Affinage

SPAG1

Sperm-associated antigen 1 · UniProt Q07617

Length
926 aa
Mass
103.6 kDa
Annotated
2026-04-28
16 papers in source corpus 10 papers cited in narrative 10 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SPAG1 is a cytoplasmic dynein axonemal assembly factor that scaffolds the R2SP complex (with RUVBL1, RUVBL2, and PIH1D2) to facilitate the pre-assembly and folding of axonemal dynein heavy chains and their association with intermediate chains before transport into cilia (PMID:24055112, PMID:35178554). SPAG1 contains three TPR domains, two of which recruit HSP70 and HSP90 chaperones by binding their C-terminal tails, as defined by NMR structures and biophysical assays; SPAG1 itself lacks efficient GTPase activity and instead coordinates the ATPase activities of its RUVBL1/2 and chaperone partners (PMID:31118266, PMID:33739091). Loss-of-function mutations in SPAG1 cause primary ciliary dyskinesia with combined outer and inner dynein arm defects (PMID:24055112). In reproductive cells, SPAG1 knockdown impairs meiotic spindle morphogenesis in oocytes and modulates PI3K/AKT and ERK/MAPK signaling in Sertoli cells, indicating additional roles in germ cell proliferation and meiotic progression (PMID:27053660, PMID:29119857).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2001 Medium

    Initial biochemical characterization identified SPAG1 as a testis-expressed, TPR- and P-loop-containing protein with apparent GTP-binding and GTPase activity, establishing its domain architecture and spermatozoon localization but leaving its cellular function unknown.

    Evidence GTP overlay assay, in vitro GTPase/PKC assays, immunostaining of human spermatozoa

    PMID:11517287

    Open questions at the time
    • GTPase activity was later challenged by NMR/biochemical studies
    • no physiological substrate or pathway identified
    • no loss-of-function data
  2. 2006 Medium

    Interaction with Gβ1 and demonstration that SPAG1 fragments activate ERK1/2 via PKC in a TPR- and P-loop-dependent manner suggested SPAG1 could modulate MAPK signaling, expanding its functional scope beyond sperm antigen to a signaling scaffold.

    Evidence Yeast two-hybrid, co-IP in HEK293 cells, ERK1/2 activation with domain deletion analysis

    PMID:16368546

    Open questions at the time
    • single lab, not replicated independently
    • overexpression system; physiological relevance in vivo unclear
    • Gβ1 interaction not validated in motile cilia biology
  3. 2013 High

    Identification of SPAG1 mutations in primary ciliary dyskinesia families established that SPAG1 is a dynein axonemal assembly factor required for both outer and inner dynein arm assembly; fractionation showed it acts in the cytoplasm rather than as an axonemal structural component.

    Evidence Immunofluorescence, axoneme fractionation, analysis of 14 affected individuals, zebrafish morpholino knockdown

    PMID:24055112

    Open questions at the time
    • precise molecular mechanism of dynein pre-assembly unknown
    • identity of direct dynein client proteins not established
    • relationship to R2TP/chaperone machinery not yet defined
  4. 2016 Medium

    RNAi depletion in mouse oocytes revealed a role for SPAG1 in meiotic spindle morphogenesis via regulation of MAPK phosphorylation, γ-tubulin distribution, and AMPK-dependent metabolic signaling, extending SPAG1 function beyond motile cilia.

    Evidence RNAi in mouse oocytes, live imaging, cAMP/ATP measurement, immunofluorescence

    PMID:27053660

    Open questions at the time
    • single lab; mechanism linking SPAG1 to AMPK/cAMP unclear
    • whether this reflects TPR/chaperone scaffolding or an independent function is unknown
    • no genetic knockout confirmation
  5. 2017 Medium

    Validation of SPAG1 as a miR-638 target in Sertoli cells showed that SPAG1 knockdown suppresses PI3K/AKT signaling and proliferation, indicating SPAG1 supports spermatogenic cell survival through this pathway.

    Evidence Luciferase reporter assay, siRNA knockdown, Western blot for PI3K/AKT in porcine Sertoli cells

    PMID:29119857

    Open questions at the time
    • porcine model; relevance to human spermatogenesis not confirmed
    • mechanism by which SPAG1 activates PI3K/AKT not defined
    • single lab
  6. 2019 High

    NMR structure of a SPAG1 TPR domain and biophysical analysis resolved how two of three TPR domains recruit HSP70/HSP90 via their C-terminal tails, while demonstrating that SPAG1 itself lacks efficient GTPase activity — redefining SPAG1 as a chaperone-recruiting scaffold rather than an enzyme.

