Affinage

XKR8

XK-related protein 8 · UniProt Q9H6D3

Length
395 aa
Mass
44.7 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

XKR8 is a caspase-activated phospholipid scramblase that irreversibly externalizes phosphatidylserine on apoptotic cells, providing the principal 'eat-me' signal for efferocytic clearance and thereby preventing autoimmunity, supporting spermatogenesis, and enabling developmental axon pruning. XKR8 resides at the plasma membrane as a heteromeric complex with the single-pass chaperones basigin (BSG) or neuroplastin (NPTN), which are required for its surface delivery; upon apoptotic caspase-3 cleavage of a C-terminal autoinhibitory plug domain—or, independently, via activating phosphorylation at three identified sites—the complex reorganizes into a heterotetrameric state that opens a charged intramolecular translocation pathway lined by six polar residues and gated by a conserved tryptophan (PMID:27503893, PMID:34625749, PMID:38364890, PMID:30718401). Xkr8 knockout mice exhibit defective phosphatidylserine exposure on apoptotic hematopoietic cells leading to lupus-like autoimmune disease, male infertility from failed germ-cell clearance, brain hyperconnectivity from impaired synaptic pruning, and defective neutrophil extracellular trap formation with compromised antifungal defense (PMID:29440417, PMID:31712393, PMID:37211968, PMID:41781710). A nonsense variant (p.W237X) that impairs XKR8 surface localization causes late-onset auditory neuropathy in a transgenic mouse model, linking XKR8 membrane trafficking to inner-ear neural homeostasis (PMID:37101210).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 2016 High

    Establishing that XKR8 requires BSG or NPTN as obligate chaperone partners for plasma-membrane delivery and that the activated complex assembles into a heterotetramer resolved the long-standing question of how XKR8 reaches the cell surface and acquires apoptosis-dependent scramblase competence.

    Evidence Reciprocal co-IP, BSG/NPTN double-KO cells, BSG transmembrane glutamic-acid mutagenesis, native PAGE and size-exclusion chromatography

    PMID:27503893

    Open questions at the time
    • Structural basis of heterotetramer assembly unknown at this stage
    • Role of NPTN versus BSG in tissue-specific contexts not resolved
    • Activation mechanism downstream of caspase cleavage not molecularly defined
  2. 2018 High

    Demonstrating that Xkr8 is the sole caspase-dependent scramblase in hematopoietic cells and that its loss causes lupus-like autoimmunity established XKR8 as physiologically essential for apoptotic cell clearance and immune homeostasis.

    Evidence Xkr8 knockout mouse, annexin V flow cytometry, Tim4/MerTK phagocytosis assays, immunological characterization

    PMID:29440417

    Open questions at the time
    • Whether other XKR family members compensate in non-hematopoietic tissues not addressed
    • Downstream signaling linking impaired efferocytosis to autoimmunity not dissected
  3. 2019 High

    Identifying phosphorylation as a caspase-independent activation mechanism for XKR8, and showing that flippase inactivation cooperates with XKR8 activation, revealed a dual-switch model for phosphatidylserine externalization.

    Evidence Phos-tag PAGE, mass spectrometry of phosphosites, phosphomimic mutagenesis, kinase/phosphatase inhibitor treatments, flippase gene deletion

    PMID:30718401

    Open questions at the time
    • Identity of the responsible kinase(s) not determined
    • Whether phosphorylation-mediated activation occurs physiologically during non-apoptotic processes not shown
  4. 2020 High

    Showing that Xkr8 knockout causes male infertility through failed PtdSer-dependent clearance of apoptotic germ cells by Sertoli cells extended XKR8's physiological role beyond the immune system to reproductive biology.

    Evidence Xkr8 KO mouse testes analysis, electron and fluorescence microscopy, phagocytosis assays

    PMID:31712393

    Open questions at the time
    • Whether XKR8 functions in female reproductive cell clearance not examined
    • Molecular cues linking Sertoli-cell recognition to XKR8-exposed PtdSer not fully defined
  5. 2021 High

    The first atomic-resolution structure of the XKR8–BSG complex revealed a cuboid transmembrane architecture with a charged intramolecular translocation pathway and a tryptophan gate, providing a molecular mechanism for how the scramblase translocates phospholipids.

