Affinage

SPEF2

Sperm flagellar protein 2 · UniProt Q9C093

Length
1822 aa
Mass
209.8 kDa
Annotated
2026-06-10
22 papers in source corpus 13 papers cited in narrative 13 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SPEF2 is a large (>200 kDa) microtubule-associated protein required for the assembly of motile cilia and sperm flagella, originally identified as a gene upregulated during airway ciliogenesis and expressed stage-specifically in spermatocytes and round spermatids (PMID:10100999). During spermatid differentiation it localizes to the Golgi complex, manchette, basal body, and sperm tail midpiece and serves as a linker for cytoplasmic dynein 1-mediated cargo transport: it binds cytoplasmic dynein 1 and GOLGA3, co-localizes with dynein in the manchette, and is required for dynein 1-dependent transport of the intraflagellar transport protein IFT20 from the Golgi to the manchette, with its loss causing basal body duplication, failed manchette migration, and abnormal sperm head shape (PMID:19889948, PMID:28619825). Loss of SPEF2 function disrupts axonemal and accessory-structure assembly, shortens sperm flagella, and reduces ciliary beat frequency in respiratory epithelium (PMID:21715716), and it is needed for correct targeting of other flagellar proteins such as CFAP69 (PMID:31048344). In motile cilia and sperm flagella SPEF2 is a central pair (CP) apparatus-associated protein whose ciliary localization depends on functional HYDIN, and its loss results in absence of the central pair complex (PMID:31545650, PMID:38568462). Loss-of-function mutations affecting the long testis-specific, IFT20-binding isoforms cause multiple morphological abnormalities of the sperm flagella (MMAF) in humans, and SPEF2 deficiency is associated with primary ciliary dyskinesia (PMID:31151990, PMID:31048344, PMID:36873931). Beyond its ciliary roles, SPEF2 also supports osteoblast differentiation and bone formation (PMID:29339787).

Mechanistic history

Synthesis pass · year-by-year structured walk · 12 steps
  1. 1999 Medium

    Established SPEF2 (KPL2) as a ciliogenesis-associated gene, raising the question of what role its large multidomain protein plays in ciliated and germ cells.

    Evidence Differential display and cDNA cloning with stage-specific expression analysis in airway epithelium and testis

    PMID:10100999

    Open questions at the time
    • No functional assay linking expression to a ciliary mechanism
    • Domain functions (CH domain, P-loop) not tested
  2. 2006 High

    Showed SPEF2 is genetically required for sperm flagellar axoneme development by linking a disruptive intronic insertion to the porcine immotile short-tail sperm defect.

    Evidence Positional cloning and RT-PCR splice analysis in pigs, with comparative mapping

    PMID:16549801

    Open questions at the time
    • Molecular mechanism of SPEF2 in axoneme assembly not defined
    • Protein-level consequences not characterized
  3. 2007 Medium

    Defined the causal lesion as a full-length LINE-1 retrotransposon disrupting splice signals, clarifying how an isoform-specific defect arises.

    Evidence Cloning, junction analysis, and target-site repeat characterization of the insertion

    PMID:17610085

    Open questions at the time
    • Does not address SPEF2 protein function
    • Isoform-specific requirement not mapped to a functional domain
  4. 2009 Medium

    Connected SPEF2 to intraflagellar transport by demonstrating it localizes to Golgi, manchette, basal body, and midpiece and binds IFT20.

    Evidence Immunofluorescence, yeast two-hybrid, and co-immunoprecipitation in testis

    PMID:19889948

    Open questions at the time
    • Directionality/role of the IFT20 interaction not resolved
    • No motor or transport mechanism yet identified
  5. 2011 High

    Established SPEF2 as required for both sperm tail assembly and cilia motility using a loss-of-function mouse with shortened flagella and reduced ciliary beat frequency.

    Evidence Positional cloning of bgh nonsense mutation with histology, EM, and video microscopy

    PMID:21715716

    Open questions at the time
    • Reduced beat frequency despite normal cilia ultrastructure not mechanistically explained
    • Molecular cargo/transport role still unconfirmed
  6. 2017 High

    Defined the molecular mechanism: SPEF2 links cytoplasmic dynein 1 and GOLGA3 to drive dynein-dependent IFT20 transport from Golgi to manchette during spermatid differentiation.

