LRGUK is an intracellular 825-amino-acid protein that contains nine leucine-rich repeats, an LRR C-terminal domain and a guanylate kinase-like domain. It supports spermatid morphogenesis and sperm-flagellum assembly along the acrosome-acroplaxome-manchette-basal-body axis. A biallelic truncating human variant that removes the C-terminal region is associated with absent LRGUK in sperm, severe multiple morphological abnormalities of the sperm flagella, axonemal central-pair disruption, sperm-head defects and male infertility. Mouse Lrguk-1 binds HOOK-family cytoskeletal transport proteins, RIMBP3 and KLC3; its GUK-like and LRR regions mediate different partner interactions, consistent with a scaffold role in manchette microtubule organization and early axoneme growth. No guanylate kinase catalytic activity has been demonstrated, and neither a catalytic-enzyme nor a definitive pseudokinase assignment is warranted. No functionally distinct human isoforms have been established. The shorter Tetrahymena Cfap246 ortholog lacks the C-terminal GUK-like region present in mammalian LRGUK.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0001669 acrosomal vesicle | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference places LRGUK in the acrosomal vesicle using the exact mouse Lrguk ortholog and a supporting PANTHER node. Reason: Mouse Lrguk localizes to the acrosome during spermiogenesis and is required for acrosome attachment and sperm-head assembly. The donor is the exact ortholog, so this is a well-supported core localization transfer. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:1921604 · mouse Lrguk SUPPORTS TRANSFER PANTHER:PTN002750528 · PANTHER acrosomal-localization node SUPPORTS TRANSFER |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: A broad guanylate-kinase-family IBA places LRGUK in the cytoplasm. Reason: Cytoplasmic localization is compatible with the exact mouse Lrguk donor and with LRGUK's specialized cytoskeletal localizations in spermatids. The other donors are conventional guanylate kinases from several lineages, so this broad compartment is defensible but does not define LRGUK's core sperm-assembly role. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: MGI:MGI:1921604 · mouse Lrguk SUPPORTS TRANSFER MGI:MGI:95871 · mouse Guk1 SUPPORTS TRANSFER PANTHER:PTN000563396 · broad guanylate-kinase phylogenetic node SUPPORTS TRANSFER UniProtKB:P60546 · Escherichia coli Gmk SUPPORTS TRANSFER UniProtKB:Q16774 · human GUK1 SUPPORTS TRANSFER UniProtKB:Q389H1 · Trypanosoma guanylate kinase SUPPORTS TRANSFER UniProtKB:Q38AS6 · Trypanosoma guanylate kinase SUPPORTS TRANSFER |
| GO:0007283 spermatogenesis | IBA GO_REF:0000033 | ACCEPT | Summary: LRGUK is inferred to function in spermatogenesis from the exact mouse ortholog and an LRGUK-specific phylogenetic node. Reason: Mouse Lrguk deficiency causes male infertility with defective acrosome attachment, manchette function, sperm-head shaping and flagellum formation. These conserved, gene-specific phenotypes directly support the broad spermatogenesis process for human LRGUK. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:1921604 · mouse Lrguk SUPPORTS TRANSFER PANTHER:PTN008591047 · LRGUK phylogenetic node SUPPORTS TRANSFER |
| GO:0035082 axoneme assembly | IBA GO_REF:0000033 | ACCEPT | Summary: LRGUK is inferred to participate in axoneme assembly from the exact mouse ortholog and an LRGUK-specific phylogenetic node. Reason: Mouse Lrguk is required for basal-body attachment, axonemal microtubule extension and initiation of sperm-tail growth. The exact-ortholog transfer therefore captures a core sperm-flagellum assembly role. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:1921604 · mouse Lrguk SUPPORTS TRANSFER PANTHER:PTN008591047 · LRGUK phylogenetic node SUPPORTS TRANSFER |
| GO:0001669 acrosomal vesicle | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location vocabulary mapping assigns LRGUK to the acrosome. Reason: The mapped UniProt location is based on the exact mouse Lrguk ortholog, whose acrosomal localization and role in acrosome attachment are experimentally established. This agrees with the independent IBA. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB-SubCell:SL-0007 · acrosome controlled vocabulary SUPPORTS TRANSFER |
| GO:0005856 cytoskeleton | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: UniProt vocabulary mapping assigns the broad cytoskeleton location to LRGUK. Reason: Exact-ortholog evidence places mouse Lrguk at the manchette and basal body, both microtubule-based structures, so the broad cytoskeleton term is correct. The more specific manchette localization better represents the specialized core context. