ATP9A

ATP9A is a CDC50-independent P4-ATPase lipid flippase that translocates negatively charged phospholipids—including phosphatidylserine and phosphoinositides—from the exoplasmic to the cytoplasmic membrane leaflet, thereby maintaining phospholipid asymmetry at endosomes and the trans-Golgi network (PMID:40876594, PMID:27733620). At early and recycling endosomes, ATP9A modulates RAB5 and RAB11 GTPase activation to drive endosomal recycling of cargo such as transferrin receptor and GLUT1 to the plasma membrane, and participates in an evolutionarily conserved MON2–DOPEY2 complex that supports SNX3-retromer-mediated Wntless sorting and Wnt secretion (PMID:27733620, PMID:36604604, PMID:30213940). ATP9A also forms homomeric and heteromeric complexes with ATP9B at the TGN to facilitate Golgi-to-plasma-membrane exocytic transport, suppresses exosome release, and is required for dendritic spine maturation and neuronal morphology, with loss-of-function mutations causing synaptic transmission defects in mice (PMID:40234049, PMID:30947313, PMID:36604604, PMID:40226306). High-resolution cryo-EM reveals that ATP9A operates as a monomer with a unique outward-open gating mechanism driven by TM6–10 helix rearrangement, distinct from the canonical TM1-2/A-domain gate of other P4-ATPases, and possesses an enlarged cavity capable of accommodating lipids with headgroups larger than phosphatidylserine (PMID:40876594).

1. 2016 High

   Establishing where ATP9A acts and its first cellular function: ATP9A was shown to reside on PS-positive early/recycling endosomes and to be required for recycling cargo back to the plasma membrane, linking a class-5 P4-ATPase to a specific membrane trafficking step.

   Evidence siRNA knockdown in HeLa cells with transferrin recycling and GLUT1 trafficking assays

   PMID:27733620

   Open questions at the time

   • Whether ATP9A flippase activity is directly required for the recycling phenotype was not tested
   • Mechanism by which ATP9A promotes recycling (direct lipid flipping vs. effector recruitment) was unknown
2. 2018 High

   Identifying ATP9A's protein complex and a role in retrograde sorting: ATP9A was found to form an endosomal complex with MON2 and DOPEY2, which collaborates with SNX3-retromer to sort Wntless, connecting ATP9A flippase activity to a specific cargo sorting pathway required for Wnt secretion.

   Evidence Reciprocal co-IP/MS, C. elegans RNAi epistasis, and ATPase-dead mutant phenocopy

   PMID:30213940

   Open questions at the time

   • Direct biochemical demonstration of lipid flipping by ATP9A was still lacking
   • How flippase-generated membrane asymmetry mechanistically promotes SNX3-retromer tubule formation was not resolved
3. 2019 Medium

   Extending ATP9A function to extracellular vesicle biogenesis: loss of ATP9A was shown to increase exosome release, establishing ATP9A as a negative regulator of EV secretion independently of caspase-3.

   Evidence siRNA knockdown in hepatoma cells with nanoparticle tracking analysis of EVs

   PMID:30947313

   Open questions at the time

   • Single lab observation; independent confirmation in other cell types was not reported
   • Whether the exosome phenotype is a direct consequence of altered lipid asymmetry or secondary to recycling defects was unresolved
4. 2023 High

   Revealing ATP9A's mechanism in endosomal recycling and its neuronal importance: ATP9A was demonstrated to control RAB5/RAB11 GTPase activation at endosomes, and Atp9a-null mice exhibited impaired neurite morphology and synaptic transmission, establishing a physiological requirement in the nervous system.

   Evidence Atp9a knockout mouse, RAB5/RAB11 GTP-pulldown assays, primary neuron culture, synaptic electrophysiology

   PMID:36604604

   Open questions at the time

   • Whether ATP9A directly activates or inhibits RAB GEFs/GAPs was not determined
   • The specific phospholipid substrates mediating the neuronal phenotype were unknown
5. 2023 Medium

   Linking ATP9A to nutrient scavenging: ATP9A was found to interact with V-ATPase subunit ATP6V1A and promote its delivery to the plasma membrane, driving cholesterol accumulation and RAC1-dependent macropinocytosis under nutrient stress in hepatocellular carcinoma cells.

   Evidence Co-IP, confocal imaging, cholesterol staining, RAC1 activity assay in HCC cells

   PMID:36715683

   Open questions at the time

   • Single lab study in one cancer cell type; generalizability unclear
   • Whether the ATP6V1A interaction depends on ATP9A flippase activity was not tested
6. 2025 High

   Defining ATP9A's role in the exocytic pathway: ATP9A and ATP9B were shown to form homomeric and heteromeric complexes at the TGN, and flippase activity of both was required for VSVG transport from Golgi to the plasma membrane, establishing a new role for ATP9A beyond endosomal recycling.

   Evidence Reciprocal co-IP, VSVG transport assay with flippase-dead mutants, fluorescence localization

   PMID:40234049

   Open questions at the time

   • The stoichiometry and structure of the ATP9A–ATP9B heteromeric complex are unknown
   • Whether the heteromeric complex has distinct substrate specificity from homomers was not addressed
7. 2025 High

   Solving the structural basis of ATP9A catalysis: cryo-EM at 2.2 Å revealed ATP9A operates as a CDC50-independent monomer with a novel outward gating mechanism (TM6–10 movement) and an enlarged lipid-binding cavity that accommodates substrates beyond PS, including phosphoinositides.

   Evidence Cryo-EM structure determination, in vitro ATPase assays with defined lipid substrates, molecular dynamics simulations

   PMID:40876594

   Open questions at the time

   • Structure was captured only in the E2P state; conformational cycle intermediates (E1, E2) remain unresolved
   • No structure with a bound phospholipid substrate was obtained
8. 2025 Medium

   Establishing ATP9A's neuronal cell-biological function: missense mutations and shRNA knockdown demonstrated that ATP9A is required for dendritic spine maturation and neuronal arborization, linking its flippase activity to synaptic structure.

   Evidence Overexpression of missense mutants and shRNA knockdown in HeLa cells and primary neurons with spine/morphology quantification

   PMID:40226306

   Open questions at the time

   • Single lab study; the specific lipid substrate changes underlying spine defects are unknown
   • Whether dendritic spine defects are cell-autonomous was not definitively established in mammalian neurons

• Key open questions: the full conformational cycle of ATP9A during lipid translocation remains structurally unresolved; the mechanism by which flippase-driven lipid asymmetry activates RAB GTPases or promotes membrane tubulation for cargo sorting is unknown; whether ATP9A loss-of-function mutations cause a defined human neurological disorder has not been established through human genetic studies.
• No substrate-bound structural intermediate has been captured
• The link between lipid flipping and RAB GEF/GAP regulation is mechanistically unresolved
• Human disease causality from ATP9A mutations has not been demonstrated in family studies