Affinage

ATP8A2

Phospholipid-transporting ATPase IB · UniProt Q9NTI2

Length
1188 aa
Mass
133.6 kDa
Annotated
2026-06-09
29 papers in source corpus 14 papers cited in narrative 14 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

ATP8A2 is a P4-ATPase phospholipid flippase that uses ATP hydrolysis to actively translocate phosphatidylserine, and to a lesser degree phosphatidylethanolamine, from the exoplasmic to the cytoplasmic leaflet of membranes, thereby maintaining the lipid asymmetry of photoreceptor outer segments and neuronal membranes (PMID:19778899, PMID:24413176). It functions as an obligate heteromer with the β-subunit CDC50A, whose transmembrane and exocytoplasmic domains and N-linked glycosylation are required for stable expression, ER export, folding, and flippase activity of the complex (PMID:21454556). Mechanistic dissection places the lipid substrate on a translocation pathway running between transmembrane segments M2 and M4, with N359 (M4) recognizing the lipid head group on the exoplasmic side and a hydrophobic gate centered on I364 controlling release into the cytoplasmic leaflet, while N905 in M6 is essential for the lipid-substrate-induced dephosphorylation (E2P→E2) step of the reaction cycle (PMID:24706822, PMID:30760526, PMID:37678495). Translocation of the negatively charged phosphatidylserine head group is electrogenic, with the principal charge movement occurring as the lipid is released and reoriented near positively charged residues at the cytoplasmic bilayer border (PMID:31371510, PMID:35945663); activity is further tuned by C-terminal autoinhibitory and anti-autoinhibitory domains that also govern folding and ER exit (PMID:27932490, PMID:39662833). ATP8A2 acts with CDC50A to promote neurite outgrowth, and loss of the flippase causes shortened photoreceptor outer segments, photoreceptor and spiral ganglion degeneration, phosphatidylserine externalization, and immune-mediated neurodegeneration (PMID:22641037, PMID:24413176, PMID:41394670). Missense variants from patients with the neurodevelopmental disorder CAMRQ4 abolish ATPase activity either by misfolding and proteasomal degradation or by retaining folding while lacking catalytic function (PMID:31397519, PMID:31612321).

Mechanistic history

Synthesis pass · year-by-year structured walk · 13 steps
  1. 2009 High

    Established that ATP8A2 is itself a phospholipid flippase, resolving whether a purified P4-ATPase can directly translocate aminophospholipids rather than merely being associated with such activity.

    Evidence Immunoaffinity purification from photoreceptor membranes, ATPase assays, and reconstitution into liposomes with fluorescent lipid transport readout

    PMID:19778899

    Open questions at the time
    • Did not identify an obligate subunit required for activity
    • Transmembrane translocation pathway and gating residues unmapped
  2. 2011 High

    Identified CDC50A as the obligate β-subunit, explaining how ATP8A2 achieves stable folding, ER export, and catalytic competence.

    Evidence Mass spectrometry, reciprocal co-IP, chimeric CDC50 mutagenesis, and reconstituted ATPase/flippase assays in HEK293T cells

    PMID:21454556

    Open questions at the time
    • Structural basis of the ATP8A2–CDC50A interface not resolved
    • Role of glycans in catalysis vs. stability not separated
  3. 2012 Medium

    Linked flippase activity to a cellular phenotype by showing ATP8A2/CDC50A promotes neurite outgrowth, implicating the pump in neuronal morphogenesis.

    Evidence Overexpression in PC12 cells and primary hippocampal neurons with CDC50A RNAi knockdown and neurite length measurement

    PMID:22641037

    Open questions at the time
    • No in vitro reconstitution linking lipid flipping to outgrowth
    • Downstream signaling from membrane asymmetry to cytoskeleton unknown
  4. 2014 High

    Defined the in vivo physiological role of ATP8A2 in sensory neurons, showing its flippase function maintains photoreceptor and spiral ganglion integrity through phospholipid composition.

