ABCB4

Summary: ABCB4 binds ATP through the nucleotide-binding apparatus that powers transport.

Reason: Recombinant PGP4 shows nucleotide photoaffinity labeling and Mg/vanadate-dependent nucleotide trapping. ATP binding is an integral mechanistic property of the ABC carrier, not a peripheral incidental ligand interaction.

Summary: The AtSubP nuclear assignment overextends sequence prediction for the intact ABCB4 membrane transporter.

Reason: AtSubP is an ISM compartment prediction; experimental-curator deference does not apply. ABCB4 live-cell imaging and photobleaching directly support strong plasma-membrane retention and slow endocytosis toward the lytic vacuolar pathway (PMID:22492845), consistent with its multipass carrier topology. Assigning the intact carrier to the nucleus from the sequence classifier alone exceeds the target-specific localization evidence and does not account for the established membrane architecture and trafficking. MARK_AS_OVER_ANNOTATED reflects that unsupported compartment assignment, not a demonstrated absence from every nuclear-envelope pool. A hypothetical nuclear membrane pool or cleaved fragment is not positive support for this row; no such target-specific mechanism is established by the cited studies.

Summary: The cited detergent-solubilized microsomal experiment does not establish a soluble cytosolic ABCB4 pool.

Reason: Inspection of all PMID28887381 supplements identifies ABCB4 in the Mapp multi-protein group as a true membrane protein with a plasma-membrane SUBAcon assignment and no dual-location flag. Cholate solubility describes extraction of microsomal membranes, not cytosolic localization. The cytosol claim exceeds that evidence; this is not a categorical exclusion based only on transmembrane topology.

Summary: ABCB4 is a plasma-membrane transporter.

Reason: Native root-cell imaging, FRAP and transport studies independently support the PM assignment across the proteomic, direct and electronic rows. Strong PM retention does not imply absolute absence of endocytosis or a uniform apical/basal orientation in all root tissues.

Summary: ABCB4 is a plasma-membrane transporter.

Reason: Native root-cell imaging, FRAP and transport studies independently support the PM assignment across the proteomic, direct and electronic rows. Strong PM retention does not imply absolute absence of endocytosis or a uniform apical/basal orientation in all root tissues.

Summary: ABCB4 is a plasma-membrane transporter.

Reason: Native root-cell imaging, FRAP and transport studies independently support the PM assignment across the proteomic, direct and electronic rows. Strong PM retention does not imply absolute absence of endocytosis or a uniform apical/basal orientation in all root tissues.

Summary: ABCB4 is a plasma-membrane transporter.

Reason: Native root-cell imaging, FRAP and transport studies independently support the PM assignment across the proteomic, direct and electronic rows. Strong PM retention does not imply absolute absence of endocytosis or a uniform apical/basal orientation in all root tissues.

Summary: ABCB4 is a plasma-membrane transporter.

Reason: Native root-cell imaging, FRAP and transport studies independently support the PM assignment across the proteomic, direct and electronic rows. Strong PM retention does not imply absolute absence of endocytosis or a uniform apical/basal orientation in all root tissues.

Summary: ABCB4 is a plasma-membrane transporter.

Reason: Native root-cell imaging, FRAP and transport studies independently support the PM assignment across the proteomic, direct and electronic rows. Strong PM retention does not imply absolute absence of endocytosis or a uniform apical/basal orientation in all root tissues.

Summary: The cited substrate screen does not demonstrate ABC-type xenobiotic export.

Reason: PMID16243904 measures vanadate trapping stimulated by candidate compounds and direct IAA uptake; stimulation is not itself translocation of those compounds. GO:0008559 specifically encodes ATP-coupled xenobiotic movement from inside to outside. Later direct 2,4-D uptake supports xenobiotic transport as a process but explicitly not its efflux, so it cannot rescue this directional molecular-function assertion.

Summary: ABCB4 was detected in a plasmodesmata-enriched proteomic fraction.

Reason: The actual TableS1 entry AT2G47000.1 contains four unique peptides and4.67% coverage. Retain the existing HDA localization at that evidential resolution, with the authors' fraction-purity caveat. No target fluorescent-fusion validation, exclusive plasmodesmal residence or transport-through-plasmodesmata role is claimed.

Summary: ABCB4-dependent basipetal auxin flow modulates root gravitropic curvature.

Reason: The mdr4 mutants have enhanced gravitropic curvature in this study, despite reduced basipetal flow. Retain gravitropism without converting the phenotype into an invariant positive-promoting role or conflating acropetal and basipetal streams.

Summary: ABCB4 responds to auxin and directly regulates cellular auxin levels.

Reason: The original expression time course is corroborated functionally by concentration-dependent IAA uptake/efflux and auxin-dependent root phenotypes. This is not retained merely because its transcript is inducible.

Summary: Kinetin transiently induces ABCB4 expression, without an established cytokinin-response function.

Reason: The original time course measures an expression change. It does not show that ABCB4 transports cytokinin or performs a step required for the cytokinin response. Retain the observation in provenance while judging the product-level process assertion as beyond these data.

Summary: ABCB4 contributes to polar auxin movement near the root apex.

Reason: Native mutant and overexpression transport assays support a role in the root-tip redirective stream. Cellular uptake versus efflux depends on concentration and assay context, so tissue-level shootward movement does not require a universal outward cellular mode.

Summary: ABCB4 can export auxin under efflux-favoring conditions.

