A collaborative study led by Dr. Chi Chen (Xing Wang Deng Lab, Peking University Institute of Advanced Agricultural Sciences), Dr. Daohua Jiang (Institute of Physics, Chinese Academy of Sciences), and Professor Xiaoguang Lei (Peking University) has been published in Molecular Plant under the title "Structural basis of auxin recognition and transport in plant influx carrier AUX1". Using high-resolution cryo-electron microscopy (cryo-EM), the team resolved the structures of AUX1 in both inward-open and inward-occluded states, providing near-atomic-level insights into how this key transporter recognizes and transports auxin.

Auxin, the first identified plant hormone, plays a central role in regulating development from seed germination to fruit maturation. Its function relies on directional cell-to-cell polar transport, mediated by specific influx (AUX/LAX family) and efflux (PIN/ABCB family) carriers. AUX1 is a major influx transporter responsible for importing both natural and synthetic auxins into cells. Loss-of-function mutations in AUX1 lead to severe developmental defects, including impaired root gravitropism and inhibition of primary root elongation. However, the molecular mechanism underlying auxin recognition and transport by AUX1 has remained unclear.

 

With a molecular weight of only 54 kDa, AUX1 is challenging for structural study by cryo-EM. By screening various truncated constructs, the authors obtained a homogeneous and stable protein sample suitable for structural analysis. They determined the structures of Arabidopsis AUX1 in both the apo form and in complex with the auxin analog 2,4-dichlorophenoxyacetic acid (2,4-D) at resolutions sufficient to resolve side-chain details (Fig. 1A). AUX1 comprises 11 transmembrane helices (TMs), with the first 10 adopting a canonical LeuT fold, while TM11 — along with TM1, TM3, and TM10 — helps stabilize the inward-open conformation (Fig. 1B). The 2,4-D-bound structure clearly defines the auxin-binding pocket and identifies key residues involved in substrate recognition. Functional assays supported by mutagenesis confirmed the essential role of these residues (Fig. 1C).

 

Fig. 1. Structural features of AUX1.

 

The researchers identified a putative cation-binding site that contributes to stabilizing the inward-facing conformation. Binding of 2,4-D induces significant conformational changes in TM1 and TM6, most notably a 180° flip of His249. Mutation of this residue completely abolished transport activity, suggesting a critical role in gating (Fig. 2).

 

Fig. 2. Conformational changes in AUX1.

 

Integrating structural and functional data, the authors propose a mechanistic model for AUX1-mediated transport (Fig. 3): (1) In the outward-open state, His249 forms a hydrogen bond with Asn142. (2) Extracellular auxin and protons enter the central cavity. (3) Protonation of His249 disrupts the His249–Asn142 interaction, triggering conformational changes in TM1 and TM6 that transition the transporter to an inward-open state. (4) Auxin and protons are released into the cytoplasm, completing the transport cycle.

 

Fig. 3. Proposed model of AUX1-mediated auxin transport.

 

These findings significantly advance our understanding of plant hormone transport and provide a structural basis for the rational design of novel auxin-based herbicides.

 

Academician Xing Wang Deng (Peking University Institute of Advanced Agricultural Sciences), Dr. Daohua Jiang (Institute of Physics, Chinese Academy of Sciences (CAS)), and Professor Xiaoguang Lei (Peking University) are the co-corresponding authors. The co-first authors are Dr. Huiwen Chen (visiting postdoctoral researcher at the Institute of Physics, CAS), Associate Researcher Junping Fan (Peking University), and Dr. Cheng Chi. Other key contributors include Dr. Di Wu (Institute of Physics, CAS), Dr. Jun Zhao (Microstructure Research Platform), and Senior Engineer Bin Xu. The Peking University Institute of Advanced Agricultural Sciences / Shandong Laboratory of Advanced Agricuture Sciences in Weifang is the first affiliation.

 

This research was supported by the Shandong Provincial Natural Science Foundation, the National Key R&D Program of China, the Shandong Provincial Key R&D Program, the National Natural Science Foundation of China, the Taishan Scholars Young Expert Program, and the New Cornerstone Science Foundation.

 

Note: On 15 May 2025, a study by the teams of Profs. Linfeng Sun, Xin Liu, and Shutang Tan at the University of Science and Technology of China was published in Cell, reporting the first structure of AUX1 using a different protein engineering strategy. Despite methodological differences, both studies reveal highly similar AUX1 architectures, auxin-binding modes, and transport mechanisms — converging on His249 as a key protonation site and gate — thus providing a mutual validation.

 

Original article:

https://www.sciencedirect.com/science/article/pii/S1674205225002084