Affinage

SPAG1

Sperm-associated antigen 1 · UniProt Q07617

Length
926 aa
Mass
103.6 kDa
Annotated
2026-06-10
16 papers in source corpus 10 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 4/4 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SPAG1 is a cytoplasmic dynein axonemal assembly factor (DNAAF) required for the preassembly of ciliary dynein arms; in airway epithelia it is present in the cytoplasm but absent from assembled axonemes, and loss-of-function mutations cause primary ciliary dyskinesia with combined loss of outer and inner dynein arm proteins from cilia (PMID:24055112). Mechanistically, SPAG1 acts as a scaffold for an R2TP-like chaperone assembly, interacting with multiple DNAAFs, dynein heavy chains (DHCs), dynein intermediate chains (DICs), and canonical R2TP components; in SPAG1-mutant tissue DHC levels fall and DHC–DIC association is disrupted, establishing that SPAG1 promotes the folding and loading of dynein heavy chains onto the intermediate-chain complex during dynein arm assembly (PMID:35178554). SPAG1 carries tetratricopeptide-repeat (TPR) domains, two of whose three TPR units recruit the C-terminal tails of HSP70 and HSP90, an interaction defined at atomic resolution and providing the chaperone-recruitment basis for client folding (PMID:31118266, PMID:33739091); structural analysis of the SPAG1-containing R2SP complex (RUVBL1, RUVBL2, SPAG1, PIH1D2) shows SPAG1 and PIH1D2 serving as client-binding adaptors that drive quaternary assembly. Beyond ciliary assembly, RNAi studies implicate SPAG1 in meiotic spindle morphogenesis and germinal vesicle breakdown in oocytes through AMPK and MAPK signaling (PMID:27053660) and in PI3K/AKT-driven Sertoli cell proliferation (PMID:29119857).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 2001 Medium

    Initial characterization established SPAG1 as a TPR- and P-loop-containing, PKC-phosphorylated protein expressed in male germ cells, framing it as a candidate signaling/scaffold molecule.

    Evidence GTP-binding blot overlay, in vitro GTPase and PKC phosphorylation assays, and immunostaining of human spermatozoa and seminiferous epithelium

    PMID:11517287

    Open questions at the time
    • GTPase activity inferred from blot overlay was not validated by quantitative kinetics
    • No cellular function assigned at this stage
    • Reproductive localization not connected to a defined pathway
  2. 2002 Low

    A yeast two-hybrid screen began to define SPAG1 binding partners, identifying the G-protein β1 subunit C-terminus as an interactor and mapping the interaction to an intact bait domain.

    Evidence Yeast two-hybrid screen of a human ovary cDNA library with the HSD-0.7 fragment

    PMID:12905684

    Open questions at the time
    • Yeast two-hybrid only, with no in-cell confirmation in this study
    • Functional consequence of the interaction not tested
    • Single bait fragment used
  3. 2006 Medium

    The SPAG1–Gβ1 interaction was validated in cells and linked to a functional signaling output, showing SPAG1 fragments activate ERK1/2 through a PKC-dependent route requiring both TPR and P-loop motifs.

    Evidence Co-immunoprecipitation and co-localization in HEK293 cells with ERK1/2 activation and domain-deletion assays

    PMID:16368546

    Open questions at the time
    • Used overexpressed fragments rather than full-length endogenous protein
    • ERK activation not connected to ciliary or reproductive phenotype
    • Single lab
  4. 2013 High

    Discovery of loss-of-function SPAG1 mutations in primary ciliary dyskinesia, combined with its cytoplasmic (non-axonemal) localization, repositioned SPAG1 as a cytoplasmic dynein arm assembly/trafficking factor rather than a structural cilia component.

    Evidence Exome sequencing, subcellular fractionation, immunofluorescence of patient cilia, and zebrafish morpholino knockdown

    PMID:24055112

    Open questions at the time
    • Did not define molecular partners or the biochemical assembly step
    • Mechanism linking SPAG1 loss to absent ODA and IDA proteins unresolved
  5. 2016 Medium

    RNAi in oocytes extended SPAG1 function beyond cilia, implicating it in meiotic spindle morphogenesis and germinal vesicle breakdown via cAMP/ATP balance, AMPK activation, and MAPK signaling.

