title: Arabidopsis ABCB21 is a facultative auxin importer/exporter regulated by cytoplasmic auxin concentration.
abstract: The phytohormone auxin is critical for plant growth and many developmental processes. Members of the P-glycoprotein (PGP/ABCB) subfamily of ATP-binding cassette (ABC) transporters have been shown to function in the polar movement of auxin by transporting auxin over the plasma membrane in both monocots and dicots. Here, we characterize a new Arabidopsis member of the ABCB subfamily, ABCB21/PGP21, a close homolog of ABCB4, for which conflicting transport directionalities have been reported. ABCB21 is strongly expressed in the abaxial side of cotyledons and in junctions of lateral organs in the aerial part, whereas in roots it is specifically expressed in pericycle cells. Membrane fractionation by sucrose density gradient centrifugation followed by Western blot showed that ABCB21 is a plasma membrane-localized ABC transporter. A transport assay with Arabidopsis protoplasts suggested that ABCB21 was involved in IAA transport in an outward direction, while naphthalene acetic acid (NAA) was a less preferable substrate for ABCB21. Further functional analysis of ABCB21 using yeast import and export assays showed that ABCB21 mediates the 1-N-naphthylphthalamic acid (NPA)-sensitive translocation of auxin in an inward direction when the cytoplasmic IAA concentration is low, whereas this transporter mediates outward transport under high internal IAA. An increase in the cytoplasmic IAA concentration by pre-loading of IAA into yeast cells abolished the IAA uptake activity by ABCB21 as well as ABCB4. These findings suggest that ABCB21 functions as a facultative importer/exporter controlling auxin concentrations in plant cells.
authors.affiliation: Laboratory of Plant Gene Expression, Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho Uji, 611-0011 Japan.
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Kamimoto
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Terasaka
authors.forename: Masafumi
authors.initials: M
authors.lastname: Hamamoto
authors.forename: Kojiro
authors.initials: K
authors.lastname: Takanashi
authors.forename: Shoju
authors.initials: S
authors.lastname: Fukuda
authors.forename: Nobukazu
authors.initials: N
authors.lastname: Shitan
authors.forename: Akifumi
authors.initials: A
authors.lastname: Sugiyama
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Suzuki
authors.forename: Daisuke
authors.initials: D
authors.lastname: Shibata
authors.forename: Bangjun
authors.initials: B
authors.lastname: Wang
authors.forename: Stephan
authors.initials: S
authors.lastname: Pollmann
authors.forename: Markus
authors.initials: M
authors.lastname: Geisler
authors.forename: Kazufumi
authors.initials: K
authors.lastname: Yazaki
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D017360:Arabidopsis / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D001692:Biological Transport; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D055785:Gene Knockdown Techniques; D007210:Indoleacetic Acids / Q000378:metabolism*; D009154:Mutation; D009280:Naphthaleneacetic Acids / Q000378:metabolism; D009928:Organ Specificity; D010641:Phenotype; D010802:Phylogeny; D010937:Plant Growth Regulators / Q000378:metabolism*; D018517:Plant Roots / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D030821:Plants, Genetically Modified; D011523:Protoplasts; D034622:RNA Interference; D013379:Substrate Specificity
chemical_list: C583140:ABCB21 protein, Arabidopsis; D018528:ATP-Binding Cassette Transporters; D029681:Arabidopsis Proteins; D007210:Indoleacetic Acids; D009280:Naphthaleneacetic Acids; D010937:Plant Growth Regulators