The phytohormone auxin triggers transcriptional reprogramming through a well-characterized perception machinery in the nucleus. By contrast, mechanisms that underlie fast effects of auxin, such as the regulation of ion fluxes, rapid phosphorylation of proteins or auxin feedback on its transport, remain unclear1-3. Whether auxin-binding protein 1 (ABP1) is an auxin receptor has been a source of debate for decades1,4. Here we show that a fraction of Arabidopsis thaliana ABP1 is secreted and binds auxin specifically at an acidic pH that is typical of the apoplast. ABP1 and its plasma-membrane-localized partner, transmembrane kinase 1 (TMK1), are required for the auxin-induced ultrafast global phospho-response and for downstream processes that include the activation of H+-ATPase and accelerated cytoplasmic streaming. abp1 and tmk mutants cannot establish auxin-transporting channels and show defective auxin-induced vasculature formation and regeneration. An ABP1(M2X) variant that lacks the capacity to bind auxin is unable to complement these defects in abp1 mutants. These data indicate that ABP1 is the auxin receptor for TMK1-based cell-surface signalling, which mediates the global phospho-response and auxin canalization.

• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• Phosphorylation MeSH
• Hydrogen-Ion Concentration MeSH
• Indoleacetic Acids * metabolism   MeSH
• Mutation MeSH
• Protein Serine-Threonine Kinases * genetics   metabolism   MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Proton-Translocating ATPases metabolism   MeSH
• Cytoplasmic Streaming MeSH
• Plant Growth Regulators metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• AT1G66150 protein, Arabidopsis MeSH Browser
• auxin-binding protein 1 MeSH Browser
• Indoleacetic Acids * MeSH
• Protein Serine-Threonine Kinases * MeSH
• Arabidopsis Proteins * MeSH
• Proton-Translocating ATPases MeSH
• Plant Growth Regulators MeSH

Much of plant development depends on cell-to-cell redistribution of the plant hormone auxin, which is facilitated by the plasma membrane (PM) localized PIN FORMED (PIN) proteins. Auxin export activity, developmental roles, subcellular trafficking, and polarity of PINs have been well studied, but their structure remains elusive besides a rough outline that they contain two groups of 5 alpha-helices connected by a large hydrophilic loop (HL). Here, we focus on the PIN1 HL as we could produce it in sufficient quantities for biochemical investigations to provide insights into its secondary structure. Circular dichroism (CD) studies revealed its nature as an intrinsically disordered protein (IDP), manifested by the increase of structure content upon thermal melting. Consistent with IDPs serving as interaction platforms, PIN1 loops homodimerize. PIN1 HL cytoplasmic overexpression in Arabidopsis disrupts early endocytic trafficking of PIN1 and PIN2 and causes defects in the cotyledon vasculature formation. In summary, we demonstrate that PIN1 HL has an intrinsically disordered nature, which must be considered to gain further structural insights. Some secondary structures may form transiently during pairing with known and yet-to-be-discovered interactors.

• Keywords
• PIN1, dimerization, hydrophilic hoop, intrinsic disorder, subcellular trafficking,
• MeSH
• Arabidopsis * metabolism   MeSH
• Biological Transport MeSH
• Plant Roots metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Membrane Transport Proteins genetics   metabolism   MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Intrinsically Disordered Proteins * genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Membrane Transport Proteins MeSH
• PIN1 protein, Arabidopsis MeSH Browser
• Arabidopsis Proteins * MeSH
• Intrinsically Disordered Proteins * MeSH

Polar subcellular localization of the PIN exporters of the phytohormone auxin is a key determinant of directional, intercellular auxin transport and thus a central topic of both plant cell and developmental biology. Arabidopsis mutants lacking PID, a kinase that phosphorylates PINs, or the MAB4/MEL proteins of unknown molecular function display PIN polarity defects and phenocopy pin mutants, but mechanistic insights into how these factors convey PIN polarity are missing. Here, by combining protein biochemistry with quantitative live-cell imaging, we demonstrate that PINs, MAB4/MELs, and AGC kinases interact in the same complex at the plasma membrane. MAB4/MELs are recruited to the plasma membrane by the PINs and in concert with the AGC kinases maintain PIN polarity through limiting lateral diffusion-based escape of PINs from the polar domain. The PIN-MAB4/MEL-PID protein complex has self-reinforcing properties thanks to positive feedback between AGC kinase-mediated PIN phosphorylation and MAB4/MEL recruitment. We thus uncover the molecular mechanism by which AGC kinases and MAB4/MEL proteins regulate PIN localization and plant development.

• Keywords
• Arabidopsis, cell polarity, lateral diffusion, plant development, polar auxin transport, positive feedback, protein phosphorylation,
• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• Biological Transport MeSH
• Plant Roots metabolism   MeSH
• Indoleacetic Acids MeSH
• Membrane Transport Proteins genetics   MeSH
• Cell Polarity MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Cells metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Membrane Transport Proteins MeSH
• Arabidopsis Proteins * MeSH