    Evidence RNAi knockdown in mouse oocytes with live imaging, immunofluorescence, cAMP/ATP measurement, and AMPK/MAPK readouts

    PMID:27053660

    Open questions at the time
    • Direct molecular targets of SPAG1 at the spindle not identified
    • Relationship to the ciliary assembly role unclear
    • Single lab
  6. 2017 Medium

    SPAG1 was identified as a miR-638 target that sustains PI3K/AKT signaling to drive Sertoli cell proliferation, adding a proliferative signaling role.

    Evidence miRNA target validation and SPAG1 siRNA knockdown with Western blot of PI3K/AKT and cell cycle factors in porcine Sertoli cells

    PMID:29119857

    Open questions at the time
    • How SPAG1 engages the PI3K/AKT pathway mechanistically not defined
    • Direct binding partners in this context unknown
  7. 2019 High

    Biochemical and NMR analysis established the chaperone-recruitment basis of SPAG1, showing two of its three TPR domains bind HSP70/HSP90 C-terminal tails, and directly challenged the earlier intrinsic GTPase claim.

    Evidence ITC, NMR spectroscopy, MD simulations, and an in vitro GTPase assay on the P-loop fragment

    PMID:31118266

    Open questions at the time
    • Did not resolve full-length complex architecture
    • Client specificity of chaperone recruitment not addressed
  8. 2021 High

    Atomic-resolution structure of SPAG1-TPR1 detailed the specific motifs positioning HSP70/HSP90 peptides, refining the chaperone-binding mechanism.

    Evidence NMR structure determination of an optimized SPAG1-TPR1 variant with biophysical binding assays

    PMID:33739091

    Open questions at the time
    • Single TPR domain in isolation
    • Does not capture client (dynein) engagement
  9. 2022 High

    Co-IP in human airway epithelia placed SPAG1 within an R2TP-like scaffold that promotes DHC folding and DHC–DIC assembly, and identified a 60 kDa SPAG1 isoform capable of partial ODA rescue.

    Evidence Reciprocal immunoprecipitation, Western blot in control vs PCD epithelia, and isoform/ultrastructural analysis

    PMID:35178554

    Open questions at the time
    • Stoichiometry and assembly order not fully resolved
    • Functional role of the 60 kDa isoform inferred from patient ultrastructure only
  10. 2025 High

    Cryo-EM of the human R2SP complex defined how SPAG1 and PIH1D2 act as client-binding adaptors and how RUVBL1/2 ATPase behavior differs from canonical R2TP, providing the structural framework for SPAG1-mediated quaternary assembly.

    Evidence Cryo-EM, NMR, structural mass spectrometry, reconstitution, and ATPase assays (preprint)

    Open questions at the time
    • Preprint, not yet peer-reviewed
    • Dynein client engagement within the intact complex not directly visualized

Open questions

Synthesis pass · forward-looking unresolved questions
  • How SPAG1's chaperone-scaffolding role mechanistically integrates with its reported signaling functions (AMPK/MAPK, PI3K/AKT, ERK) across reproductive and proliferative contexts remains unresolved.
  • No unifying mechanism connecting dynein assembly scaffold to signaling outputs
  • Direct signaling substrates/effectors of SPAG1 undefined
  • Tissue-specific isoform contributions not mapped

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0044183 protein folding chaperone 2 GO:0060090 molecular adaptor activity 2 GO:0140096 catalytic activity, acting on a protein 2
Localization
GO:0005829 cytosol 2 GO:0005856 cytoskeleton 1
Pathway
R-HSA-1852241 Organelle biogenesis and maintenance 2 R-HSA-392499 Metabolism of proteins 2
Partners
GNB1HSP70HSP90PIH1D2RUVBL1RUVBL2
Complex memberships
R2SP complex (RUVBL1, RUVBL2, SPAG1, PIH1D2)R2TP-like dynein assembly scaffold