    Evidence Cryo-EM and X-ray crystallography at 3.8 Å, Trp-to-Ala mutagenesis yielding constitutive activity, scramblase assay

    PMID:34625749

    Open questions at the time
    • Structure captured in detergent, not native lipid bilayer
    • Conformational change accompanying caspase cleavage not visualized
    • Lipid selectivity of the translocation pathway not mechanistically explained
  6. 2023 High

    Demonstrating that Xkr8 loss prevents PtdSer-dependent glial pruning of excess synapses and axonal projections in the developing brain established XKR8 as a central effector of developmental neural circuit refinement.

    Evidence Xkr8 KO mouse brain, confocal synapse density analysis, axon tracing, hippocampal electrophysiology

    PMID:37211968

    Open questions at the time
    • Upstream cues triggering XKR8 activation in neurons not identified
    • Whether phenotype is fully cell-autonomous versus involving non-neuronal XKR8 not resolved
  7. 2023 Medium

    Identification of a human nonsense variant (p.W237X) that impairs XKR8 surface localization and causes late-onset auditory neuropathy in transgenic mice linked XKR8 membrane trafficking to inner-ear neural homeostasis.

    Evidence HEK293T surface-localization assay, transgenic mouse ABR audiometry, cochlear immunolabeling

    PMID:37101210

    Open questions at the time
    • Single variant in a single lab; independent replication in human cohorts lacking
    • Mechanism connecting impaired XKR8 localization to spiral ganglion neuron degeneration not defined
    • Whether scramblase activity is required or merely surface presence unclear
  8. 2024 High

    A cryo-EM structure in lipid nanodiscs resolved the autoinhibitory mechanism: the XKR8 C-terminal tail plugs the cytoplasmic groove via polar and van der Waals contacts, and disruption of these contacts activates scrambling, unifying caspase cleavage and phosphorylation as convergent plug-release mechanisms.

    Evidence Cryo-EM at 3.66 Å in lipid nanodiscs, C-terminal tail contact-residue mutagenesis, scramblase activity assay

    PMID:38364890

    Open questions at the time
    • Active-state structure after plug removal not yet captured
    • How phosphorylation physically displaces the plug not structurally resolved
  9. 2025 Medium

    Showing that XKR8 knockout in breast cancer cells selectively suppresses macrophage efferocytosis and tumor growth in immune-competent hosts distinguished the apoptotic XKR8 scramblase pathway from TMEM16F in the tumor immune microenvironment.

    Evidence CRISPR KO in breast cancer cells, orthotopic implantation in syngeneic versus immunodeficient mice, efferocytosis assays

    PMID:41198619

    Open questions at the time
    • Single tumor model and single lab; generalizability across cancer types not established
    • Mechanism by which efferocytosis promotes immune evasion not molecularly dissected
  10. 2026 High

    Revealing that caspase-3-mediated XKR8 cleavage during NETosis disrupts membrane lipid asymmetry, altering lipid tension to open mechanosensitive Ca²⁺ channels, established an unexpected non-apoptotic role for XKR8 scramblase in innate immune defense.

    Evidence Caspase-3 cleavage-site mutagenesis, Xkr8 KO mice, Ca²⁺ channel pharmacology, NET quantification, in vivo Candida albicans pulmonary infection model

    PMID:41781710

    Open questions at the time
    • Identity of the mechanosensitive channel(s) activated by XKR8-driven lipid scrambling not determined
    • Whether XKR8 participates in other forms of regulated cell death (e.g., pyroptosis, ferroptosis) unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • A structure of the fully activated (plug-released) XKR8–BSG complex has not been captured, leaving the conformational transition that opens the lipid translocation pathway and the basis for phospholipid selectivity unresolved.
  • No active-state atomic structure available
  • Lipid headgroup selectivity of the translocation pathway not mechanistically explained
  • Upstream kinase(s) responsible for activating phosphorylation not identified

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 3 GO:0008289 lipid binding 2
Localization
GO:0005886 plasma membrane 4
Pathway
R-HSA-5357801 Programmed Cell Death 4 R-HSA-168256 Immune System 3
Partners
BSGNPTN
Complex memberships
XKR8–Basigin heterotetramerXKR8–Neuroplastin heterotetramer

