Affinage

SPEF2

Sperm flagellar protein 2 · UniProt Q9C093

Length
1822 aa
Mass
209.8 kDa
Annotated
2026-04-28
22 papers in source corpus 12 papers cited in narrative 12 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SPEF2 is a central pair-associated protein essential for the assembly and function of motile cilia and sperm flagella. It contains a calponin homology domain and functions as a linker for cytoplasmic dynein 1-mediated cargo transport along manchette microtubules during spermiogenesis, interacting with IFT20, GOLGA3, RSPH9, and HYDIN to coordinate intraflagellar transport and axonemal organization (PMID:19889948, PMID:28619825, PMID:34755699, PMID:31545650). Loss of SPEF2 disrupts axonemal central pair formation, accessory structure assembly, and mitochondrial sheath integrity in sperm, and reduces ciliary beat frequency in respiratory epithelium, causing multiple morphological abnormalities of the sperm flagella (MMAF) and primary ciliary dyskinesia in humans (PMID:31151990, PMID:31545650, PMID:38568462, PMID:21715716). SPEF2 localization to the central pair depends on functional HYDIN, and its deficiency additionally impairs osteoblast differentiation and bone formation (PMID:31545650, PMID:29339787).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 1999 Medium

    Identification of SPEF2 (KPL2) as a novel ciliogenesis-associated gene resolved the question of what transcripts accompany ciliated cell differentiation and spermatogenesis, establishing it as a candidate motile cilia gene with a calponin homology domain.

    Evidence Differential display and Northern blot on differentiating airway epithelial cells and testis

    PMID:10100999

    Open questions at the time
    • No functional data; domain predictions only
    • No loss-of-function evidence
    • Protein localization not determined
  2. 2006 High

    Positional cloning of a retrotransposon insertion in SPEF2 demonstrated that disruption of a testis-specific exon causes defective sperm axoneme development, establishing SPEF2 as functionally required for flagellar assembly.

    Evidence Positional cloning and RT-PCR in a naturally occurring bovine male-infertility model

    PMID:16549801

    Open questions at the time
    • Mechanism of axoneme disruption unknown
    • Whether respiratory cilia are affected was untested
    • No protein interaction data
  3. 2009 High

    Localization of SPEF2 to the Golgi, manchette, basal body, and sperm tail midpiece, together with its physical interaction with IFT20, revealed that SPEF2 participates in intraflagellar transport-related protein trafficking during spermatid differentiation.

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

    PMID:19889948

    Open questions at the time
    • Motor protein involvement unknown
    • Functional consequence of IFT20 interaction not tested in vivo
  4. 2011 High

    A mouse nonsense mutation in SPEF2 (bgh) proved that SPEF2 is required not only for sperm flagella integrity but also for normal respiratory cilia beat frequency, establishing SPEF2 as a primary ciliary dyskinesia gene.

    Evidence Positional cloning, electron microscopy, video microscopy, and Western blot in bgh mutant mice

    PMID:21715716

    Open questions at the time
    • Respiratory cilia ultrastructure appeared normal despite reduced beat frequency — structural basis unclear
    • Central pair involvement not yet identified
  5. 2017 High

    Conditional knockout and interaction studies established SPEF2 as a molecular linker between cytoplasmic dynein 1 and cargo (including IFT20) along manchette microtubules, explaining how it coordinates protein transport and head shaping during spermiogenesis.

    Evidence Male germ cell-specific Spef2 knockout, co-immunoprecipitation with dynein 1 and GOLGA3, dynein inhibition assay

    PMID:28619825

    Open questions at the time
    • Direct binding domains for dynein 1 and GOLGA3 not mapped
    • Structural basis of SPEF2 bridging function unknown
  6. 2018 Medium

    Discovery that Spef2 knockout mice exhibit impaired osteoblast differentiation and reduced bone formation extended SPEF2 function beyond cilia/flagella to skeletal development.

    Evidence Spef2 knockout mouse with X-ray analysis and in vitro osteoblast differentiation assay

    PMID:29339787

    Open questions at the time
    • Mechanism linking SPEF2 to osteoblast differentiation unknown
    • Whether this is cilia-dependent or cilia-independent is unresolved
    • Single study without independent replication
  7. 2019 High

    Identification of loss-of-function SPEF2 mutations in human infertile men confirmed SPEF2 as a causal gene for MMAF with axonemal and mitochondrial sheath defects, and demonstrated that disease-causing mutations selectively affect the long testis-expressed isoform encoding the IFT20-binding domain.