    Evidence ATP8A2 knockout mouse with electroretinography, electron microscopy, lipid composition analysis, and auditory brainstem responses

    PMID:24413176

    Open questions at the time
    • Mechanism connecting altered lipid composition to cell death not defined
    • Did not address CNS or motor phenotypes
  5. 2014 High

    Mapped the lipid translocation pathway by identifying I364 as a hydrophobic gate and N359 as an exoplasmic head-group recognition residue, providing a structural basis for substrate movement.

    Evidence Site-directed mutagenesis with kinetics of partial reactions, homology modeling, and molecular dynamics simulations

    PMID:24706822

    Open questions at the time
    • Pathway inferred from modeling without experimental structure
    • Full set of pathway-lining residues not yet enumerated
  6. 2016 High

    Revealed C-terminal autoinhibitory and anti-autoinhibitory domains that regulate activity, folding, and ER exit, defining an intramolecular regulatory layer.

    Evidence C-terminal deletion mutagenesis with ATPase, flippase, localization, neurite, and phosphorylation analyses

    PMID:27932490

    Open questions at the time
    • Physiological trigger for autoinhibition relief unidentified
    • Functional consequence of CaMKII-motif phosphorylation not established
  7. 2019 High

    Demonstrated that phosphatidylserine translocation is electrogenic and that the disease residue I364 is required for charge movement, mechanistically distinguishing P4-ATPases from ion-pumping P2-ATPases.

    Evidence Solid-supported membrane electrophysiology with E198Q and I364M mutants and transient current measurement

    PMID:31371510

    Open questions at the time
    • Precise spatial origin of the charge movement not yet localized
    • Single-lab electrophysiological approach
  8. 2019 High

    Identified N905 in M6 as essential for lipid-induced dephosphorylation, connecting the flippase reaction cycle to canonical P-type ATPase chemistry.

    Evidence Systematic M5–M6 mutagenesis with ATPase kinetics and dephosphorylation assays

    PMID:30760526

    Open questions at the time
    • How the peripheral lipid path triggers the centrally located dephosphorylation step not fully explained
  9. 2019 Medium

    Characterized CAMRQ4 disease variants, distinguishing two loss-of-function classes: folded-but-catalytically-dead versus misfolded and proteasomally degraded.

    Evidence HEK293T expression with CDC50A, Western blot, immunofluorescence, ATPase assays, and MG132 treatment across multiple variants (and parallel enzyme assays on patient variants)

    PMID:31397519 PMID:31612321

    Open questions at the time
    • Genotype–phenotype correlation across variants not established
    • Single-lab functional characterization
  10. 2022 Medium

    Localized the principal electrogenic event to phosphatidylserine release at the cytoplasmic leaflet, where charged residues guide head-group reorientation for insertion.

    Evidence Solid-supported membrane electrophysiology with mutagenesis of charged residues near the cytoplasmic bilayer border

    PMID:35945663

    Open questions at the time
    • Individual contributions of specific charged residues not fully resolved
    • Single study extending prior work
  11. 2023 High

    Defined the lipid translocation route as an M2–M4 path through comprehensive mutagenesis, showing conformational coupling to the exoplasmic entry site and a basis for lipid specificity.

    Evidence Systematic mutagenesis of 130 residues across M1–M4 with quantitative ATPase kinetics

    PMID:37678495

    Open questions at the time
    • No experimental structure of substrate-bound intermediate
    • Coupling mechanism between entry site and M2–M4 path not visualized
  12. 2024 Medium

    Characterized species-specific N-terminal and conserved C-terminal (GYAFS motif) determinants of expression, folding, and autoinhibition.

    Evidence Mass spectrometry of the N-terminus plus GYAFS-motif mutagenesis with ATPase assays and immunofluorescence

    PMID:39662833

    Open questions at the time
    • Functional significance of the extended N-terminus in vivo unknown
    • Single-lab study
  13. 2025 Medium

    Connected ATP8A2 loss to immune-mediated neurodegeneration and placed it downstream of TDP-43 cryptic-exon regulation, identifying phosphatidylserine externalization driving glial/macrophage attack as a primary disease mechanism.