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2016 XKR8 forms a complex with basigin (BSG) or neuroplastin (NPTN); BSG and NPTN chaperone XKR8 to the plasma membrane, and their absence causes XKR8 to localize intracellularly, abolishing apoptosis-induced phosphatidylserine exposure. The atypical glutamic acid in BSG's transmembrane region is required for BSG association with XKR8. Upon apoptotic signaling, XKR8 is cleaved at the C-terminus and the XKR8/BSG complex forms a higher-order complex, likely a heterotetramer of two XKR8 and two BSG/NPTN molecules. Co-immunoprecipitation, BSG/NPTN double-knockout cell lines, mutational analysis of BSG transmembrane glutamic acid, native PAGE/size-exclusion to determine complex stoichiometry Proceedings of the National Academy of Sciences of the United States of America High 27503893
2019 XKR8 scramblase activity is activated by phosphorylation: kinase inhibitors suppress and phosphatase inhibitors enhance PtdSer exposure; mass spectrometry and mutational analysis identified three phosphorylation sites whose phosphomimic mutations render XKR8 resistant to kinase inhibition. Flippase activity counteracts XKR8-mediated PtdSer exposure, so combined phosphorylation of XKR8 and flippase downregulation cooperate to achieve maximal PtdSer externalization. Phos-tag PAGE, mass spectrometry, site-directed mutagenesis of phosphorylation sites, kinase/phosphatase inhibitor treatments, flippase gene deletion Proceedings of the National Academy of Sciences of the United States of America High 30718401
2018 XKR8 is the sole caspase-dependent scramblase expressed in mouse hematopoietic cells; Xkr8 knockout mice show strongly reduced PtdSer exposure on apoptotic thymocytes, splenocytes, and neutrophils, impairing their phagocytic clearance by Tim4/MerTK-expressing macrophages in vitro and in vivo, and leading to lupus-like autoimmune disease. Xkr8 knockout mouse model, flow cytometry for PtdSer exposure (annexin V), in vitro phagocytosis assays with Tim4/MerTK-expressing phagocytes, histological/immunological characterization Proceedings of the National Academy of Sciences of the United States of America High 29440417
2021 Cryo-EM and X-ray crystallography of the human XKR8-Basigin complex at 3.8 Å revealed a cuboid transmembrane region with 22 charged residues stabilized by salt bridges; phosphatidylcholine binds in a hydrophobic cleft on the extracellular surface; six charged residues arranged top-to-bottom inside the molecule form a translocation pathway for phospholipids. A critical tryptophan at the exoplasmic end of the path gates scramblase activity — its mutation to alanine renders the complex constitutively active. Cryo-EM, X-ray crystallography, site-directed mutagenesis (W→A), scramblase activity assay Nature structural & molecular biology High 34625749
2020 XKR8-mediated PtdSer exposure is essential for phagocytic clearance of apoptotic germ cells by Sertoli cells during spermatogenesis; Xkr8-/- male mice are infertile due to impaired PtdSer-dependent 'eat-me' signaling and secondary accumulation of unengulfed dead germ cells. Xkr8 knockout mouse model, annexin V flow cytometry, in vitro phagocytosis assay with Tim4/MerTK cells, fluorescence and electron microscopy of testes Molecular and cellular biology High 31712393
2024 Cryo-EM structure of the human XKR8-Basigin complex in lipid nanodiscs at 3.66 Å shows that the C-terminal tail of XKR8 engages the cytoplasmic groove of XKR8 via polar and van der Waals interactions, maintaining the inactive state; point mutations disrupting these interactions strongly enhance scramblase activity, establishing that the C-terminal tail functions as a plug that is released upon caspase cleavage or phosphorylation to activate XKR8. Cryo-EM in lipid nanodiscs, site-directed mutagenesis of C-terminal tail contact residues, scramblase activity assay The Journal of biological chemistry High 38364890
2023 XKR8 is required for developmental axon pruning in the mammalian brain: Xkr8 knockout mice show failure of PtdSer exposure in hippocampal neurons, excess excitatory nerve terminals, increased cortico-cortical and cortico-spinal projections, aberrant hippocampal electrophysiology, and global brain hyperconnectivity, identifying XKR8-mediated phospholipid scrambling as the mechanism labeling developing projections for glial phagocytic elimination. Xkr8 knockout mouse model, annexin V staining for PtdSer exposure, confocal microscopy of synapse density, axon tracing, electrophysiology (hippocampal recordings) The EMBO journal High 37211968
2026 During neutrophil extracellular trap (NET) formation, XKR8 is cleaved by caspase-3, disrupting plasma membrane lipid asymmetry; the resulting phospholipid scrambling alters membrane lipid tension, promoting Ca2+ influx through mechanosensitive channels that drives NET formation. Mutation of the caspase-3 cleavage site in XKR8 impairs NET formation, and Xkr8-deficient mice show defective NETs and compromised control of Candida albicans pulmonary infection. Caspase-3 cleavage site mutagenesis, Xkr8 knockout mice, calcium channel inhibition at different stages, NET quantification, in vivo fungal infection model Nature immunology High 41781710
2025 In a CRISPR-Cas9 knockout breast cancer model, XKR8 KO specifically suppressed macrophage-mediated efferocytosis and tumor growth in immune-competent (but not immune-deficient) mice, establishing that XKR8-driven apoptotic PtdSer externalization promotes tumor immune evasion via efferocytosis, distinct from the calcium-activated TMEM16F scramblase pathway. CRISPR/Cas9 knockout, orthotopic tumor implantation in immune-competent and immune-deficient (NOD/SCID, RAG-KO) mice, efferocytosis assays Cell death discovery Medium 41198619
2023 A nonsense variant (p.W237X) in XKR8 impairs surface localization of XKR8 protein in HEK293T cells, and transgenic mice carrying this variant show late-onset auditory neuropathy with altered XKR8 protein localization in spiral ganglion neurons, linking proper XKR8 membrane trafficking to inner ear neural homeostasis. Plasmid expression in HEK293T cells, immunolabeling for surface localization, transgenic mouse audiometry (ABR), in situ hybridization and immunolabeling in cochlea Journal of translational medicine Medium 37101210