    Evidence Germ-cell-specific Spef2 knockout, Co-IP, co-localization, and dynein inhibition assay

    PMID:28619825

    Open questions at the time
    • Whether SPEF2 transports cargoes beyond IFT20 not established
    • Structural basis of dynein/GOLGA3 binding unknown
  7. 2018 Medium

    Revealed a non-ciliary role for SPEF2 in osteoblast differentiation and bone formation.

    Evidence Spef2 knockout mouse with X-ray, bone histology, and in vitro osteoblast differentiation assays

    PMID:29339787

    Open questions at the time
    • Molecular pathway linking SPEF2 to osteoblast differentiation unknown
    • Whether the bone phenotype is cilia-dependent not determined
  8. 2019 High

    Established SPEF2 as a human MMAF gene and showed it is needed for correct targeting of other axonemal proteins.

    Evidence Whole-exome sequencing in two cohorts, TEM, Western blot, and CFAP69 immunofluorescence on patient sperm

    PMID:31048344 PMID:31151990

    Open questions at the time
    • Mechanism by which SPEF2 directs CFAP69 localization unknown
    • Genotype-phenotype relationship across isoforms incompletely mapped
  9. 2020 Medium

    Placed SPEF2 within the central pair apparatus of respiratory cilia and showed its ciliary localization depends on HYDIN.

    Evidence Immunofluorescence on respiratory epithelium from HYDIN- and SPEF2-mutant PCD patients

    PMID:31545650

    Open questions at the time
    • Direct HYDIN-SPEF2 physical interaction not demonstrated here
    • Hierarchy of central pair assembly not fully resolved
  10. 2022 Medium

    Broadened the interaction network to RSPH9 and confirmed IFT20 binding, while proteomics revealed downstream flagellar assembly proteins depleted upon SPEF2 loss.

    Evidence In vitro Co-IP plus mass spectrometry and Western blot on SPEF2-mutant sperm

    PMID:34755699

    Open questions at the time
    • Direct versus indirect nature of proteomic changes not separated
    • RSPH9 interaction not validated in vivo
  11. 2023 Medium

    Reinforced a SPEF2-HYDIN relationship in sperm flagella by showing co-loss of the two proteins in mutant flagella.

    Evidence Immunofluorescence, TEM, and high-speed video microscopy across multiple patients

    PMID:36873931

    Open questions at the time
    • Does not establish direct physical interaction
    • Whether co-loss reflects shared assembly pathway or mutual stabilization unclear
  12. 2024 Medium

    Linked SPEF2 directly to central pair complex integrity by showing variants abolish the central pair in sperm flagella and deplete SPEF2 in both sperm and respiratory cilia.

    Evidence Whole-exome sequencing, sperm TEM, immunofluorescence of spermatozoa and nasal cilia, in vitro variant analysis

    PMID:38568462

    Open questions at the time
    • Whether SPEF2 is structural to or assembles the central pair not distinguished
    • Mechanistic link between central pair loss and motility defect not detailed

Open questions

Synthesis pass · forward-looking unresolved questions
  • How SPEF2 mechanistically connects its dynein/IFT20 cargo-transport role in the manchette to its central pair apparatus role within the assembled axoneme remains unresolved.
  • No structural model of SPEF2 or its binding interfaces
  • Whether central pair and transport functions are separable across isoforms unknown
  • Molecular basis of the osteoblast phenotype uncharacterized

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0005198 structural molecule activity 2 GO:0060090 molecular adaptor activity 2 GO:0008092 cytoskeletal protein binding 1
Localization
GO:0005929 cilium 3 GO:0005794 Golgi apparatus 2 GO:0005815 microtubule organizing center 2 GO:0005856 cytoskeleton 2
Pathway
R-HSA-1852241 Organelle biogenesis and maintenance 3 R-HSA-1643685 Disease 2 R-HSA-1474165 Reproduction 1 R-HSA-9609507 Protein localization 1
Partners
CFAP69DYNC1H1GOLGA3HYDINIFT20RSPH9
Complex memberships
central pair apparatusmanchette