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2013 SPAG1 is present in human airway epithelial cell lysates but absent from isolated axonemes; immunofluorescence showed absence of ODA and IDA proteins in cilia from affected individuals with SPAG1 mutations, indicating SPAG1 plays a role in cytoplasmic assembly and/or trafficking of axonemal dynein arms rather than being a structural cilia component. Zebrafish morpholino knockdown of spag1 produced cilia-related phenotypes consistent with cytoplasmic assembly factor function. Subcellular fractionation, immunofluorescence, zebrafish morpholino knockdown, exome sequencing with loss-of-function mutation analysis American journal of human genetics High 24055112
2022 SPAG1 interacts (by immunoprecipitation) with multiple DNAAFs (dynein axonemal assembly factors), dynein heavy chains (DHCs), dynein intermediate chains (DICs), and canonical components of the R2TP complex. In SPAG1 mutants, protein levels of DHCs were reduced and interactions between DHCs and DICs were diminished, demonstrating that SPAG1 scaffolds R2TP-like complexes to facilitate folding/binding of DHCs to the DIC complex during dynein arm assembly. Immunoprecipitation, protein interaction studies, Western blot in control vs. PCD human airway epithelia Journal of cell science High 35178554
2022 A previously uncharacterized 60 kDa SPAG1 isoform can partially compensate for the absence of full-length SPAG1 to assemble a reduced number of outer dynein arms, as identified in PCD subjects with atypical ultrastructural defects. Protein isoform identification in patient-derived airway epithelia, ultrastructural analysis Journal of cell science Medium 35178554
2019 The TPR domains of SPAG1 recruit HSP70 and HSP90 chaperones: only two of the three TPR domains are capable of recruiting these chaperones. NMR-driven docking and MD simulations defined the binding interface between SPAG1-TPR1 and the C-terminal tails of HSP70 and HSP90. Additionally, a SPAG1 sub-fragment containing a putative P-loop motif cannot efficiently bind or hydrolyze GTP in vitro, challenging prior claims of intrinsic GTPase activity. Biochemical assays, isothermal titration calorimetry (ITC), NMR spectroscopy, molecular dynamics simulations, in vitro GTPase assay The Biochemical journal High 31118266
2021 Structural and biophysical analysis of the first TPR domain of human SPAG1 (using an optimized variant) revealed with atomistic precision how the C-terminal tails of HSP70 and HSP90 bind the SPAG1-TPR1 domain, identifying specific motifs in the TPR sequence that drive HSP peptide positioning. Protein sequence optimization, NMR structure determination, biophysical binding assays Biochemistry High 33739091
2001 The HSD-3.8 protein (SPAG1) contains tetratricopeptide repeat (TPR) motifs, a P-loop sequence, and phosphorylation sites. GTP-binding was demonstrated by blot overlay assay with [α-32P]GTP; the protein possesses GTPase activity and is phosphorylated by PKC in vitro. The protein localizes to the postacrosomal zone surface of human spermatozoa and to germ cells in seminiferous epithelium. GTP-binding blot overlay assay, in vitro GTPase assay, in vitro PKC phosphorylation assay, immunostaining Molecular human reproduction Medium 11517287
2006 HSD-3.8 (SPAG1) interacts with the C-terminal 144 amino acids of G-protein β1 subunit (Gβ1), as identified by yeast two-hybrid and confirmed by co-immunoprecipitation in HEK293 cells (where both proteins co-localized in the cytoplasm). Overexpression of the HSD-0.7 fragment activated ERK1/2 in a PKC-dependent (not Ras-dependent) manner; deletion of either the TPR domain or P-loop abolished ERK1/2 activation. Yeast two-hybrid, co-immunoprecipitation, co-transfection in HEK293 cells, ERK1/2 activation assay, domain deletion analysis Frontiers in bioscience Medium 16368546
2002 Yeast two-hybrid screening with the 0.7 kb fragment of HSD-3.8 (SPAG1) identified the C-terminal 144 amino acids of human G-protein β1 subunit as an interacting partner; truncated bait plasmids lacking parts of the HSD-0.7 sequence did not interact, indicating the interaction requires the intact bait domain. Yeast two-hybrid screen of human ovary cDNA library Acta Academiae Medicinae Sinicae Low 12905684
2016 In mouse oocytes, SPAG1 associates with meiotic spindles. RNAi-mediated depletion of SPAG1 impaired germinal vesicle breakdown (GVBD) by increasing intracellular cAMP and decreasing ATP production, activating AMPK. SPAG1 depletion also disrupted spindle morphogenesis by reducing γ-tubulin function and MAPK phosphorylation at spindle poles, and decreased actin expression with disruption of actin caps, cortical granule-free domains, and the contractile ring. RNAi knockdown in mouse oocytes, live imaging, immunofluorescence, cAMP/ATP measurement, AMPK activation assay, MAPK phosphorylation analysis Molecular biology of the cell Medium 27053660
2017 SPAG1 is a direct target of miR-638 in porcine immature Sertoli cells; SPAG1 RNAi reduced phospho-PI3K and phospho-AKT levels, and downregulated cell cycle factors (c-MYC, CCND1, CCNE1, CDK4), demonstrating that SPAG1 sustains PI3K/AKT pathway activation to promote Sertoli cell proliferation. miRNA target validation (luciferase or equivalent), SPAG1 siRNA knockdown, Western blot for PI3K/AKT pathway components Cell cycle Medium 29119857
2022 SPAG1 knockdown in AML cells reduced proliferation and survival, downregulated SMC3 expression, and suppressed ERK/MAPK signaling pathway activation. Inhibiting SPAG1 also altered AML cell susceptibility to venetoclax. RNA interference knockdown in AML cell lines, proliferation/survival assays, Western blot for ERK/MAPK pathway Neoplasma Low 35951456
2025 Cryo-EM and structural mass spectrometry revealed the 3D organization of the human R2SP complex (RUVBL1, RUVBL2, SPAG1, PIH1D2), showing a structure similar to the canonical R2TP complex but with differences in RUVBL1/2 ATPase mode of action and in how adaptors SPAG1 and PIH1D2 bind. SPAG1 and PIH1D2 function as adaptors that interact with specific clients to promote quaternary assembly. Cryo-EM, NMR, structural mass spectrometry, biochemical reconstitution, ATPase assay bioRxivpreprint High