    Evidence Whole-exome sequencing, transmission electron microscopy, Western blot, and immunofluorescence in independent human cohorts

    PMID:31048344 PMID:31151990

    Open questions at the time
    • Genotype-phenotype correlations across different mutation types incomplete
    • Isoform-specific rescue not performed
  8. 2020 High

    Demonstration that SPEF2 is a central pair-associated protein whose localization depends on HYDIN identified its precise axonemal substructure and placed it within the HYDIN-dependent central pair assembly pathway, linking SPEF2 mutations to PCD with central pair defects.

    Evidence Immunofluorescence on respiratory cells from 189 individuals including HYDIN-mutant patients, with whole-exome sequencing

    PMID:31545650

    Open questions at the time
    • Whether SPEF2 is a structural component or regulatory factor at the central pair is unclear
    • Direct SPEF2–HYDIN binding not demonstrated
  9. 2022 Medium

    Proteomic and interaction studies expanded the SPEF2 interactome to include RSPH9 and revealed that SPEF2 deficiency broadly reduces levels of flagellar assembly proteins, indicating SPEF2 acts upstream in flagellar protein homeostasis.

    Evidence Co-immunoprecipitation and proteomic analysis of human spermatozoa

    PMID:34755699

    Open questions at the time
    • Whether reduced protein levels reflect transcriptional, transport, or stability defects is unknown
    • RSPH9 interaction awaits in vivo validation
  10. 2023 Medium

    The SPEF2–HYDIN dependency was confirmed in sperm flagella (extending the respiratory cilia finding), establishing that central pair integrity in both cilia and flagella requires coordinated SPEF2–HYDIN interaction.

    Evidence Immunofluorescence on sperm from HYDIN-mutant individuals

    PMID:36873931

    Open questions at the time
    • Direct physical interaction between SPEF2 and HYDIN not biochemically confirmed
    • Single study

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include the structural basis of SPEF2 at the central pair, the molecular domains mediating its interactions with dynein 1 and HYDIN, the mechanism connecting SPEF2 to osteoblast differentiation, and whether short SPEF2 isoforms have distinct ciliary functions.
  • No high-resolution structural data for SPEF2 or its complexes
  • Cilia-independent vs cilia-dependent roles in bone not delineated
  • No reconstituted in vitro transport assay with purified SPEF2

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0008092 cytoskeletal protein binding 2 GO:0060090 molecular adaptor activity 1
Localization
GO:0005794 Golgi apparatus 2 GO:0005856 cytoskeleton 2 GO:0005929 cilium 2
Pathway
R-HSA-1852241 Organelle biogenesis and maintenance 4 R-HSA-1474165 Reproduction 3
Partners
DYNC1H1GOLGA3HYDINIFT20RSPH9