Evidence

Reading pass · 13 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1999 KPL2 (SPEF2) is a novel gene upregulated during ciliogenesis of airway epithelial cells; it encodes a protein >200 kDa containing a calponin homology domain, nuclear localization signals, a P-loop, and a proline-rich region, and is expressed in a stage-specific manner in spermatocytes and round spermatids of the testis. Differential display, Northern blot, RACE-based cDNA sequencing, Southern blot, protein domain analysis American journal of respiratory cell and molecular biology Medium 10100999
2006 A retrotransposon insertion within an intron of KPL2 (SPEF2) causes aberrant splicing (exon skipping or intronic sequence inclusion) specifically of a testis-expressed isoform, leading to premature translation termination and the immotile short-tail sperm defect in pigs, demonstrating that SPEF2 is required for correct axonemal development in sperm flagella. Positional cloning, porcine-human comparative mapping, RT-PCR splice analysis, sequence analysis Proceedings of the National Academy of Sciences of the United States of America High 16549801
2007 The intronic insertion in KPL2 (SPEF2) is a full-length LINE-1 retrotransposon capable of retrotransposition; the aberrant splicing it causes involves interference with intronic splice signals and activation of a cryptic splice site. Cloning and sequencing of the insertion, junction analysis, characterization of direct target-site repeats Molecular genetics and genomics Medium 17610085
2009 SPEF2 protein localizes to the Golgi complex, manchette, basal body, and midpiece of the sperm tail in differentiating germ cells, and physically interacts with the intraflagellar transport protein IFT20 in testis, as demonstrated by yeast two-hybrid assay and co-immunoprecipitation. Immunofluorescence localization, yeast two-hybrid assay, co-immunoprecipitation Biology of reproduction Medium 19889948
2011 Loss of SPEF2 function in mice (bgh mutation, nonsense variant) causes shortened sperm flagella with disorganized axonemal and accessory structures, reduced numbers of elongating spermatids, and reduced cilia beat frequency in respiratory epithelium (with ultrastructurally normal cilia), establishing SPEF2 as required for cilia motility and sperm tail assembly. Positional cloning, Western blot (protein loss validation), histopathology, immunofluorescence, electron microscopy, video microscopy of cilia beat frequency Biology of reproduction High 21715716
2017 SPEF2 functions in microtubule-mediated protein transport during spermatid differentiation: it interacts with cytoplasmic dynein 1 and GOLGA3 (novel interaction partners), co-localizes with dynein 1 in the manchette, and is required for dynein 1-dependent transport of IFT20 from the Golgi to the manchette; loss of SPEF2 also causes basal body duplication and failure of manchette migration, resulting in abnormal sperm head shape. Male germ cell-specific Spef2 knockout mouse, co-immunoprecipitation, immunofluorescence co-localization, dynein inhibition assay Development (Cambridge, England) High 28619825
2018 SPEF2 is expressed in osteoblasts and cartilage; Spef2 knockout mice exhibit growth retardation, shorter long bones, reduced bone mineral content, and compromised in vitro osteoblast differentiation, revealing a role for SPEF2 in bone formation and osteoblast differentiation. Spef2 knockout mouse model, X-ray analysis, bone histology, in vitro osteoblast differentiation assay Scientific reports Medium 29339787
2019 Loss-of-function mutations in SPEF2 (affecting long testis-specific isoforms encoding the IFT20-binding domain) cause multiple morphological abnormalities of the sperm flagella (MMAF) in humans, with disrupted axonemal structure, mitochondrial sheath defects, and reduced SPEF2 protein in spermatozoa. Whole exome sequencing, transmission electron microscopy, Western blot, immunofluorescence on patient spermatozoa Journal of medical genetics High 31048344 31151990
2019 SPEF2 deficiency in MMAF patients alters the localization of the axonemal protein CFAP69, indicating that SPEF2 is required for proper targeting of other axonemal proteins in the sperm flagellum. Immunofluorescence assay on patient spermatozoa Journal of medical genetics Medium 31048344
2020 SPEF2 is a central pair (CP)-associated protein in respiratory cilia; in HYDIN-mutant cells, SPEF2 is absent from cilia, demonstrating that SPEF2 localization to the CP apparatus depends on functional HYDIN. Immunofluorescence microscopy on respiratory epithelial cells from PCD patients with HYDIN and SPEF2 mutations American journal of respiratory cell and molecular biology Medium 31545650
2022 SPEF2 interacts with RSPH9 (radial spoke head component) and IFT20 in vitro; proteomic analysis of SPEF2-mutant patient spermatozoa reveals downregulation of multiple flagellar assembly proteins including SPAG6, DYNLT1, RSPH1, TOM20, EFHC1, MNS1, and IFT20. Co-immunoprecipitation (in vitro), proteomic analysis (mass spectrometry), Western blot validation Asian journal of andrology Medium 34755699
2023 Pathogenic variants in SPEF2 cause absence or severe reduction of SPEF2 protein in sperm flagella and co-occur with loss of HYDIN in flagella of HYDIN-mutant individuals, providing further evidence for an interaction between HYDIN and SPEF2 in sperm flagella. Immunofluorescence microscopy on sperm flagella, transmission electron microscopy, high-speed video microscopy, genetic testing Frontiers in genetics Medium 36873931
2024 SPEF2 variants cause absence of the central pair complex in sperm flagella, as demonstrated by ultrastructural analysis, and abolish SPEF2 protein in both spermatozoa and respiratory cilia, linking SPEF2 to central pair apparatus integrity. Whole-exome sequencing, transmission electron microscopy of sperm, immunofluorescence of spermatozoa and nasal cilia, in vitro variant analysis Journal of assisted reproduction and genetics Medium 38568462

