The Hydrophilic Loop of Arabidopsis PIN1 Auxin Efflux Carrier Harbors Hallmarks of an Intrinsically Disordered Protein

Int J Mol Sci. 2022 Jun 6;23(11):6352. doi: 10.3390/ijms23116352.


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 terms

  • Arabidopsis Proteins* / genetics
  • Arabidopsis Proteins* / metabolism
  • Arabidopsis* / metabolism
  • Biological Transport
  • Indoleacetic Acids / metabolism
  • Intrinsically Disordered Proteins* / genetics
  • Intrinsically Disordered Proteins* / metabolism
  • Membrane Transport Proteins / genetics
  • Membrane Transport Proteins / metabolism
  • Plant Roots / metabolism


  • Arabidopsis Proteins
  • Indoleacetic Acids
  • Intrinsically Disordered Proteins
  • Membrane Transport Proteins
  • PIN1 protein, Arabidopsis