming a role for nonsense-mediated decay components in neurocognitive development. European journal of human genetics : EJHG 12 35087184
2017 Structure of a SMG8-SMG9 complex identifies a G-domain heterodimer in the NMD effector proteins. RNA (New York, N.Y.) 11 28389433
2011 Role of a tyrosine phosphorylation of SMG-9 in binding of SMG-9 to IQGAP and the NMD complex. Biochemical and biophysical research communications 7 21640080
2021 Expanding the phenotypic and allelic spectrum of SMG8: Clinical observations reveal overlap with SMG9-associated disease trait. American journal of medical genetics. Part A 5 34761517
2019 MicroRNA 4651 regulates nonsense-mediated mRNA decay by targeting SMG9 mRNA. Gene 5 30902786
2021 SMG9 Serves as an Oncogene to Promote the Tumor Progression via EMT and Wnt/β-Catenin Signaling Pathway in Hepatocellular Carcinoma. Frontiers in pharmacology 4 34456727
2022 Identification of a novel compound heterozygous SMG9 variants in a Chinese family with heart and brain malformation syndrome using whole exome sequencing. BMC medical genomics 2 35321723
2026 SMG1:SMG8:SMG9-complex integrity supports efficient execution of nonsense-mediated mRNA decay. Nucleic acids research 0 41830328

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