Source papers

Stage 0 corpus · 22 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2011 Loss of SPEF2 function in mice results in spermatogenesis defects and primary ciliary dyskinesia. Biology of reproduction 106 21715716
2008 Partial duplication of the PRLR and SPEF2 genes at the late feathering locus in chicken. BMC genomics 80 18713476
2006 An intronic insertion in KPL2 results in aberrant splicing and causes the immotile short-tail sperm defect in the pig. Proceedings of the National Academy of Sciences of the United States of America 78 16549801
2009 Expression of SPEF2 during mouse spermatogenesis and identification of IFT20 as an interacting protein. Biology of reproduction 76 19889948
2019 Loss-of-function mutations in SPEF2 cause multiple morphological abnormalities of the sperm flagella (MMAF). Journal of medical genetics 65 31151990
2019 Homozygous mutations in SPEF2 induce multiple morphological abnormalities of the sperm flagella and male infertility. Journal of medical genetics 60 31048344
2020 SPEF2- and HYDIN-Mutant Cilia Lack the Central Pair-associated Protein SPEF2, Aiding Primary Ciliary Dyskinesia Diagnostics. American journal of respiratory cell and molecular biology 58 31545650
2017 SPEF2 functions in microtubule-mediated transport in elongating spermatids to ensure proper male germ cell differentiation. Development (Cambridge, England) 58 28619825
1999 Cloning and characterization of KPL2, a novel gene induced during ciliogenesis of tracheal epithelial cells. American journal of respiratory cell and molecular biology 41 10100999
2023 Pathogenic gene variants in CCDC39, CCDC40, RSPH1, RSPH9, HYDIN, and SPEF2 cause defects of sperm flagella composition and male infertility. Frontiers in genetics 25 36873931
2018 Cilia-related protein SPEF2 regulates osteoblast differentiation. Scientific reports 24 29339787
2011 L1 insertion within SPEF2 gene is associated with increased litter size in the Finnish Yorkshire population. Journal of animal breeding and genetics = Zeitschrift fur Tierzuchtung und Zuchtungsbiologie 21 22394230
2022 Sperm flagellar 2 (SPEF2) is essential for sperm flagellar assembly in humans. Asian journal of andrology 19 34755699
2007 Infertile Finnish Yorkshire boars carry a full-length LINE-1 retrotransposon within the KPL2 gene. Molecular genetics and genomics : MGG 17 17610085
2023 circ_SPEF2 Regulates the Balance of Treg Cells by Regulating miR-16-5p/BACH2 in Lymphoma and Participates in the Immune Response. Tissue engineering and regenerative medicine 10 37801226
2022 Novel SPEF2 Variant in a Japanese Patient with Primary Ciliary Dyskinesia: A Case Report and Literature Review. Journal of clinical medicine 6 36615117
2014 A recent L1 insertion within SPEF2 gene is associated with changes in PRLR expression in sow reproductive organs. Animal genetics 5 24712415
2025 Novel variants of FSIP2 and SPEF2 cause varying degrees of spermatozoa damage in MMAF patients and favorable ART outcomes. Journal of assisted reproduction and genetics 4 39753944
2021 Search for Associations of FSHR, INHA, INHAB, PRL, TNP2 and SPEF2 Genes Polymorphisms with Semen Quality in Russian Holstein Bulls (Pilot Study). Animals : an open access journal from MDPI 4 34679903
2022 The ciliary protein Spef2 stimulates acinar Ampkα/Sirt1 signaling and ameliorates acute pancreatitis and associated lung injury. Annals of translational medicine 3 35965828
2024 Novel SPEF2 variants cause male infertility and likely primary ciliary dyskinesia. Journal of assisted reproduction and genetics 2 38568462
2024 Retraction: The ciliary protein Spef2 stimulates acinar Ampkα/Sirt1 signaling and ameliorates acute pancreatitis and associated lung injury. Annals of translational medicine 0 40949679

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