Acid soils are prevalent worldwide, comprising around half of all potentially arable land, where aluminum (Al) toxicity is a major limitation for crop growth. The plant Aluminum (Al)-activated Malate Transporter ALMT1 mediates the efflux of malate to chelate the Al in acidic soils and underlies the plant Al resistance. Revealing the structure of ALMT1 and elucidating the molecular mechanism of aluminum-activated malate transport are of great significance for the cultivation of aluminum-tolerant crops and the improvement of crop yields in acid soils.

In recent years, the research team led by Prof. GUO Jiangtao from Zhejiang University School of Medicine has conducted systematic research into the structures and Al-activated malate transport mechanisms of ALMT1 in Arabidopsis. The latest research results were published in the journal Cell Research on November 19, entitled “Structural Basis of ALMT1-Mediated Aluminum Resistance in Arabidopsis”.

Researchers presented cryo-electron microscopy (cryo-EM) structures of Arabidopsis thaliana ALMT1 (AtALMT1) in the apo, malate-bound and Al-bound states in neutral and (or) acidic pH up to 3.0 Å resolution. The AtALMT1 dimer assembles an anion channel and each subunit contains six transmembrane helices (TMs) and six cytosolic α-helices (Fig.1).

Fig.1. Cryo-EM structure of AtALMT1

In the malate-bound structure, researchers identified two pairs of Arg residues (R80, R165) in the center of the ion conduction pore that directly coordinated the malate (Fig.2). Using molecular dynamic simulations, electrophysiology and in vivo data, researchers further validated the malate binding site and thus elucidated the molecular basis for the malate recognition and transport. In addition, in the Al³⁺-bound structure, researchers observed that the Al³⁺ bound at extracellular side between TM1-2 linker (D49) and TM6 (E156, D160) (Fig.2).

Fig. 2. The malate and Al³⁺ binding sites of AtALMT1

By comparing the apo closed state and Al³⁺-bound open state structures of AtALMT1, researchers deciphered the Al activation mechanism of AtALMT1. Under the acidic condition, the Al binds at the extracellular side and is coordinated by three acidic residues (Asp40, Glu156, and Asp160), inducing conformational changes of TM1-2 loop and TM5-6 loop, resulting in the outward movement of Ile53 residues and opening of the extracellular gate (Fig. 3).

Fig. 3. Extracellular Al³⁺ activation mechanism of AtALMT1

These mechanistic insights into the molecular basis for the Al-activated malate transport not only explain the highly efficient malate secretion in Al-resistance plants in response to the Al stress in the acidic soil, but also provide the structural basis for the rational design of gain-of-function mutant of ALMT1 to enhance the Al-dependent malate secretion and Al-resistance of crops (Fig. 4).