A possible CO2 conducting and concentrating mechanism in plant stomata SLAC1 channel

Abstract

Background: The plant SLAC1 is a slow anion channel in the membrane of stomatal guard cells, which controls the turgor pressure in the aperture-defining guard cells, thereby regulating the exchange of water vapour and photosynthetic gases in response to environmental signals such as drought, high levels of carbon dioxide, and bacterial invasion. Recent study demonstrated that bicarbonate is a small-molecule activator of SLAC1. Higher CO(2) and HCO(3)(-) concentration activates S-type anion channel currents in wild-type Arabidopsis guard cells. Based on the SLAC1 structure a theoretical model is derived to illustrate the activation of bicarbonate to SLAC1 channel. Meanwhile a possible CO(2) conducting and concentrating mechanism of the SLAC1 is proposed.

Methodology: The homology structure of Arabidopsis thaliana SLAC1 (AtSLAC1) provides the structural basis for study of the conducting and concentrating mechanism of carbon dioxide in SLAC1 channels. The pK(a) values of ionizable amino acid side chains in AtSLAC1 are calculated using software PROPKA3.0, and the concentration of CO(2) and anion HCO(3)(-) are computed based on the chemical equilibrium theory.

Conclusions: The AtSLAC1 is modeled as a five-region channel with different pH values. The top and bottom layers of channel are the alkaline residue-dominated regions, and in the middle of channel there is the acidic region surrounding acidic residues His332. The CO(2) concentration is enhanced around 10(4) times by the pH difference between these regions, and CO(2) is stored in the hydrophobic region, which is a CO(2) pool. The pH driven CO(2) conduction from outside to inside balances the back electromotive force and maintain the influx of anions (e.g. Cl(-) and NO(3)(-)) from inside to outside. SLAC1 may be a pathway providing CO(2) for photosynthesis in the guard cells.

Figure 1. The structural alignment of Arabidopsis thaliana SLAC1 (AtSLAC1) homology structure and the template structure Haemophilius influenzae TehA (HiTehA).

The backbone of AtSLAC1 is shown in red and the HiTehA is in green. (A) A side view of AtSLAC1 and HiTehA alignment. The backbones of two structures overlap very nicely. (B) A top view of AtSLAC1 and HiTehA alignment. The ten helical hairpins are arranged in two layers with quasi-five-fold symmetry. The residue Phe262 (colored in yellow) is in the center of channel, which is the gate of the channel. The ten helices of the two layers in SLAC1 channel are connected by flexible loops. It is anticipated that the diameter of the SLAC1 channel can be adjusted by pressure change in the guard cell.

Figure 2. Amino acid distributions in Arabidopsis thaliana SLAC1 (AtSLAC1) and in Haemophilius influenzae TehA (HiTehA).

(A) Amino acid distributions of AtSLAC1. The acidic residues are shown in blue space filling render, and the acidic residues in pink space filling render. The polar residues are in light blue line drawing, and the hydrophobic residues are in light green line drawing. (B) Amino acid distributions of HiTehA. The acidic residues (pink) and the alkaline residues (blue) are concentrated in top and bottom layers, and the hydrophobic residues (light green) are arranged in the middle of the channel. (C) The transmembrane model of AtSLAC1. The top and bottom layers are filled by water molecules. In the channel there are two water-rich regions, in which the water molecules are shown in space filling drawing. The dark gray ball indicates the hydrophobic region, where is an empty cavity.

Figure 3. Acidic residues (pink) and alkaline residues (blue) in the top and bottom layers of AtSLAC1 channel.

(A) A top view of the residue distribution in the top layer. (B) A side view of the residue distribution in the top layer. In the top layer there are five acidic residues (His219, His293, Asp351, Asp412, and Glu464) and 14 alkaline residues (Lys211, Arg289, Lys290, Tyr291, Lys347, Lys355, Tyr408, Cys414, Arg416, Cys418, Lys61, Tyr462, Tyr469, and Arg472). (C) A bottom view of the residue distribution in the bottom layer. (D) A side view of the residue distribution in the bottom layer. In the bottom layer there are 7 acidic residues (Glu252, Glu257, His260, Glu380, Glu385, His387, and His496) and 20 alkaline residues (Cys192, Tyr243, Lys246, Cys247, Tyr250, Lys255, Arg256, Tyr258, Arg263, Lys310, Lys320, Arg321, Arg322, Cys324, Lys325, Tyr373, Arg375, Lys384, Tyr390, and Lys440). Both top and bottom layers are alkaline residue-dominated region, and filled by water molecules.

Figure 4. The cartoon model of the AtSLAC1 channel for illustrating CO2 conducting mechanism and concentrating mechanism.

(A) Five regions of AtSLAC1 channel. The top layer and bottom layer are the region 1 and region 5, respectively. Inside the channel the water-rich region below the top layer is the region 2, and another water rich-region surrounding acidic residue His332 is region 3. The hydrophobic region is the region 4. (B) The cartoon model of plant AtSLAC1 channel. The region 1 and region 5 (top and bottom layer) are modeled as alkaline solution (pH≈9.0). The region 2 is neural aqueous solution (pH≈7.0), and region 3 is an acidic region (pH≈5.0), because the residue His332 possesses the very lower pK_a value (pK_a = 3.65). The hydrophobic region 4 is a CO₂ storage pool.