Intercellular flow of the phytohormone auxin underpins multiple developmental processes in plants. Plant-specific pin-formed (PIN) proteins and several phosphoglycoprotein (PGP) transporters are crucial factors in auxin transport-related development, yet the molecular function of PINs remains unknown. Here, we show that PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors. Conditional gain-of-function alleles and quantitative measurements of auxin accumulation in Arabidopsis and tobacco cultured cells revealed that the action of PINs in auxin efflux is distinct from PGP, rate-limiting, specific to auxins, and sensitive to auxin transport inhibitors. This suggests a direct involvement of PINs in catalyzing cellular auxin efflux.

SummaryDirectional transport of the phytohormone auxin is required for the establishment and maintenance of plant polarity, but the underlying molecular mechanisms have not been fully elucidated. Plant homologs of human multiple drug resistance/P-glycoproteins (MDR/PGPs) have been implicated in auxin transport, as defects in MDR1 (AtPGP19) and AtPGP1 result in reductions of growth and auxin transport in Arabidopsis (atpgp1, atpgp19), maize (brachytic2) and sorghum (dwarf3). Here we examine the localization, activity, substrate specificity and inhibitor sensitivity of AtPGP1. AtPGP1 exhibits non-polar plasma membrane localization at the shoot and root apices, as well as polar localization above the root apex. Protoplasts from Arabidopsis pgp1 leaf mesophyll cells exhibit reduced efflux of natural and synthetic auxins with reduced sensitivity to auxin efflux inhibitors. Expression of AtPGP1 in yeast and in the standard mammalian expression system used to analyze human MDR-type proteins results in enhanced efflux of indole-3-acetic acid (IAA) and the synthetic auxin 1-naphthalene acetic acid (1-NAA), but not the inactive auxin 2-NAA. AtPGP1-mediated efflux is sensitive to auxin efflux and ABC transporter inhibitors. As is seen in planta, AtPGP1 also appears to mediate some efflux of IAA oxidative breakdown products associated with apical sites of high auxin accumulation. However, unlike what is seen in planta, some additional transport of the benzoic acid is observed in yeast and mammalian cells expressing AtPGP1, suggesting that other factors present in plant tissues confer enhanced auxin specificity to PGP-mediated transport.

Directional transport of the phytohormone auxin is established primarily at the point of cellular efflux and is required for the establishment and maintenance of plant polarity. Studies in whole plants and heterologous systems indicate that PIN-FORMED (PIN) and P-glycoprotein (PGP) transport proteins mediate the cellular efflux of natural and synthetic auxins. However, aromatic anion transport resulting from PGP and PIN expression in nonplant systems was also found to lack the high level of substrate specificity seen in planta. Furthermore, previous reports that PGP19 stabilizes PIN1 on the plasma membrane suggested that PIN-PGP interactions might regulate polar auxin efflux. Here, we show that PGP1 and PGP19 colocalized with PIN1 in the shoot apex in Arabidopsis thaliana and with PIN1 and PIN2 in root tissues. Specific PGP-PIN interactions were seen in yeast two-hybrid and coimmunoprecipitation assays. PIN-PGP interactions appeared to enhance transport activity and, to a greater extent, substrate/inhibitor specificities when coexpressed in heterologous systems. By contrast, no interactions between PGPs and the AUXIN1 influx carrier were observed. Phenotypes of pin and pgp mutants suggest discrete functional roles in auxin transport, but pin pgp mutants exhibited phenotypes that are both additive and synergistic. These results suggest that PINs and PGPs characterize coordinated, independent auxin transport mechanisms but also function interactively in a tissue-specific manner.

Null-mutations of the Arabidopsis FKBP-like immunophilin TWISTED DWARF1 (TWD1) gene cause a pleiotropic phenotype characterized by reduction of cell elongation and disorientated growth of all plant organs. Heterologously expressed TWD1 does not exhibit cis-trans-peptidylprolyl isomerase (PPIase) activity and does not complement yeast FKBP12 mutants, suggesting that TWD1 acts indirectly via protein-protein interaction. Yeast two-hybrid protein interaction screens with TWD1 identified cDNA sequences that encode the C-terminal domain of Arabidopsis multidrug-resistance-like ABC transporter AtPGP1. This interaction was verified in vitro. Mapping of protein interaction domains shows that AtPGP1 surprisingly binds to the N-terminus of TWD1 harboring the cis-trans peptidyl-prolyl isomerase-like domain and not to the tetratrico-peptide repeat domain, which has been shown to mediate protein-protein interaction. Unlike all other FKBPs, TWD1 is shown to be an integral membrane protein that colocalizes with its interacting partner AtPGP1 on the plasma membrane. TWD1 also interacts with AtPGP19 (AtMDR1), the closest homologue of AtPGP1. The single gene mutation twd1-1 and double atpgp1-1/atpgp19-1 (atmdr1-1) mutants exhibit similar phenotypes including epinastic growth, reduced inflorescence size, and reduced polar auxin transport, suggesting that a functional TWD1-AtPGP1/AtPGP19 complex is required for proper plant development.

The immunophilin-like protein TWISTED DWARF1 (TWD1/FKBP42) has been shown to physically interact with the multidrug resistance/P-glycoprotein (PGP) ATP-binding cassette transporters PGP1 and PGP19 (MDR1). Overlapping phenotypes of pgp1/pgp19 and twd1 mutant plants suggested a positive regulatory role of TWD1 in PGP-mediated export of the plant hormone auxin, which controls plant development. Here, we provide evidence at the cellular and plant levels that TWD1 controls PGP-mediated auxin transport. twd1 and pgp1/pgp19 cells showed greatly reduced export of the native auxin indole-3-acetic acid (IAA). Constitutive overexpression of PGP1 and PGP19, but not TWD1, enhanced auxin export. Coexpression of TWD1 and PGP1 in yeast and mammalian cells verified the specificity of the regulatory effect. Employing an IAA-specific microelectrode demonstrated that IAA influx in the root elongation zone was perturbed and apically shifted in pgp1/pgp19 and twd1 roots. Mature roots of pgp1/pgp19 and twd1 plants revealed elevated levels of free IAA, which seemed to account for agravitropic root behavior. Our data suggest a novel mode of PGP regulation via FK506-binding protein-like immunophilins, implicating possible alternative strategies to overcome multidrug resistance.

Polarized transport of the plant hormone auxin influences multiple growth processes in plants and is regulated by plasma-membrane-localized efflux and uptake carriers. The PGP (P-glycoprotein) ABC transporters (ATP-binding-cassette transporters), PIN (pin-formed) subfamily of major facilitator proteins and members of AUX/LAX families have been shown to independently transport auxin both in planta and in heterologous systems. However, PIN- and PGP-mediated transport in heterologous systems exhibits decreased substrate specificity and inhibitor-sensitivity compared with what is seen in plants and plant cells. To determine whether PIN-PGP interactions enhance transport specificity, we analysed interactions of the representative auxin-transporting PGPs with PIN1 and AUX1 in planta and in heterologous systems. Here, we provide evidence that PINs and PGPs interact and function both independently and co-ordinately to control polar auxin transport and impart transport specificity and directionality. These interactions take place in protein complexes stabilized by PGPs in detergent-resistant microdomains.