Source papers

Stage 0 corpus · 16 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2013 Mutations in SPAG1 cause primary ciliary dyskinesia associated with defective outer and inner dynein arms. American journal of human genetics 113 24055112
1994 Polymorphism of SPAG-1, a candidate antigen for inclusion in a sub-unit vaccine against Theileria annulata. Molecular and biochemical parasitology 44 7838169
1993 Immunoglobulin M reactivity towards the immunologically active region sp75 of the core protein of hepatitis C virus (HCV) in chronic HCV infection. Journal of medical virology 42 8388029
2017 miR-638 Inhibits immature Sertoli cell growth by indirectly inactivating PI3K/AKT pathway via SPAG1 gene. Cell cycle (Georgetown, Tex.) 37 29119857
1994 Theileria annulata sporozoite surface antigen (SPAG-1) contains neutralizing determinants in the C terminus. Parasite immunology 26 7517029
2006 Vaccination of calves with an attenuated cell line of Theileria annulata and the sporozoite antigen SPAG-1 produces a synergistic effect. Veterinary parasitology 24 16870344
2016 The GTPase SPAG-1 orchestrates meiotic program by dictating meiotic resumption and cytoskeleton architecture in mouse oocytes. Molecular biology of the cell 20 27053660
2001 Expression and function of the HSD-3.8 gene encoding a testis-specific protein. Molecular human reproduction 15 11517287
2022 The role of SPAG1 in the assembly of axonemal dyneins in human airway epithelia. Journal of cell science 14 35178554
2019 Binding properties of the quaternary assembly protein SPAG1. The Biochemical journal 13 31118266
2006 A sperm component, HSD-3.8 (SPAG1), interacts with G-protein beta 1 subunit and activates extracellular signal-regulated kinases (ERK). Frontiers in bioscience : a journal and virtual library 13 16368546
1999 A tetratricopeptide repeat-containing protein gene, tpis, whose expression is induced with differentiation of spermatogenic cells. Biochemical and biophysical research communications 13 10527845
1996 SP75 is encoded by the DP87 gene and belongs to a family of modular Dictyostelium discoideum outer layer spore coat proteins. Microbiology (Reading, England) 13 8760935
2021 Optimizing the First TPR Domain of the Human SPAG1 Protein Provides Insight into the HSP70 and HSP90 Binding Properties. Biochemistry 7 33739091
2022 SPAG1 promotes the development of AML by activating the ERK/MAPK signaling pathway and affects the chemotherapy sensitivity of venetoclax. Neoplasma 3 35951456
2002 [Study on the function of HSD-3.8 gene encoding a testis-specific protein with yeast two-hybrid system]. Zhongguo yi xue ke xue yuan xue bao. Acta Academiae Medicinae Sinicae 1 12905684

Missed literature

Know a paper Affinage missed for SPAG1? Flag it for the maintainers and the community.

No submissions yet.