Identification of an ABCB/P-glycoprotein-specific Inhibitor of Auxin Transport by Chemical Genomics

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

“…A chaperone-like function for TWD1 during ER to plasma membrane trafficking of ABCBs was suggested based on the finding that ABCB1,4,19 are retained on the ER and degraded in the twd1 mutant [8,9]. ABCB-type auxin transporters and TWD1 have been both identified as targets of NPA [37,41,62,111,112]. Micromolar concentrations of NPA were also shown to disrupt the ABCB1-TWD1 interaction (see +NPA) supporting the hypothesis that NPA binds at the ABCB-TWD1 interface [44,130].…”

“…Although plant ABCBs belong to the large superfamily of multidrug resistance transporters, they were found to own a high degree of substrate specificity toward only a few auxinic compounds but not to transport closely related substances (such as the anti-auxin 2-NAA or benzoic acid [6]) or classical substrates of mammalian ABCBs (such as rhodamin123, daunomycin and vinblastine [43]). ABCB-mediated auxin transport was dependent on ATP hydrolysis [6,37] as expected for primary active pumps, and sensitive to inhibitors of auxin efflux (such as NPA, and flavonols [6,37,43,44]) along with known inhibitors of mammalian multidrug ABC transporters (such as cyclosporine A and verapamil [6,43]). Recently, expression of ABCB19 in HeLa cells was also reported to result in anion currents that were inhibited by applying an anion channel blocker [45].…”

“…Recent analyses in heterologous, non-plant auxin-transport systems (such as yeast, HeLa cells and Xenopus oocytes) demonstrated that AUX1/LAX influx, and ABCB and PIN-mediated IAA efflux activity is substrate-specific and rate limiting [6,19,25,26,[37][38][39][40][41][42], providing experimental evidence that these proteins are bona fide auxin transporters. Although plant ABCBs belong to the large superfamily of multidrug resistance transporters, they were found to own a high degree of substrate specificity toward only a few auxinic compounds but not to transport closely related substances (such as the anti-auxin 2-NAA or benzoic acid [6]) or classical substrates of mammalian ABCBs (such as rhodamin123, daunomycin and vinblastine [43]).…”

“…At low micromolar concentrations, auxin transport inhibitors are thought to impair directly the activity of auxin transporters [37,41,101], whereas at higher concentrations (>50 M) auxin transport inhibitors seem to interfere with the bundling status of the actin cytoskeleton, thus affecting the presence of auxin exporters on the plasma membrane in this manner [7,[102][103][104][105]. These two modes of actions for NPA on auxin transport, which could be even interconnected [7], might be represented by two distinct NPA binding affinities, as reported for zucchini plasma membrane and speculated to correspond to the auxin transporter itself and an additional NPA-binding regulatory subunit [106].…”

“…ABCBs-type auxin transporters and TWD1 are valid candidates because both have been demonstrated to bind NPA. However, while the connection of NPA binding to ABCBs and inhibition of auxin transport appears straightforward, the importance of NPA binding to TWD1 is more difficult to validate [37,41,62,111,112]. Root gravitropic response, an auxin-transport regulated process, is NPA-insensitive in the twd1 root [44] supporting the idea that TWD1 and NPA act in a common pathway.…”