ATP8B2

ATP8B2 is a plasma membrane class-1 P4-ATPase (phospholipid flippase) that translocates phospholipids from the exoplasmic to the cytoplasmic leaflet to maintain membrane lipid asymmetry (PMID:25315773, PMID:25947375). Its activity and trafficking depend on the CDC50 accessory subunits CDC50A and CDC50B: CDC50 association is required for ER export and for phosphorylation of the catalytically essential Asp residue, and CDC50A co-expression markedly increases ATP8B2 plasma membrane abundance, while the P4-ATPase subunit itself dictates the localization of the complex (PMID:20961850, PMID:20947505). ATP8B2 preferentially flips phosphatidylcholine in an ATPase-dependent manner (PMID:25315773, PMID:25947375), and also enriches plasmalogens in the cytoplasmic leaflet, where it controls plasmalogen homeostasis through plasmalogen-dependent degradation of the biosynthetic enzyme FAR1 and supports AKT phosphorylation and cell growth (PMID:35832735). Together with ATP8B1 it additionally translocates phosphatidylinositol and sustains plasma membrane PtdIns(4,5)P2 levels upon Gq-coupled receptor activation (PMID:39258799). These lipid-handling functions are linked to glucose-stimulated insulin secretion in pancreatic beta cells, where the flippase machinery localizes to insulin secretory granules (PMID:26240149).

1. 2010 High

   Established that ATP8B2 is not an autonomous enzyme but requires a CDC50 partner, defining the subunit composition of the functional flippase and the determinant of its plasma membrane delivery.

   Evidence Co-IP, ER export assay, and catalytic Asp phosphorylation assay in co-expression systems

   PMID:20947505 PMID:20961850

   Open questions at the time

   • Did not resolve the relative physiological roles of CDC50A versus CDC50B
   • No structural model of the ATP8B2/CDC50 complex
2. 2010 Medium

   Showed the P4-ATPase subunit, not CDC50, dictates where the complex resides, answering whether targeting information is carried by the catalytic or accessory subunit.

   Evidence Immunofluorescence, co-expression, and subcellular fractionation

   PMID:20947505

   Open questions at the time

   • Targeting motif within ATP8B2 not mapped
   • Single-lab observation
3. 2014 High

   Identified the substrate specificity of ATP8B2 as phosphatidylcholine and demonstrated catalytic dependence, moving from a putative flippase to a defined PC-translocating enzyme.

   Evidence Fluorescent lipid analog flippase assay in stable cell lines with ATPase-deficient mutant controls

   PMID:25315773 PMID:25947375

   Open questions at the time

   • Did not establish full substrate repertoire beyond PC
   • Kinetics and selectivity relative to other P4-ATPases not quantified
4. 2015 Medium

   Connected flippase activity to a physiological secretory process by showing ATP8B2 is needed for glucose-stimulated insulin secretion.

   Evidence siRNA silencing and glucose-stimulated insulin secretion assays in beta cells and human islets

   PMID:26240149

   Open questions at the time

   • Mechanistic link between lipid flipping and granule docking/fusion not directly demonstrated
   • Relative contribution of ATP8B2 versus ATP8B1 in granules unresolved
5. 2022 Medium

   Extended substrate range to plasmalogens and linked ATP8B2 to a lipid-homeostatic feedback loop and AKT-dependent growth, explaining a downstream signaling consequence of leaflet asymmetry.

   Evidence siRNA knockdown with fluorescent lipid localization, FAR1 stability and AKT phosphorylation Western blots, and growth assays

   PMID:35832735

   Open questions at the time

   • Direct biochemical flipping of plasmalogens not reconstituted
   • How leaflet asymmetry mechanistically couples to AKT not defined
   • Single-lab study
6. 2024 Medium

   Broadened ATP8B2 function to phosphoinositide homeostasis, showing it (with ATP8B1) maintains plasma membrane PtdIns(4,5)P2 during receptor signaling.

   Evidence CDC50A and ATP8B1/ATP8B2 double-knockout cells with flippase assays and live-cell PtdIns(4,5)P2 biosensor imaging

   PMID:39258799

   Open questions at the time

   • Direct flipping of phosphoinositides versus indirect effects not fully separated
   • Redundancy with ATP8B1 limits attribution to ATP8B2 alone
7. 2020 Low

   First in vivo loss-of-function evidence implicated ATP8B2 in osteoclast-mediated bone mass regulation.

   Evidence Global Atp8b2 knockout mouse with bone phenotype analysis

   PMID:32553114

   Open questions at the time

   • No molecular mechanism linking flippase activity to bone phenotype established in this study
   • Cell-type-specific contribution not dissected
8. 2025 Low

   Associated ATP8B2 loss with defects in the autophagy-lysosomal pathway and mitochondrial function in macrophage/foam cell models.

   Evidence siRNA knockdown with LC3-II/p62 and CD63 readouts, ox-LDL degradation, autophagosome quantification, and JC-1 mitochondrial potential assays

   PMID:40148707 PMID:40402302

   Open questions at the time

   • No rescue experiment and no direct mechanistic link from phospholipid flipping to autophagy/lysosomal fusion
   • Single knockdown approach in a single cell model

• How ATP8B2-driven leaflet asymmetry is mechanistically transduced into downstream signaling, secretion, and membrane fusion events remains unresolved.
• No structure of the ATP8B2/CDC50 complex
• Causal chain from specific lipid substrates to AKT, insulin granule fusion, and autophagy not reconstituted
• No human disease mutation linked to ATP8B2 in the available corpus