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

In recent years, the research team led byProf. GUO Jiangtao from Zhejiang University School ofMedicine has conducted systematic research into the structures and Al-activatedmalate transport mechanisms of ALMT1 in Arabidopsis. The latest researchresults were published in the journal Cell Research on November 19, entitled “StructuralBasis of ALMT1-Mediated Aluminum Resistance in Arabidopsis”.

Researcherspresented cryo-electron microscopy (cryo-EM) structures of Arabidopsis thaliana ALMT1 (AtALMT1) in the apo, malate-bound andAl-bound states in neutral and (or) acidic pH up to 3.0 Å resolution. TheAtALMT1 dimer assembles an anion channel and each subunit contains sixtransmembrane helices (TMs) and six cytosolic α-helices (Fig.1).

Fig.1. Cryo-EM structureof AtALMT1

In the malate-bound structure, researchersidentified two pairs of Arg residues (R80, R165) in the center of the ionconduction pore that directly coordinated the malate (Fig.2). Using moleculardynamic simulations, electrophysiology and in vivo data, researchersfurther validated the malate binding site and thus elucidated the molecularbasis for the malate recognition and transport. In addition, in the Al³⁺-boundstructure, researchersobserved that the Al³⁺ bound at extracellular side between TM1-2linker (D49) and TM6 (E156, D160) (Fig.2).

Fig. 2. The malateand Al³⁺ binding sites of AtALMT1

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

Fig. 3. ExtracellularAl³⁺ activation mechanism of AtALMT1

These mechanistic insights into the molecularbasis for the Al-activated malate transport not only explain the highlyefficient malate secretion in Al-resistance plants in response to the Al stressin the acidic soil, but also provide the structural basis for the rationaldesign of gain-of-function mutant of ALMT1 to enhance the Al-dependent malatesecretion and Al-resistance of crops (Fig. 4).