Molecular basis for the R-type anion channel QUAC1 activity in guard cells

Abstract

The rapid (R)-type anion channel plays a central role in controlling stomatal closure in plant guard cells, thus regulating the exchange of water and photosynthetic gas (CO2) in response to environmental stimuli. The activity of the R- type anion channel is regulated by malate. However, the molecular basis of the R-type anion channel activity remains elusive. Here, we describe the first cryo-EM structure of the R-type anion channel QUAC1 at 3.5-angstrom resolution in the presence of malate. The structure reveals that the QUAC1 is a symmetrical dimer, forming a single electropositive T-shaped pore for passing anions across the membrane. The transmembrane and cytoplasmic domains are assembled into a twisted bi-layer architecture, with the associated dimeric interfaces nearly perpendicular. Our structural and functional analyses reveal that QUAC1 functions as an inward rectifying anion channel and suggests a mechanism for malate-mediated channel activation. Altogether, our study uncovers the molecular basis for a novel class of anion channels and provides insights into the gating and modulation of the R-type anion channel.

Structural and Functional Insights into the Role of Guard Cell Ion Channels in Abiotic Stress-Induced Stomatal Closure

“…Unlike AtALMT1, the activity of AtALMT12 is not governed by Al 3+ [16,84]. Recently, the structural basis of the ALMT12 channel was revealed to be a symmetrical dimer that forms a pore with a T-shape which is responsible for passing anions across the membrane [85]. The ALMT12 structure is composed of two layers that can be divided into the TM and CH (cytoplasmic helical) domains.…”

“…The TM domain consists of 6 V-shaped TM helices and the CH domain comprises 7 helices. Interestingly, there is a region in the CH domain enriched with serine/threonine residues that may contain a phosphorylation site for ALMT12 regulation [85].…”