    Evidence TDP-43 depletion and splicing analysis in human neurons/ALS-FTD brains, Atp8a2 knockout mouse PS exposure assays, and macrophage/microglia depletion with lifespan and behavioral rescue (preprint)

    PMID:41394670

    Open questions at the time
    • Preprint not yet peer-reviewed
    • Identity of immune sensors recognizing externalized PS not defined
    • Relevance of TDP-43–ATP8A2 axis to human ALS-FTD pathology not established

Open questions

Synthesis pass · forward-looking unresolved questions
  • A high-resolution experimental structure of the ATP8A2–CDC50A complex with bound lipid substrate is still needed to unify the modeled M2–M4 pathway, gating, electrogenic release, and dephosphorylation steps into a single conformational cycle.
  • No experimental structure of a lipid-bound translocation intermediate
  • Coupling between catalytic and lipid-translocation steps inferred from kinetics/modeling only

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0008289 lipid binding 3 GO:0140104 molecular carrier activity 3 GO:0140657 ATP-dependent activity 3 GO:0016787 hydrolase activity 2
Localization
GO:0005783 endoplasmic reticulum 2 GO:0005886 plasma membrane 2 GO:0005768 endosome 1 GO:0005794 Golgi apparatus 1 GO:0005929 cilium 1
Pathway
R-HSA-1643685 Disease 3 R-HSA-1266738 Developmental Biology 2 R-HSA-1430728 Metabolism 2 R-HSA-382551 Transport of small molecules 2
Partners
CDC50A
Complex memberships
ATP8A2–CDC50A flippase complex

