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.

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

“…Auxin exporters are primarily represented by plasma membrane localized PINs (so-called long PINs, see below) and members of the ABCB family, which have so far all been found to be plasma membrane-embedded [6,[25][26][27]. However, it should be mentioned that recently ABCB4 and ABCB21 were characterized as facultative importer/exporters whose transport directionality seems to be triggered by intracellular auxin levels [28].…”

Structure and mechanism of ATP-dependent phospholipid transporters

“…It is possible that eukaryotic ABC "importers" present the same fold as "exporters", but have evolved an inverse affinity switch that allow them to transport the substrate inwards. Indeed, it has been demonstrated that some plant ABC transporters are capable of switching between the importer and exporter mode depending on substrate concentrations [68,69].…”

Mechanisms of Biotin Transport

“…ABC transporters are classified as importers (those that mediate inward translocation of substrates into the cell) or exporters (those that facilitate extrusion of solutes from the cell). The former are predominantly present in prokaryotes [15][16][17], whereas the latter are found in both prokaryotes and eukaryotes [18]. ABC importers can be grouped into 2 further subtypes based on their topology in the membrane [19,20].…”