Reason: Plant-cell efflux assays and concentration-dependent uptake-to-efflux switching independently support the outward process. The ABCB21-focused source also directly includes ABCB4; its paralog-centered title is not grounds for rejecting the annotation.

Summary: ABCB4 can export auxin under efflux-favoring conditions.

Reason: Plant-cell efflux assays and concentration-dependent uptake-to-efflux switching independently support the outward process. The ABCB21-focused source also directly includes ABCB4; its paralog-centered title is not grounds for rejecting the annotation.

Summary: ABCB4 also supports auxin influx at low cellular auxin.

Reason: Direct mammalian-cell uptake and concentration-dependent yeast experiments support the inward activity. Preserve this alongside efflux because the source explicitly resolves condition-dependent direction rather than treating the two functions as contradictory.

Summary: ABCB4 has auxin efflux activity in defined concentration and cell contexts.

Reason: The cellular transport studies establish outward IAA movement. This claim coexists with directly measured influx at other auxin levels; no universal high-affinity inward or constitutive outward mode is implied.

Summary: ABCB4 has auxin efflux activity in defined concentration and cell contexts.

Reason: The cellular transport studies establish outward IAA movement. This claim coexists with directly measured influx at other auxin levels; no universal high-affinity inward or constitutive outward mode is implied.

Summary: ABCB4 contributes specifically to root basipetal auxin transport.

Reason: The mdr4 transport assay loses approximately half of the basipetal stream while preserving the measured acropetal stream. This is a native directional phenotype, distinct from the direction of one transporter molecule in an isolated heterologous cell.

Summary: The membrane assignment can be refined to plasma membrane.

Reason: Direct native-cell imaging and FRAP locate ABCB4 at the PM, independently corroborating the broad proteomic/domain/ancestral membrane claim. Slow endocytosis remains compatible with the refinement and is not described as PIN-like rapid recycling.

Proposed replacements: plasma membrane

Summary: The membrane assignment can be refined to plasma membrane.

Reason: Direct native-cell imaging and FRAP locate ABCB4 at the PM, independently corroborating the broad proteomic/domain/ancestral membrane claim. Slow endocytosis remains compatible with the refinement and is not described as PIN-like rapid recycling.

Proposed replacements: plasma membrane

Summary: The membrane assignment can be refined to plasma membrane.

Reason: Direct native-cell imaging and FRAP locate ABCB4 at the PM, independently corroborating the broad proteomic/domain/ancestral membrane claim. Slow endocytosis remains compatible with the refinement and is not described as PIN-like rapid recycling.

Proposed replacements: plasma membrane

Summary: The membrane assignment can be refined to plasma membrane.

Reason: Direct native-cell imaging and FRAP locate ABCB4 at the PM, independently corroborating the broad proteomic/domain/ancestral membrane claim. Slow endocytosis remains compatible with the refinement and is not described as PIN-like rapid recycling.

Proposed replacements: plasma membrane

Summary: ATP hydrolysis is integral to the ABCB4 transport cycle.

Reason: The nucleotide-binding cassette architecture and Mg/vanadate-dependent nucleotide trapping support the ATPase chemical step. This mechanistic property is retained as core without claiming purified substrate-stimulated turnover kinetics were measured in every assay.

Summary: ABCB4 is specifically an ABC-type ATP-powered transporter.

Reason: The PTN000657189 assignment of ATPase-coupled transport is sound. Two nucleotide-binding cassettes and the ABC transmembrane architecture support the existing more specific ABC-type transporter activity, while uptake/efflux assays independently resolve auxin substrates.

Proposed replacements: ABC-type transporter activity

Summary: ABCB4 transports the synthetic auxin herbicide 2,4-D under defined conditions.

Reason: Independent PMID21992190 directly reports 2,4-D uptake in cells lacking AUX1, supporting this nondirectional xenobiotic-transport process despite overextension of the original xenobiotic-export MF. Do not generalize to broad drug efflux, and distinguish uptake measurements from inhibitor binding.

Summary: ABCB4-dependent auxin availability modulates root-hair elongation.

Reason: The cited root-hair-specific overexpression assay links enhanced efflux to shorter hairs, with auxin/efflux-inhibitor rescue. Later low-concentration uptake behavior explains why the physiological effect need not always have the same sign. Retain the contextual elongation role without an exclusive native requirement.

Summary: The broad transport process resolves to cellular auxin import and export.

Reason: Both directions are experimentally supported in different concentration/cellular contexts. Refine the inherited/electronic general transport term to both specific processes, not just the efflux mode.

Proposed replacements: auxin import into cell auxin export across the plasma membrane

Summary: The broad transport process resolves to cellular auxin import and export.

Reason: Both directions are experimentally supported in different concentration/cellular contexts. Refine the inherited/electronic general transport term to both specific processes, not just the efflux mode.

Proposed replacements: auxin import into cell auxin export across the plasma membrane

Summary: ABCB4 can import auxin into cells.

Reason: The original comparative study explicitly reports loss of ABCB4 uptake after IAA preloading, and independent assays show low-concentration uptake. The import process is conditional, not an exclusive direction.

Summary: ABCB4 is an ABC-type transporter with ATP-dependent nucleotide chemistry.

Reason: Its two-cassette architecture and recombinant nucleotide-trapping experiment support the ABC mechanism. Retain this mechanistic MF alongside substrate-specific auxin transport; it does not alone specify every xenobiotic substrate.