Evidence

Reading pass · 14 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2009 ATP8A2 (Atp8a2) localizes to outer segment disc membranes of rod and cone photoreceptor cells. Purified ATP8A2 exhibits ATPase activity stimulated by phosphatidylserine and phosphatidylethanolamine but not phosphatidylcholine. When reconstituted into liposomes, purified ATP8A2 flips fluorescent-labeled phosphatidylserine from the inner (exocytoplasmic) to the outer (cytoplasmic) leaflet in an ATP-dependent manner, providing the first direct biochemical evidence that a purified P4-ATPase can translocate aminophospholipids across membranes. Immunoaffinity purification, immunofluorescence microscopy, subcellular fractionation, ATPase activity assay, functional reconstitution into liposomes with fluorescent lipid transport assay The Journal of biological chemistry High 19778899
2011 CDC50A is the obligate β-subunit of ATP8A2. Mass spectrometry and Western blotting showed CDC50A co-purifies with ATP8A2 from photoreceptor membranes. In HEK293T cells, ATP8A2 assembles with CDC50A (but not CDC50B) to form a heteromeric complex required for stable expression, ER export, correct folding, and phosphatidylserine flippase activity. Both transmembrane and exocytoplasmic domains of CDC50A are required for a functional complex. N-linked oligosaccharides on CDC50A are required for stable expression of active complex. Mass spectrometry, Western blotting, co-immunoprecipitation, chimeric CDC50 protein mutagenesis, HEK293T expression, ATPase and flippase activity assays The Journal of biological chemistry High 21454556
2012 ATP8A2 overexpression in NGF-induced PC12 cells and primary rat hippocampal neurons increases neurite outgrowth length. Co-overexpression of ATP8A2 with CDC50A enhances this effect, and RNAi knockdown of CDC50A in hippocampal neurons reduces neurite outgrowth, establishing that ATP8A2 acts in synergy with CDC50A to promote neurite outgrowth. Overexpression in PC12 cells, primary hippocampal neuron culture, RNA interference knockdown of CDC50A, neurite length measurement FEBS letters Medium 22641037
2014 Isoleucine-364 (I364) in transmembrane segment M4 of ATP8A2 is critical for release of phosphatidylserine into the cytosolic leaflet, functioning as part of a hydrophobic gate. Asparagine-359 (N359) in M4 is involved in lipid substrate recognition on the exoplasmic side. Mutagenesis and kinetic analysis of partial ATPase reaction steps, supported by structural homology modeling and molecular dynamics simulations, indicate the lipid head group passes near I364 and that I364 and adjacent hydrophobic residues gate a pathway outlined by transmembrane segments M1, M2, M4, and M6. Site-directed mutagenesis, ATPase activity assays (kinetics of overall and partial reactions), structural homology modeling, molecular dynamics simulations Proceedings of the National Academy of Sciences of the United States of America High 24706822
2014 ATP8A2-deficient mice have shortened photoreceptor outer segments, reduced photoresponses, decreased photoreceptor viability, altered phosphatidylserine and phosphatidylethanolamine composition in outer segments, and reduced rhodopsin content. ATP8A2 deficiency also causes auditory brainstem response threshold elevation and spiral ganglion cell degeneration, establishing roles in photoreceptor and spiral ganglion cell function and survival via phospholipid composition maintenance. ATP8A2 knockout mouse analysis, electroretinography, electron microscopy, lipid composition analysis, auditory brainstem response, histology Journal of cell science High 24413176
2016 The C-terminus of ATP8A2 contains an autoinhibitory domain and an anti-autoinhibitory domain. Deletion of the C-terminal 33 residues reduces phosphatidylserine-dependent ATPase activity and flippase activity; these reductions are reversed by larger deletions (60–80 residues). The C-terminus is also important for efficient protein folding and ER exit. Unlike yeast Drs2, ATP8A2 is not regulated by phosphoinositides but undergoes phosphorylation on a serine within a CaMKII target motif. C-terminal deletion mutagenesis, ATPase activity assay, flippase activity assay, immunofluorescence, PC12 neurite outgrowth assay, phosphorylation analysis Molecular biology of the cell High 27932490
2019 Phosphatidylserine translocation by ATP8A2 is electrogenic: ATP8A2 generates a transient electrical current in the presence of ATP and phosphatidylserine (negatively charged substrate), whereas phosphatidylethanolamine produces only a diminutive current. The mutation E198Q (blocks dephosphorylation) abolishes this current, and I364M (disease-associated) strongly interferes with electrogenic lipid translocation. No charged substrate is countertransported, distinguishing P4-ATPases from P2-ATPases. Solid-supported membrane electrophysiology, site-directed mutagenesis (E198Q, I364M), transient current measurement Proceedings of the National Academy of Sciences of the United States of America High 31371510