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1999 KPL2 (SPEF2) encodes a novel protein upregulated during ciliogenesis of airway epithelial cells, containing a calponin homology domain, nuclear localization signals, a P-loop, and a proline-rich region; its expression is stage-specific in spermatocytes and round spermatids in the testis and closely parallels axonemal dynein heavy chain expression during ciliated cell differentiation. Differential display, Northern blot, Southern blot, cDNA cloning and sequencing, immunohistochemistry American journal of respiratory cell and molecular biology Medium 10100999
2006 A retrotransposon insertion within an intron of KPL2 (SPEF2) causes aberrant splicing specifically of a testis-expressed exon, leading to frameshifts and premature termination, resulting in defective axoneme development in sperm flagella while cilia in other tissues are unaffected; this demonstrates that SPEF2 is important for correct axoneme development. Positional cloning, comparative genomic mapping, RT-PCR, sequence analysis, tissue expression analysis Proceedings of the National Academy of Sciences of the United States of America High 16549801
2009 SPEF2 protein localizes to the Golgi complex, manchette, basal body, and midpiece of the sperm tail during spermatid differentiation, and interacts with the intraflagellar transport protein IFT20 in the testis. Immunofluorescence microscopy, yeast two-hybrid assay, coimmunoprecipitation Biology of reproduction High 19889948
2011 Loss of SPEF2 function in mice (bgh mutation — a nonsense mutation) causes shortened flagella, disorganized axonemal and accessory structures in sperm, and reduced cilia beat frequency in respiratory epithelium (ultrastructurally normal cilia), establishing SPEF2 as required for cilia function and a genetic cause of primary ciliary dyskinesia. Positional cloning, histopathology, immunofluorescence, electron microscopy, video microscopy, Western blot Biology of reproduction High 21715716
2017 SPEF2 functions in microtubule-mediated protein transport in elongating spermatids via the manchette; it interacts with cytoplasmic dynein 1 and GOLGA3, colocalizes with dynein 1 in the manchette, and is required for IFT20 transport from the Golgi to the manchette. Loss of SPEF2 also causes basal body duplication and defective manchette migration leading to abnormal sperm head shape. Male germ cell-specific Spef2 knockout mouse, coimmunoprecipitation, immunofluorescence, dynein inhibition assay Development (Cambridge, England) High 28619825
2018 SPEF2 is required for osteoblast differentiation and bone formation; Spef2 knockout mice show shorter long bones, reduced bone mineral content, and compromised in vitro osteoblast differentiation. Spef2 knockout mouse model, X-ray analysis, in vitro osteoblast differentiation assay Scientific reports Medium 29339787
2019 Loss-of-function mutations in SPEF2 in humans cause multiple morphological abnormalities of the sperm flagella (MMAF) with disrupted axonemal structure and mitochondrial sheath defects, and result in significantly decreased SPEF2 protein levels in spermatozoa. Whole exome sequencing, transmission electron microscopy, Western blot, immunofluorescence Journal of medical genetics High 31048344 31151990
2019 SPEF2 mutations affect the long SPEF2 transcripts expressed in the testis that encode the IFT20-binding domain; deficiency of SPEF2 alters the localization of other axonemal proteins, including CFAP69, in spermatozoa. Whole exome sequencing, immunofluorescence assay, genetic analysis of isoform specificity Journal of medical genetics Medium 31048344
2020 SPEF2 is a central pair (CP)-associated protein in motile cilia; SPEF2 is absent in HYDIN-mutant cilia, revealing that SPEF2 localization to the central pair depends on functional HYDIN; loss-of-function mutations in SPEF2 itself cause PCD with central pair defects. Immunofluorescence microscopy on respiratory cells, genetic analysis (whole-exome/next-generation sequencing) American journal of respiratory cell and molecular biology High 31545650
2022 SPEF2 interacts with RSPH9 and IFT20 in vitro (in addition to the previously identified IFT20 interaction), and its deficiency leads to reduced expression of proteins related to flagellar assembly including SPAG6, DYNLT1, RSPH1, TOM20, EFHC1, MNS1, and IFT20 in human spermatozoa. Proteomic analysis, co-immunoprecipitation, Western blot Asian journal of andrology Medium 34755699
2023 SPEF2 and HYDIN interact in sperm flagella; absence of SPEF2 protein in sperm flagella of HYDIN-mutant individuals reveals a dependency relationship between these central pair-associated proteins in sperm flagella (as distinct from respiratory cilia). Immunofluorescence microscopy on sperm cells, next-generation sequencing Frontiers in genetics Medium 36873931
2024 SPEF2 variants cause absence of the central pair complex in sperm flagella, confirmed by ultrastructural analysis; immunostaining of spermatozoa and respiratory cilia validates pathogenicity of SPEF2 variants in both tissues. Whole-exome sequencing, electron microscopy, immunofluorescence, in vitro analyses 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 105 21715716
2008 Partial duplication of the PRLR and SPEF2 genes at the late feathering locus in chicken. BMC genomics 79 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 76 16549801
2009 Expression of SPEF2 during mouse spermatogenesis and identification of IFT20 as an interacting protein. Biology of reproduction 75 19889948
2019 Loss-of-function mutations in SPEF2 cause multiple morphological abnormalities of the sperm flagella (MMAF). Journal of medical genetics 64 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 57 31545650
2017 SPEF2 functions in microtubule-mediated transport in elongating spermatids to ensure proper male germ cell differentiation. Development (Cambridge, England) 57 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 24 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 18 34755699
2007 Infertile Finnish Yorkshire boars carry a full-length LINE-1 retrotransposon within the KPL2 gene. Molecular genetics and genomics : MGG 16 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 9 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
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
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 2 39753944
2024 Novel SPEF2 variants cause male infertility and likely primary ciliary dyskinesia. Journal of assisted reproduction and genetics 1 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