Cryo-EM structure and electrophysiological characterization of ALMT from <i>Glycine max</i> reveal a previously uncharacterized class of anion channels.

Abstract

Aluminum-activated malate transporters (ALMTs) form an anion channel family that plays essential roles in diverse functions in plants. <i>Arabidopsis</i> ALMT12, also named QUAC1 (quick anion channel 1), regulates stomatal closure in response to environmental stimuli. However, the molecular basis of ALMT12/QUAC1 activity remains elusive. Here, we describe the cryo-EM structure of ALMT12/QUAC1 from <i>Glycine max</i> at 3.5-Å resolution. <i>Gm</i>ALMT12/QUAC1 is a symmetrical dimer, forming a single electropositive T-shaped pore across the membrane. The transmembrane and cytoplasmic domains are assembled into a twisted two-layer architecture, with their associated dimeric interfaces nearly perpendicular. <i>Gm</i>ALMT12/QUAC1-mediated currents display rapid kinetics of activation/deactivation and a bell-shaped voltage dependency, reminiscent of the rapid (R)-type anion currents. Our structural and functional analyses reveal a domain-twisting mechanism for malate-mediated activation. Together, our study uncovers the molecular basis for a previously uncharacterized class of anion channels and provides insights into the gating and modulation of the ALMT12/QUAC1 anion channel.