2019 Asparagine-905 (N905) in transmembrane segment M6 of ATP8A2 is essential for lipid substrate-induced dephosphorylation (E2P→E2 step). All N905 substitution mutants (N905A/D/E/H/L/Q/R) showed very low activity and dramatic insensitivity to lipid substrate. Valine-906 is approached by the lipid substrate during translocation. N905 aligns with key ion-binding residues of P2-ATPase ion pumps, supporting a mechanistic resemblance despite the peripheral location of the flippase translocation pathway. Systematic mutagenesis of M5–M6 region (multiple point mutants), ATPase activity kinetics (Vmax, apparent lipid affinity), dephosphorylation assays The Journal of biological chemistry High 30760526
2019 Disease-associated missense variants p.Ile376Met and p.Lys429Met express at normal levels and localize preferentially to Golgi-recycling endosomes but are devoid of ATPase activity, indicating folding without catalytic function. Variants p.Lys429Asn, p.Ala544Pro, p.Arg625Trp, and p.Trp702Arg express poorly, localize to the ER, lack ATPase activity, and are stabilized by proteasome inhibitor MG132, indicating misfolding and proteasomal degradation. HEK293T expression of mutant ATP8A2 with CDC50A, Western blot, immunofluorescence microscopy, ATPase activity assay, proteasome inhibitor treatment Human mutation Medium 31397519
2019 Functional studies of missense mutations in ATP8A2 from CAMRQ4 patients showed four of five missense variants had very low protein levels and lacked phosphatidylserine-activated ATPase activity; one variant (p.Ile215Leu) expressed at normal levels and retained phospholipid-activated ATPase activity similar to wild-type. Expression studies in cell lines, ATPase activity assay, Western blot Journal of neurology Medium 31612321
2022 Using solid-supported membrane electrophysiology and mutagenesis, the main electrogenic event in ATP8A2 occurs during release of bound phosphatidylserine to the cytoplasmic leaflet. Positively charged lysine and arginine residues near the cytoplasmic border of the lipid bilayer interact with the phospholipid substrate during translocation and reorientation for insertion into the cytoplasmic leaflet. Solid-supported membrane electrophysiology, site-directed mutagenesis of charged residues, transient current analysis FEBS letters Medium 35945663
2023 Comprehensive mutagenesis of all residues in transmembrane segments M1, M2, M3, and M4 of ATP8A2 (130 new mutants) supports a lipid translocation pathway between M2 and M4 ('M2-M4 path'). Residues in this path have side chains capable of zipper-like ionic/hydrogen bond interactions with each other and the lipid head group. The M2-M4 path and the exoplasmic entry site are conformationally coupled, and some M2-M4 path mutations cause loss of lipid specificity. Systematic site-directed mutagenesis (130 mutants), ATPase activity kinetics (Vmax, apparent lipid affinity for each mutant) Biochimica et biophysica acta. Molecular cell research High 37678495
2024 Mass spectrometry confirmed that bovine ATP8A2, like human ATP8A2, has an extended N-terminal segment not present in the mouse ortholog; this segment enhances protein expression without affecting cellular localization or phosphatidylserine-activated ATPase activity. The conserved GYAFS motif in the C-terminal segment plays a role in autoinhibition and efficient folding of ATP8A2 into a functional protein. Mass spectrometry (N-terminus identification), cleavable C-terminal protein construct, site-directed mutagenesis of GYAFS motif, ATPase activity assay, immunofluorescence The Journal of biological chemistry Medium 39662833
2025 ATP8A2 is a target of TDP-43 cryptic exon suppression; TDP-43 depletion in human neurons and ALS-FTD patient brains leads to significant dysregulation of ATP8A2 splicing. In mice, Atp8a2 knockout increases phosphatidylserine (PS) exposure on the outer leaflet and promotes neuroinflammation. Depletion of peripheral macrophages rescues motor axon degeneration and doubles Atp8a2 knockout mouse lifespan; co-depletion of peripheral macrophages and central microglia quadruples lifespan and improves coordination, establishing that immune-mediated neurodegeneration downstream of PS externalization is a primary disease mechanism. TDP-43 depletion in human neurons, RNA splicing analysis, Atp8a2 knockout mouse, PS exposure assay, macrophage/microglia depletion experiments, lifespan and motor behavioral analysis bioRxivpreprint Medium 41394670

