Affinage

AK8

Adenylate kinase 8 · UniProt Q96MA6

Length
479 aa
Mass
54.9 kDa
Annotated
2026-06-09
21 papers in source corpus 8 papers cited in narrative 8 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

AK8 is a dual-domain adenylate kinase that supplies local adenine nucleotide homeostasis to motile cilia and sperm flagella, where it is required for proper ciliary beating and male fertility (PMID:21080915, PMID:38761355). It carries two complete, independently active AK domains within a single polypeptide; full-length AK8 and each isolated domain phosphorylate AMP, CMP, dAMP, and dCMP using ATP (or GTP) as phosphate donor, with highest affinity for AMP-ATP (PMID:21080915). Within the flagellum AK8 is concentrated in the axonemal compartment, distinct from the outer dense fibers and mitochondrial sheath, where nucleotide interconversion shapes the ADP-driven flagellar waveform (PMID:24740601). AK8 is a structural component of the radial spoke 1 (RS1) stalk, anchored there through EFCAB10, which bridges AK8 to RSPH3B; loss of EFCAB10 abolishes ciliary AK8, whereas loss of AK8 leaves EFCAB10 intact, defining a directional assembly hierarchy (PMID:41055978, PMID:39849482). AK8 acts together with AK9 to route ATP transfer to sites of high energy consumption in the flagellum, and it associates with an axonemal nucleotide-homeostasis module that includes CFAP45 and dynein ATPases (PMID:33139725, PMID:38761355). Loss of AK8 in mice produces primary ciliary dyskinesia phenotypes including congenital hydrocephalus, respiratory defects, impaired sperm motility, and infertility (PMID:21746835, PMID:38761355, PMID:41055978).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2007 Low

    Before any functional role was known, AK8 was flagged as a sperm-associated antigen, providing the first link of the protein to reproductive biology.

    Evidence 2D gel electrophoresis with MALDI-TOF/LC-MS of human sperm proteins and peptide ELISA against immunoinfertile sera

    PMID:16998854

    Open questions at the time
    • No mechanistic or enzymatic function demonstrated for AK8
    • Antigen identification does not establish localization or molecular activity
    • Association with infertility is correlative
  2. 2011 High

    Establishing the biochemistry first: AK8 was shown to be an adenylate kinase carrying two independently catalytically active AK domains in one chain, defining its core molecular activity and substrate preferences.

    Evidence In vitro enzyme and substrate-specificity assays with domain truncations plus GFP-fusion localization

    PMID:21080915

    Open questions at the time
    • Cytosolic localization in overexpression did not reveal the axonemal targeting later observed
    • Physiological substrate flux in vivo not measured
    • No interacting partners identified
  3. 2011 Medium

    Genetic knockout established that AK8 is required in vivo, linking its loss to motile-cilia dysfunction and congenital hydrocephalus.

    Evidence Ak8 knockout mouse with neuropathological analysis

    PMID:21746835

    Open questions at the time
    • Mechanism connecting AK8 loss to ciliary failure inferred from histopathology only
    • No molecular placement of AK8 within the axoneme
    • Sperm/fertility phenotype not assessed here
  4. 2014 Medium

    Subcellular resolution placed AK8 specifically in the axonemal compartment of the sperm flagellum and tied its enzymatic activity to flagellar mechanics.

    Evidence Immunolocalization in mouse sperm with detergent-modeled motility assays using defined nucleotide substrates

    PMID:24740601

    Open questions at the time
    • Direct AK8-dependence of the AMP-driven waveform not tested by genetic ablation
    • Anchoring mechanism within the axoneme unknown
    • Single-lab functional inference
  5. 2020 Medium

    AK8 was embedded in a defined axonemal module by linking it to CFAP45 and dynein ATPases, framing it as part of a nucleotide-homeostasis system regulating beating.

    Evidence Proteomic profiling of CFAP45-deficient cilia, structural modelling of an AMP-binding CFAP45-AK8 interface, and microtubule sliding rescue

    PMID:33139725

    Open questions at the time
    • AK8-CFAP45 interface validated by modelling, not by AK8 mutagenesis
    • Functional consequence of disrupting the interface untested
    • Stoichiometry of the module unresolved
  6. 2024 High

    AK8 was shown to physically associate with the radial spoke and to act with AK9 in directing ATP transfer to flagellar energy-consuming sites, connecting its enzymatic role to motility and fertility.

    Evidence Reciprocal Co-IP/MS, Ak8 knockout mice, ATP-probe imaging, metabolomics, and a human infertile-male cohort

    PMID:38761355

    Open questions at the time
    • Precise division of labor between AK8 and AK9 not fully resolved
    • Direct radial-spoke binding partner not identified in this study
    • Causal AK8 variants in human infertility not functionally proven
  7. 2025 High

    The anchoring mechanism was resolved: EFCAB10 directly binds AK8 and RSPH3B to tether AK8 to the radial spoke, with directional epistasis showing EFCAB10 is upstream of AK8 ciliary localization.