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB-SubCell:SL-0090 · cytoskeleton controlled vocabulary SUPPORTS TRANSFER |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MODIFY | Summary: The HuRI binary-interactome screen detected human LRGUK binding to HOOK2 isoform 2, but the generic protein-binding term obscures the cytoskeletal partner context. Reason: Q96ED9-2 is human HOOK2 isoform 2. The direct human HuRI IPI establishes an LRGUK-HOOK2 association but does not establish its physiological role or direct microtubule binding by LRGUK. HOOK2 is a microtubule-binding cargo adaptor rather than a cytoskeletal polymer or necessarily a structural cytoskeletal constituent. Mechanistic mouse-orthologue studies independently map Lrguk binding to HOOK2 and RIMBP3 to its GUK-like domain and binding to KLC3 to its LRR domain. Because GO:0008092 denotes binding to a protein component of the cytoskeleton rather than direct binding to microtubules, this mouse evidence licenses cytoskeletal protein binding as the most conservative informative replacement for generic protein binding, while retaining the species and assay boundary. Proposed replacements: cytoskeletal protein binding Supporting Evidence: PMID:28003339 In the present study, we showed that the GUK-like domain is essential for binding to HOOK2 and RIMBP3, and the LRR domain is essential for binding to KLC3. PMID:25781171 HOOK2 is a member of the HOOK family of proteins, which are adaptor-like proteins involved in loading cargos (including protein complexes and organelles) onto microtubules for transport [28]. PMID:32296183 the cellular function of most individual PPIs remains to be elucidated. |
| GO:0001669 acrosomal vesicle | ISS GO_REF:0000024 | ACCEPT | Summary: Human LRGUK acrosomal localization is transferred from the exact mouse Lrguk ortholog. Reason: Mouse Lrguk localizes to the acrosome and is required for acrosome attachment. Strong sequence orthology and conserved sperm-assembly biology support this core localization transfer. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q9D5S7 · mouse Lrguk SUPPORTS TRANSFER |
| GO:0002177 manchette | ISS GO_REF:0000024 | ACCEPT | Summary: Human LRGUK manchette localization is transferred from the exact mouse Lrguk ortholog. Reason: Mouse Lrguk localizes to the manchette during spermiogenesis and is required for manchette microtubule organization and sperm-head shaping. This precise, function-linked localization is a sound orthology transfer. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q9D5S7 · mouse Lrguk SUPPORTS TRANSFER |
| GO:0007283 spermatogenesis | ISS GO_REF:0000024 | ACCEPT | Summary: Human LRGUK involvement in spermatogenesis is transferred from the exact mouse Lrguk ortholog. Reason: Mouse Lrguk deficiency causes male infertility and defects across multiple steps of spermiogenesis. The term is broad but accurately captures the conserved biological process supported by the exact ortholog. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q9D5S7 · mouse Lrguk SUPPORTS TRANSFER |
| GO:0035082 axoneme assembly | ISS GO_REF:0000024 | ACCEPT | Summary: Human LRGUK involvement in axoneme assembly is transferred from the exact mouse Lrguk ortholog. Reason: Mouse Lrguk is required for basal-body attachment and initiation and extension of the sperm-tail axoneme. These gene-specific phenotypes support the exact-ortholog process transfer without implying a role in primary-cilium biogenesis, which mouse evidence indicates is not essential. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q9D5S7 · mouse Lrguk SUPPORTS TRANSFER |
| GO:0120316 sperm flagellum assembly | IMP PMID:42493466 Biallelic Truncating Variant in LRGUK Is Associated With Sev... | NEW | Summary: Direct human loss-of-function evidence supports LRGUK involvement in sperm flagellum assembly. Reason: A homozygous truncating LRGUK variant was identified in an infertile man with absent LRGUK protein in sperm, severe flagellar malformations, disorganized axonemal architecture and central-pair abnormalities. This human evidence licenses the sperm-specific assembly term without extending the claim to all motile cilia or assigning a catalytic molecular function. Supporting Evidence: PMID:42493466 Immunofluorescence showed absence of LRGUK protein in patient spermatozoa, supporting a loss-of-function effect. Semen analysis revealed impaired motility and complete teratozoospermia. Morphological and ultrastructural analyzes demonstrated severe defects affecting both sperm head and flagellum, including disorganized axonemal architecture and central pair abnormalities. |
| GO:0036064 ciliary basal body | ISS PMID:25781171 LRGUK-1 is required for basal body and manchette function du... | NEW | Summary: Proposed human LRGUK localization to the sperm ciliary basal body by sequence-similarity transfer from the exact mouse Lrguk orthologue. Reason: Mouse LRGUK is directly detected at the sperm basal body, where mouse Lrguk-1 is required for basal-body attachment and early axoneme extension. The exact human-mouse orthology supports this sperm-context localization transfer without implying direct localization evidence in human cells or an unrestricted role at every somatic ciliary basal body. Supporting Evidence: PMID:25781171 LRGUK was also evident in the sperm basal body and the sperm tail (Fig. 3E). |
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Download this section (compressed HTML)Q: Does the human LRGUK GUK-like domain bind guanine nucleotides or catalyze ATP-dependent GMP phosphorylation, or is its physiological role limited to partner recognition and scaffolding?
Q: What is the endogenous composition, stoichiometry and spatial organization of the human LRGUK complex with HOOK1-3, RIMBP3 and KLC3 during successive stages of spermiogenesis?