Source papers

Stage 0 corpus · 29 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2009 Localization, purification, and functional reconstitution of the P4-ATPase Atp8a2, a phosphatidylserine flippase in photoreceptor disc membranes. The Journal of biological chemistry 153 19778899
2011 Critical role of the beta-subunit CDC50A in the stable expression, assembly, subcellular localization, and lipid transport activity of the P4-ATPase ATP8A2. The Journal of biological chemistry 122 21454556
2012 Missense mutation in the ATPase, aminophospholipid transporter protein ATP8A2 is associated with cerebellar atrophy and quadrupedal locomotion. European journal of human genetics : EJHG 108 22892528
2014 Critical roles of isoleucine-364 and adjacent residues in a hydrophobic gate control of phospholipid transport by the mammalian P4-ATPase ATP8A2. Proceedings of the National Academy of Sciences of the United States of America 94 24706822
2014 Phospholipid flippase ATP8A2 is required for normal visual and auditory function and photoreceptor and spiral ganglion cell survival. Journal of cell science 65 24413176
2010 Disruption of the ATP8A2 gene in a patient with a t(10;13) de novo balanced translocation and a severe neurological phenotype. European journal of human genetics : EJHG 55 20683487
2019 Circ-ATP8A2 promotes cell proliferation and invasion as a ceRNA to target EGFR by sponging miR-433 in cervical cancer. Gene 37 31029604
2016 New ATP8A2 gene mutations associated with a novel syndrome: encephalopathy, intellectual disability, severe hypotonia, chorea and optic atrophy. Neurogenetics 37 27679995
2018 Recessive mutations in ATP8A2 cause severe hypotonia, cognitive impairment, hyperkinetic movement disorders and progressive optic atrophy. Orphanet journal of rare diseases 34 30012219
2012 P4-ATPase ATP8A2 acts in synergy with CDC50A to enhance neurite outgrowth. FEBS letters 34 22641037
2016 C-terminus of the P4-ATPase ATP8A2 functions in protein folding and regulation of phospholipid flippase activity. Molecular biology of the cell 32 27932490
2019 ATP8A2-related disorders as recessive cerebellar ataxia. Journal of neurology 25 31612321
2019 Phosphatidylserine flipping by the P4-ATPase ATP8A2 is electrogenic. Proceedings of the National Academy of Sciences of the United States of America 22 31371510
2019 Expression and functional characterization of missense mutations in ATP8A2 linked to severe neurological disorders. Human mutation 17 31397519
2019 Asparagine 905 of the mammalian phospholipid flippase ATP8A2 is essential for lipid substrate-induced activation of ATP8A2 dephosphorylation. The Journal of biological chemistry 16 30760526
2023 On the track of the lipid transport pathway of the phospholipid flippase ATP8A2 - Mutation analysis of residues of the transmembrane segments M1, M2, M3 and M4. Biochimica et biophysica acta. Molecular cell research 9 37678495
2022 Electrogenic reaction step and phospholipid translocation pathway of the mammalian P4-ATPase ATP8A2. FEBS letters 7 35945663
2024 Functional and in silico analysis of ATP8A2 and other P4-ATPase variants associated with human genetic diseases. Disease models & mechanisms 5 38436085
2021 Neonatal LPS exposure reduces ATP8A2 level in the prefrontal cortex in mice via increasing IFN-γ level. Brain research bulletin 5 33766557
2024 A novel missense variant in the ATPase domain of ATP8A2 and review of phenotypic variability of ATP8A2-related disorders caused by missense changes. Neurogenetics 3 39066872
2024 Structural and functional properties of the N- and C-terminal segments of the P4-ATPase phospholipid flippase ATP8A2. The Journal of biological chemistry 3 39662833
2025 Expanding the spectrum of ATP8A2 mutations: a new splicing variant and systematic review of CAMRQ4 syndrome. Molecular biology reports 1 40312603
2025 TDP-43 suppression of ATP8A2 cryptic splicing implicates phosphatidylserine-driven neuroinflammation in ALS/FTD. bioRxiv : the preprint server for biology 1 41394670
2024 ATP8A2 expression is reduced in the mPFC of offspring mice exposed to maternal immune activation and its upregulation ameliorates synapse-associated protein loss and behavioral abnormalities. Brain, behavior, and immunity 1 39681198
2026 Early-Onset Hyperkinetic Movement Disorders Define the Most Severe Presentation of the ATP8A2-Related Phenotypic Spectrum. International journal of developmental neuroscience : the official journal of the International Society for Developmental Neuroscience 0 41919976
2026 Novel Perspectives on ATP8A2 Regulation: Evidence for Parental Imprinting and Chimeric Transcript Formation. Epigenomes 0 42029640
2025 Reevaluation of the Impact of the Novel Likely Pathogenic Variant c.1286_1288delAGA in the ATP8A2 Gene: A 7-Year Follow-Up With Clinical, Genetic, and ACMG Insights in an Iranian Family. Molecular genetics & genomic medicine 0 39931767
2025 A decrease in Atp8a2 expression in Purkinje cells mediated acrylamide-induced cerebellar pathology in rats. Toxicological research 0 41769375
2024 A novel missense variant in the ATPase domain of ATP8A2 and review of phenotypic variability of ATP8A2-related disorders caused by missense changes. medRxiv : the preprint server for health sciences 0 38798571

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