Featured Article - December 2010
Short description: Guard cells rely on anion channels to regulate stomata in response to environmental cues. A new crystal structure gives insight into how stomata close.
The aptly named guard cells control the opening and closing of the leaf stomata, thereby controlling the influx of CO2 and evaporative loss of water. Proper regulation of the stomata is essential, as excessive water loss is devastating for plants, but insufficient CO2 intake is similarly harmful. It is well-established that guard cells control stomata through modulating their internal turgor pressure, which is regulated in part by anion channels such as slow anion channel 1 (SLAC1).
Only expressed in guard cells, SLAC1 has ten membrane-spanning helices, and its function is regulated via phosphorylation by OST1, a kinase that helps to mediate the cellular response to abscisic acid. SLAC1 homologs are found in plants and other organisms ranging from bacteria to fungi. Now, Hendrickson and colleagues (New York Consortium on Membrane Protein Structure (NYCOMPS) from the National Institute of General Medical Sciences Protein Structure Initiative (NIGMS-PSI) have taken advantage of one of those homologs to provide further insight into SLAC1 structure and function.
The authors examined homologs for Arabidopsis thaliana SLAC1 (AtSLAC1), and crystallized the closely related Haemophilus influenzae anion channel TehA (HiTehA). The crystal structure revealed that each protomer of trimeric HiTehA (and presumably AtSLAC1) has a novel fold: the ten transmembrane helices are organized as five tandemly repeated helical hairpins that are arranged to form a central anion pore. This pore is formed by five kinked helices and has a uniform diameter throughout. Gating of the pore is carried out by a highly conserved phenylalanine residue (HiTehA Phe262, AtSLAC1 Phe450) whose side group occludes the channel.
Two previous mutants that show impaired AtSLAC1 function have been described: slac1-1 and slac1-2. Mapping those mutations onto the structure reveals that the mutation in slac1-1 disrupts the pore structure, and that in slac1-2 blocks the pore. The authors further validate their structural model by creating an F450A mutant AtSLAC1 and showing that removal of the occluding phenyl group allows anions to flow freely through the channel, independently of phosphorylation by OST1. Additionally, as in the slac1-2 mutant the pore is blocked 'downstream' of Phe450, creating an F450A/slac1-2 double mutant results in the pore being permanently blocked. Crystal structures of HiTehA mutants F262A and G15D (mimicking AtSLAC1 F450A and slac1-2, respectively) show that the F262A mutant has a completely open pore, whereas the G15D mutant has a doubly occluded pore.
SLAC1 is most permeable to I− and NO3−, but the structure does not reveal any discernable ion-binding location, indicating that ion selectivity is determined by alternative factors. A comparison to other anion channels reveals that SLAC1 has a unique structure, consistent with its unique ion selectivity and regulation.
Questions still remain concerning SLAC1 activity. How phosphorylation by OST1 controls the flow of anions through the channel is not completely understood; the authors propose that it somehow causes shifting of the side group of Phe450 out of the pore. Also, slac1-2 protoplasts accumulate malate, but the authors saw minimal malate transport by SLAC1. How SLAC1 works in conjunction with other channels to maintain proper ionic concentrations in guard cells is also not completely clear.
From this study, it can be seen that guard cells use a phenylalanine gate to control access through the stomata. The structure of this SLAC1 homolog gives a picture of a complex channel and will form a solid base for future experiments.
Y.H. Chen, H. Lei, M. Punta, R. Bruni, B. Hillerich et al. Homologue structure of the SLAC1 anion channel for closing stomata in leaves.
Nature 467, 1074-1080 (2010). doi:10.1038/nature09487