Q: Is human LRGUK a structural or regulatory component of the sperm axonemal C1b/C1f projection, and does any equivalent function operate in somatic motile or primary cilia?
Q: Are there functionally relevant human LRGUK transcripts or proteoforms not represented as curated UniProt isoforms, and do they differ from the mouse-Lrguk-1 mechanism?
Experiment: Purify full-length human LRGUK and its isolated GUK-like domain and quantify GMP/GDP/ATP/ADP binding and ATP-dependent phosphotransfer or hydrolysis using orthogonal biochemical assays, with canonical human GUK1 and binding-defective LRGUK mutants as controls.
Hypothesis: The LRGUK GUK-like region is optimized for partner binding and lacks measurable guanylate kinase catalysis under physiological conditions.
Type: biochemical activity and nucleotide-binding assays
Experiment: Endogenously tag LRGUK in a human spermatogenic organoid or germ-cell model and combine stage-resolved super-resolution imaging, proximity labeling and quantitative co-immunoprecipitation to map LRGUK localization and its HOOK, RIMBP3, KLC3 and central-apparatus partners.
Hypothesis: Human LRGUK changes partners as it moves from the acrosome/manchette axis to the basal body and sperm flagellum, forming a defined sperm-specific assembly complex.
Type: endogenous spatial proteomics
Experiment: Generate isogenic human LRGUK-null cells or spermatogenic organoids and rescue them with wild-type LRGUK, the patient p.Arg355Ter allele, HOOK/RIMBP3-binding GUK-like-domain mutants, and KLC3-binding LRR/LRRCT mutants; quantify manchette, basal-body, head-shaping, flagellar and central-pair phenotypes.
Hypothesis: Partner-binding surfaces in both the GUK-like and LRR regions are independently required for ordered manchette transport and sperm-flagellum assembly.
Type: separation-of-function genetic rescue
Experiment: Compare airway multiciliated cells and primary-cilium-forming somatic cells from LRGUK-deficient and corrected patient-derived induced pluripotent stem cells for LRGUK localization, cilium assembly, ultrastructure and motility.
Hypothesis: The essential human LRGUK requirement is sperm-biased and does not generalize to all somatic cilia despite comparative ciliate evidence.
Type: patient-derived ciliary phenotyping
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The biochemical activity of the human LRGUK GUK-like domain is unresolved: partner binding is demonstrated in mouse LRGUK1, but nucleotide binding, phosphotransfer and ATPase activity have not been established, so neither a guanylate kinase nor a definitive pseudokinase assignment is justified.
OPEN BIOLOGYCURATION MF_DARK
What is known: The 825-aa human protein contains a predicted GUK-like domain, and mouse experiments show that this region is required for HOOK2 and RIMBP3 binding. The current evidence therefore supports a scaffold interface without excluding an additional catalytic function.
Significance: Resolving catalytic status is necessary to prevent family-name and domain-based transfer of guanylate kinase activity and to define the correct molecular function of LRGUK.
What would resolve it: Directly measure nucleotide binding and catalysis with purified human LRGUK and separation-of-function mutants, then determine whether catalytic activity is required for cellular rescue independently of HOOK/RIMBP3 binding.
Provenance (the field's own admissions):
Gap: The endogenous human LRGUK interaction complex and the causal contribution of each partner to sperm assembly remain unresolved; the direct human HOOK2 observation is a binary-interactome result, whereas the broader HOOK1-3/RIMBP3/KLC3 complex is established in mouse Lrguk-1 experiments.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Human genetics establishes LRGUK-dependent sperm morphogenesis, and mouse evidence establishes domain-specific partner interactions. Neither source defines an endogenous, stage-resolved human spermatid complex.
Significance: Identifying the physiological human complex is required to distinguish a cargo, adaptor, structural-scaffold or regulatory role and to interpret partner-binding variants mechanistically.
What would resolve it: Perform endogenous stage-specific interaction proteomics and imaging in human spermatids, followed by partner-specific knockout and interaction-defective rescue experiments.
Provenance (the field's own admissions):
Gap: The precise placement and molecular role of LRGUK within the mammalian sperm C1b/C1f central-apparatus region, and any role in non-sperm cilia, remain unresolved despite patient sperm abnormalities and ciliate comparative evidence.
OPEN BIOLOGYCURATION RESIDUAL_SUBGAP
What is known: Human loss of LRGUK is associated with central-pair defects and altered central-apparatus proteins. Tetrahymena Cfap246 is a likely C1b/C1f component, but it is a shorter partial ortholog that lacks the mammalian C-terminal GUK-like domain.
Significance: This distinction controls whether sperm-specific central-apparatus annotations can be made and whether any ciliary role may be propagated to somatic contexts.
What would resolve it: Localize endogenous LRGUK within human sperm by super-resolution microscopy or cryo-electron tomography, test C1b/C1f assembly after LRGUK loss and rescue, and independently assay somatic motile and primary cilia.
Provenance (the field's own admissions):
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