Source papers

Stage 0 corpus · 16 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2016 Xkr8 phospholipid scrambling complex in apoptotic phosphatidylserine exposure. Proceedings of the National Academy of Sciences of the United States of America 116 27503893
2022 Targeting Xkr8 via nanoparticle-mediated in situ co-delivery of siRNA and chemotherapy drugs for cancer immunochemotherapy. Nature nanotechnology 87 36424448
2019 Phosphorylation-mediated activation of mouse Xkr8 scramblase for phosphatidylserine exposure. Proceedings of the National Academy of Sciences of the United States of America 57 30718401
2018 Lupus-like autoimmune disease caused by a lack of Xkr8, a caspase-dependent phospholipid scramblase. Proceedings of the National Academy of Sciences of the United States of America 44 29440417
2021 The tertiary structure of the human Xkr8-Basigin complex that scrambles phospholipids at plasma membranes. Nature structural & molecular biology 39 34625749
2020 Infertility Caused by Inefficient Apoptotic Germ Cell Clearance in Xkr8-Deficient Male Mice. Molecular and cellular biology 15 31712393
2024 Knocking down of Xkr8 enhances chemotherapy efficacy through modulating tumor immune microenvironment. Journal of controlled release : official journal of the Controlled Release Society 11 38685385
2024 The role of the C-terminal tail region as a plug to regulate XKR8 lipid scramblase. The Journal of biological chemistry 9 38364890
2023 Phospholipid scramblase Xkr8 is required for developmental axon pruning via phosphatidylserine exposure. The EMBO journal 9 37211968
2018 Xkr8 Modulates Bipolar Cell Number in the Mouse Retina. Frontiers in neuroscience 5 30559640
2023 A dominant variant in apoptosis-related gene XKR8 is relevant to hereditary auditory neuropathy. Journal of translational medicine 2 37101210
2025 Phospholipid Scramblases TMEM16F and Xkr8 mediate distinct features of Phosphatidylserine (PS) externalization and immune suppression to promote tumor growth. bioRxiv : the preprint server for biology 1 40391322
2024 Knocking down of Xkr8 enhances chemotherapy efficacy through modulating tumor immune microenvironment. Journal of controlled release : official journal of the Controlled Release Society 1 38471639
2026 XKR8 Deletion Protects Against Noise-Induced Hearing Loss by Attenuating Apoptosis and Preserving Mitochondrial Bioenergetics in the Cochlea. Molecular neurobiology 0 41692920
2026 Lipid asymmetry disruption by XKR8 orchestrates neutrophil extracellular trap formation and inhibits fungal infection. Nature immunology 0 41781710
2025 Phospholipid scramblases TMEM16F and Xkr8 mediate distinct features of phosphatidylserine (PS) externalization and immune suppression to promote tumor growth. Cell death discovery 0 41198619