    Evidence NMR spectroscopy, ITC, biochemical GTPase assays, molecular dynamics simulations

    PMID:31118266

    Open questions at the time
    • role of the third non-chaperone-binding TPR domain unknown
    • how chaperone recruitment couples to dynein folding not shown
    • full-length SPAG1 structure not available
  7. 2021 High

    Atomistic structural detail of the first TPR domain binding HSP70/HSP90 C-terminal peptides provided a high-resolution map of the chaperone recruitment interface, refining the structural basis for SPAG1's adaptor function.

    Evidence Optimized TPR variant, NMR spectroscopy, biophysical binding assays

    PMID:33739091

    Open questions at the time
    • binding measured with isolated domains; full-length context and client-loaded complexes not characterized
    • no mutagenesis-based functional validation in cells
  8. 2022 High

    Proteomic and co-IP studies in PCD patient airway epithelia demonstrated that SPAG1 directly scaffolds the R2SP complex (RUVBL1, RUVBL2, PIH1D2) and is required for dynein heavy chain stability and heavy chain–intermediate chain interactions, providing the molecular mechanism for its dynein assembly role.

    Evidence Reciprocal co-IPs, quantitative mass spectrometry in human airway epithelia

    PMID:35178554

    Open questions at the time
    • how the 60 kDa isoform partially compensates is unclear
    • order of assembly steps (client loading, chaperone release) not resolved
    • whether SPAG1 participates in dynein transport to cilia or only cytoplasmic pre-assembly is unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • The full 3D architecture of the client-loaded R2SP complex, the precise order of dynein arm assembly steps mediated by SPAG1, and the mechanistic basis for SPAG1's roles in meiotic spindle morphogenesis and MAPK/PI3K signaling remain to be determined.
  • no full-length SPAG1 structure in complex with dynein clients
  • no reconstituted in vitro dynein assembly assay with R2SP
  • non-ciliary signaling functions lack mechanistic connection to known TPR/R2SP scaffolding

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 4 GO:0044183 protein folding chaperone 2
Localization
GO:0005829 cytosol 2 GO:0005929 cilium 1
Pathway
R-HSA-162582 Signal Transduction 3 R-HSA-1852241 Organelle biogenesis and maintenance 2 GO:0005929 cilium 1
Partners
GNB1HSP90AA1HSPA1APIH1D2RUVBL1RUVBL2
Complex memberships
R2SP (RUVBL1/RUVBL2/SPAG1/PIH1D2)