    Evidence Co-IP, Efcab10-/- and Ak8-/- knockout mice, ciliary motility assays, and immunofluorescence

    PMID:41055978

    Open questions at the time
    • Structural basis of the EFCAB10-AK8-RSPH3B interface unknown
    • Whether anchoring is required for AK8 catalytic function untested
    • Tissue specificity of the anchoring complex not fully mapped
  8. 2025 Medium

    AK8 was placed structurally within the RS1 stalk specifically, distinguishing its location among the three radial spokes.

    Evidence MS of IQUB-deficient human and mouse sperm with western blot, immunofluorescence, and Iqub knockout

    PMID:39849482

    Open questions at the time
    • No direct AK8 mutagenesis to confirm RS1 incorporation
    • Relationship between IQUB and the EFCAB10 anchoring pathway unclear
    • RS1-specific functional role of AK8 not isolated

Open questions

Synthesis pass · forward-looking unresolved questions
  • It remains unresolved how AK8's catalytic activity, its RS1 structural role, and its anchoring by EFCAB10 are mechanistically coupled to dynein-driven beating, and whether specific human AK8 variants cause primary ciliary dyskinesia or infertility.
  • No structure of the AK8-containing RS1 assembly
  • No demonstration that anchoring is required for in-axoneme catalysis
  • Causal human disease variants not functionally validated

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0016740 transferase activity 1
Localization
GO:0005856 cytoskeleton 2 GO:0005929 cilium 2 GO:0005829 cytosol 1
Pathway
R-HSA-1430728 Metabolism 2 R-HSA-1474165 Reproduction 2
Partners
AK9CFAP45EFCAB10IQUBRSPH3B
Complex memberships
EFCAB10-AK8-RSPH3B complexaxonemal CFAP45 nucleotide-homeostasis moduleradial spoke 1 (RS1) stalk

Evidence

Reading pass · 8 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2011 Human AK8 is a novel adenylate kinase with two complete and independently active AK domains within a single polypeptide chain. Full-length AK8, as well as each domain separately (AK8p1 and AK8p2), all exhibit AK enzyme activity. AK8 phosphorylates AMP, CMP, dAMP, and dCMP with ATP as phosphate donor, and AMP, CMP, and dCMP with GTP as phosphate donor, showing highest affinity for AMP with ATP. AK8 localizes to the cytosol, demonstrated by GFP-fusion protein imaging. In vitro enzyme activity assays, substrate specificity assays, GFP-fusion protein subcellular localization imaging The Biochemical journal High 21080915
2014 AK8 is present in the axoneme compartment of the mouse sperm flagellum (a third flagellar compartment distinct from outer dense fibers and mitochondrial sheath where AK1 and AK2 reside), as shown by immunolocalization. Addition of AMP to ATP-containing medium recapitulated the ADP-driven flagellar waveform, consistent with AK-mediated nucleotide interconversion in the axoneme. Immunolocalization/immunofluorescence in mouse sperm, detergent-modeled sperm motility assays with defined nucleotide substrates, digital image analysis of flagellar waveforms Biology of reproduction Medium 24740601
2011 Knockout of Ak8 in mice results in autosomal recessive congenital hydrocephalus, with dysfunctional motile cilia identified as the underlying pathogenetic mechanism, placing AK8 as required for proper motile cilia function in vivo. Knockout mouse model, neuropathological analysis Veterinary pathology Medium 21746835
2020 CFAP45 structural modelling identifies an AMP-binding interface between CFAP45 and AK8, and proteomic profiling of CFAP45-deficient cilia links CFAP45 to an axonemal module that includes AK8 and dynein ATPases. This positions AK8 as part of an axonemal adenine nucleotide homeostasis module regulating ciliary and flagellar beating. Proteomic profiling (mass spectrometry) of CFAP45-deficient cilia/flagella, structural modelling of AMP-binding interface between CFAP45 and AK8, in vitro AMP binding assay for CFAP45, microtubule sliding rescue assay Nature communications Medium 33139725
2024 AK8 interacts with the radial spoke (RS) of the sperm axoneme, as revealed by immunoprecipitation combined with mass spectrometry. AK8 knockout mice are infertile with impaired sperm motility. AK8 and AK9 cooperatively regulate ATP transfer in the axoneme and are concentrated at sites of energy consumption in the flagellum, as shown by ATP probes and metabolomic analysis. Immunoprecipitation combined with mass spectrometry, knockout mouse model, ATP probe assay, metabolomic analysis, human infertile male cohort analysis Science China. Life sciences High 38761355
2025 EFCAB10 interacts directly with AK8 and RSPH3B, anchoring AK8 to the radial spoke in motile cilia. Loss of EFCAB10 results in complete absence of AK8 from cilia, whereas loss of AK8 does not affect ciliary EFCAB10. Both Efcab10 and Ak8 knockout mice display impaired ciliary motility and PCD-related phenotypes (respiratory defects and infertility). Biochemical co-immunoprecipitation, knockout mouse models (Efcab10-/- and Ak8-/-), ciliary motility assays, immunofluorescence localization Proceedings of the National Academy of Sciences of the United States of America High 41055978
2025 AK8 is identified as a component of the stalk of radial spoke 1 (RS1) in human and mouse sperm flagella. In IQUB-deficient sperm, RS1 (but not RS2 or RS3) is lost, and AK8 is among the RS1 stalk proteins that are correspondingly absent or reduced, placing AK8 structurally within the RS1 stalk. Protein mass spectrometry of IQUB-deficient human and mouse sperm, western blotting, immunofluorescence, Iqub knockout mouse model Cell communication and signaling : CCS Medium 39849482
2007 AK8 (then designated hypothetical protein FLJ32704) is present as a sperm antigen recognized specifically by sera of immunoinfertile women but not fertile women, identified by 2D gel electrophoresis and MALDI-TOF-MS/LC-MS of human sperm proteins. An immunodominant peptide (amino acids 151–159) was identified that reacted with 90% of immunoinfertile sera. 2D gel electrophoresis, MALDI-TOF-MS/LC-MS, ELISA with synthetic peptide Molecular reproduction and development Low 16998854

