ATP9B

ATP9B is a P4-ATPase lipid flippase that operates in the exocytic pathway of the trans-Golgi network (TGN) (PMID:21914794, PMID:40234049). Unlike most P4-ATPases, ATP9B exits the endoplasmic reticulum and reaches the TGN independently of CDC50 auxiliary subunits, and its restricted TGN localization is conferred by a targeting signal in its N-terminal cytoplasmic region (PMID:21914794). ATP9B forms homomeric and/or heteromeric complexes with its paralog ATP9A, and this heteromeric association helps retain ATP9A in the Golgi; the flippase activities of both proteins are jointly required to transport cargo such as VSVG from the Golgi to the plasma membrane (PMID:40234049). Beyond its role in lipid flipping and Golgi-to-plasma-membrane cargo transport, no further substrate specificity or physiological context for ATP9B has been characterized in the available corpus.

1. 2011 High

   Established where ATP9B resides and how it gets there, distinguishing it from the broader P4-ATPase family that depends on CDC50 chaperones for ER exit.

   Evidence RNAi depletion of CDC50 proteins with fluorescence microscopy and subcellular fractionation in cultured cells

   PMID:21914794

   Open questions at the time

   • Does not define ATP9B's transported lipid substrate
   • Does not explain how ATP9B exits the ER without a CDC50 partner
2. 2011 Medium

   Localized the TGN-targeting determinant to the N-terminal cytoplasmic region, explaining the molecular basis for ATP9B's exclusive Golgi distribution.

   Evidence ATP9A/ATP9B chimeric domain-swap constructs assessed by fluorescence microscopy

   PMID:21914794

   Open questions at the time

   • The specific sequence motif and any recognizing trafficking machinery are not identified
   • Single-lab chimera result without orthogonal validation
3. 2024 Medium

   First evidence that ATP9B partners with ATP9A and that their combined flippase activity drives Golgi-to-plasma-membrane cargo transport (preprint).

   Evidence Co-immunoprecipitation, VSVG secretory transport assay, flippase activity assay, fluorescence microscopy (bioRxiv preprint)

   Open questions at the time

   • Preprint, not yet peer-reviewed
   • Stoichiometry and structural basis of the ATP9A/ATP9B complex unresolved
4. 2025 High

   Confirmed the ATP9A/ATP9B complex and showed that both flippase activities are required for VSVG exocytic transport, with heteromeric assembly controlling ATP9A Golgi retention.

   Evidence Reciprocal Co-immunoprecipitation, VSVG transport assay, flippase activity assays, and knockdown loss-of-function in cultured cells

   PMID:40234049

   Open questions at the time

   • The lipid species flipped by ATP9B in this process is not defined
   • Whether ATP9B can function independently of ATP9A is unclear
   • No structural model of ATP9B itself
5. 2025 Medium

   Defined a non-canonical P4B-type gating mechanism and phospholipid-stimulated ATPase activity for the close paralog ATP9A, inferred to extend to ATP9B.

   Evidence 2.2 Å cryo-EM structure of ATP9A, ATPase assays with various phospholipids, and molecular dynamics simulation

   PMID:40876594

   Open questions at the time

   • Mechanism is demonstrated for ATP9A; ATP9B is included only by family inference, not direct experiment
   • No experimental ATP9B structure or substrate specificity data

• The specific lipid substrate(s) of ATP9B, its independent (ATP9A-free) function, and its physiological/organismal role remain undefined.
• No ATP9B substrate identified
• No experimental ATP9B structure
• No in vivo or disease-linked phenotype reported