Evidence

Reading pass · 10 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 shows absence of both outer and inner dynein arm (ODA and IDA) proteins in cilia from SPAG1-mutant individuals, indicating SPAG1 functions in the cytoplasmic pre-assembly and/or trafficking of axonemal dynein arms rather than as a structural axonemal component. Zebrafish spag1 morpholino knockdown produced cilia-related phenotypes consistent with cytoplasmic assembly factors. Immunofluorescence, axoneme fractionation/cell lysate comparison, zebrafish morpholino knockdown American journal of human genetics High 24055112
2022 SPAG1 interacts with multiple dynein axonemal assembly factors (DNAAFs), dynein heavy chains (DHCs), dynein intermediate chains (DICs), and canonical R2TP complex components (RUVBL1, RUVBL2, PIH1D2) by immunoprecipitation. SPAG1 loss reduces DHC protein levels and impairs DHC–DIC interactions, showing SPAG1 scaffolds R2TP-like complexes to facilitate folding/binding of DHCs to DIC complexes. A 60 kDa SPAG1 isoform can partially compensate for full-length SPAG1 loss to assemble a reduced number of outer dynein arms. Immunoprecipitation, quantitative proteomics (mass spectrometry), analysis of PCD patient airway epithelia Journal of cell science High 35178554
2019 SPAG1 contains three TPR domains, but only two of them recruit the chaperones HSP70 and HSP90 via binding to their C-terminal tails. NMR structure of one TPR domain was solved, and NMR-driven docking plus molecular dynamics simulations defined the binding interface with HSP70 and HSP90 C-terminal peptides. A SPAG1 sub-fragment containing the putative P-loop motif cannot efficiently bind or hydrolyze GTP in vitro, challenging prior reports of SPAG1 GTPase activity and suggesting SPAG1 instead regulates nucleotide hydrolysis of HSP and RUVBL1/2 partners. NMR spectroscopy (3D structure), ITC, biochemical GTPase assays, molecular dynamics simulations The Biochemical journal High 31118266
2021 Structural and biophysical characterization of the first TPR domain of human SPAG1 using an optimized variant showed with atomistic precision how the C-terminal tails of HSP70 and HSP90 bind; specific motifs within the TPR sequence drive positioning of HSP peptides. Protein sequence optimization, NMR spectroscopy, biophysical binding assays Biochemistry High 33739091
2001 HSD-3.8 (SPAG1) encodes a GTP-binding protein with GTPase activity and is phosphorylated by PKC in vitro. The protein localizes to the postacrosomal zone of human spermatozoa and to pachytene primary spermatocytes. It contains TPR motifs and a P-loop sequence. Immunization of female rats with recombinant HSD-3.8 protein caused infertility. [α-32P]GTP blot overlay assay, in vitro GTPase assay, in vitro PKC phosphorylation assay, immunofluorescence/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), forming a complex in the cytoplasm in the presence of GDP (co-immunoprecipitation in HEK293 cells, yeast two-hybrid). Overexpression of HSD-0.7 (SPAG1 fragment) activates ERK1/2 via a PKC-dependent (not Ras-dependent) pathway; deletion of either the TPR domain or the P-loop abolished ERK1/2 activation. Yeast two-hybrid, co-transfection/co-immunoprecipitation in HEK293 cells, ERK1/2 activation assay, domain deletion analysis Frontiers in bioscience Medium 16368546
2016 In mouse oocytes, SPAG1 associates with meiotic spindles. RNAi depletion of SPAG1 impairs germinal vesicle breakdown (GVBD) via increased intracellular cAMP and decreased ATP, activating AMPK. SPAG1 depletion also reduces MAPK phosphorylation and causes irregular distribution of phospho-MAPK, impairing γ-tubulin function and spindle morphogenesis. Additionally, SPAG1 RNAi reduces actin expression and disrupts cortical granule-free domains, actin caps, and the contractile ring. RNAi in mouse oocytes, live imaging of spindle morphology, cAMP/ATP measurement, AMPK activation assay, immunofluorescence of γ-tubulin and phospho-MAPK Molecular biology of the cell Medium 27053660
2017 miR-638 directly targets SPAG1 in porcine immature Sertoli cells; SPAG1 knockdown (siRNA) phenocopies miR-638 overexpression by downregulating p-PI3K, p-AKT, c-MYC, CCND1, CCNE1, and CDK4, inhibiting proliferation and promoting apoptosis. SPAG1 siRNA also suppresses SOX2 and POU5F1 mRNA levels. miRNA target validation (luciferase reporter), siRNA knockdown, Western blot for PI3K/AKT pathway components, cell cycle/proliferation assays Cell cycle (Georgetown, Tex.) Medium 29119857
2022 SPAG1 knockdown in AML cells reduces proliferation and survival, and regulates expression of SMC3 while activating the ERK/MAPK signaling pathway. RNA interference knockdown, proliferation/survival assays, Western blot for ERK/MAPK pathway Neoplasma Low 35951456
2025 The human R2SP complex (RUVBL1, RUVBL2, SPAG1, PIH1D2) has a 3D organization similar to the canonical R2TP complex, determined by cryo-EM, NMR, and structural mass spectrometry. SPAG1 and PIH1D2 act as adaptors recruiting specific clients, while the RUVBL1/2 ATPase core functions as the catalytic powerhouse; differences in RUVBL1/2 ATPase activity regulation and adaptor binding mode distinguish R2SP from canonical R2TP. Cryo-EM, NMR, structural mass spectrometry, ATPase activity assays bioRxivpreprint Medium bio_10.1101_2025.01.27.635100

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.) 36 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
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
2022 The role of SPAG1 in the assembly of axonemal dyneins in human airway epithelia. Journal of cell science 12 35178554
2019 Binding properties of the quaternary assembly protein SPAG1. The Biochemical journal 12 31118266
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