Source papers

Stage 0 corpus · 21 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2011 Congenital hydrocephalus in genetically engineered mice. Veterinary pathology 115 21746835
2013 Next-generation sequencing, FISH mapping and synteny-based modeling reveal mechanisms of decreasing dysploidy in Cucumis. The Plant journal : for cell and molecular biology 59 24127692
2020 CFAP45 deficiency causes situs abnormalities and asthenospermia by disrupting an axonemal adenine nucleotide homeostasis module. Nature communications 56 33139725
2011 The characterization of human adenylate kinases 7 and 8 demonstrates differences in kinetic parameters and structural organization among the family of adenylate kinase isoenzymes. The Biochemical journal 37 21080915
2014 Adenine nucleotide metabolism and a role for AMP in modulating flagellar waveforms in mouse sperm. Biology of reproduction 30 24740601
1994 T cells which do not express membrane tumor necrosis factor-alpha activate macrophage effector function by cell contact-dependent signaling of macrophage tumor necrosis factor-alpha production. European journal of immunology 30 8056032
2019 Proteomic markers of low and high fertility bovine spermatozoa separated by Percoll gradient. Molecular reproduction and development 29 31134708
2009 Glycosaminoglycan degradation-inhibitory lactic acid bacteria ameliorate 2,4,6-trinitrobenzenesulfonic acid-induced colitis in mice. Journal of microbiology and biotechnology 27 19597321
2019 The story of promiscuous crucifers: origin and genome evolution of an invasive species, Cardamine occulta (Brassicaceae), and its relatives. Annals of botany 24 30868165
2017 RNA-Dependent Cysteine Biosynthesis in Bacteria and Archaea. mBio 22 28487430
2007 Molecular identities of human sperm proteins reactive with antibodies in sera of immunoinfertile women. Molecular reproduction and development 21 16998854
1989 Selective induction of growth factor production and growth factor receptor expression by different signals to a single T cell. European journal of immunology 21 2574679
2024 Adenylate kinase phosphate energy shuttle underlies energetic communication in flagellar axonemes. Science China. Life sciences 10 38761355
2018 Healthy Roots and Leaves: Comparative Genome Structure of Horseradish and Watercress. Plant physiology 9 30397022
2020 An insight into the genome of extensively drug-resistant and uropathogenic Citrobacter werkmanii. Journal of global antimicrobial resistance 7 32619689
2025 IQUB mutation induces radial spoke 1 deficiency causing asthenozoospermia with normal sperm morphology in humans and mice. Cell communication and signaling : CCS 6 39849482
2021 Co-expression Analysis of Genes and Tumor-Infiltrating Immune Cells in Metastatic Uterine Carcinosarcoma. Reproductive sciences (Thousand Oaks, Calif.) 4 33905082
2025 Guipi Tang alleviates vascular dementia by regulating purine metabolism via gut microbiota-derived pantothenic acid. Phytomedicine : international journal of phytotherapy and phytopharmacology 3 41043331
2024 Genetic Factors and Long-term Treatment-Related Neurocognitive Deficits, Anxiety, and Depression in Childhood Leukemia Survivors: An Exome-Wide Association Study. Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology 2 38051303
2025 EFCAB10 anchors AK8 to the radial spoke for proper ciliary motility. Proceedings of the National Academy of Sciences of the United States of America 1 41055978
2024 Studies on the Probiotic, Adhesion, and Induction Properties of Artisanal Lactic Acid Bacteria: to Customize a Gastrointestinal Niche to Trigger Anti-obesity Functions. Probiotics and antimicrobial proteins 1 39382740

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