Structural basis of auxin binding and transport by Arabidopsis thaliana AUX1

Abstract

Indole-3-acetic acid (IAA), the major form of auxin, is essential for plant growth. Auxin-resistant 1 (AUX1), the first identified auxin importer, plays a crucial role in polar auxin transport (PAT). Here we present cryo-EM structures of Arabidopsis thaliana AUX1 in the IAA-free and IAA-bound states. AUX1 exists as a monomer that contains 11 transmembrane helixes (TMs). TMs 1-5 and 6-10 constitute the two halves of a classic LeuT-fold, and TM11 interacts with both halves at the interface. In the IAA-bound state, IAA is specifically recognized in a central pocket formed by TM1, TM3, TM6, and TM8. In the presence of IAA, TM1 and TM6 undergo marked conformational changes that are critical for IAA transport. His249 stands out to be a key residue for substrate uptake and release. Our structures reveal the molecular basis for AUX1-mediated IAA binding and transport. Significance: Auxin-resistant 1 (AUX1) is a major auxin influx transporter, playing critical regulatory roles in callus growth, vascular patterning, leaf phyllotactic patterning, root aerial development, and apical hook formation. However, its working mechanism remains unclear. In this study, we report two structures of IAA-bound and IAA-free AUX1, elucidating its substrate recognition mechanism and proposing a potential substrate release model for AUX1. These findings offer important insights into the molecular mechanisms of AUX1-mediated IAA binding and transport, and lay the foundation for future structure-based functional studies of the AUX1/LAX family and the application of auxin analogs in agriculture.