Molecular Evolution of Slow and Quick Anion Channels (SLACs and QUACs/ALMTs)

Abstract

Electrophysiological analyses conducted about 25 years ago detected two types of anion channels in the plasma membrane of guard cells. One type of channel responds slowly to changes in membrane voltage while the other responds quickly. Consequently, they were named SLAC, for SLow Anion Channel, and QUAC, for QUick Anion Channel. Recently, genes SLAC1 and QUAC1/ALMT12, underlying the two different anion current components, could be identified in the model plant Arabidopsis thaliana. Expression of the gene products in Xenopus oocytes confirmed the quick and slow current kinetics. In this study we provide an overview on our current knowledge on slow and quick anion channels in plants and analyze the molecular evolution of ALMT/QUAC-like and SLAC-like channels. We discovered fingerprints that allow screening databases for these channel types and were able to identify 192 (177 non-redundant) SLAC-like and 422 (402 non-redundant) ALMT/QUAC-like proteins in the fully sequenced genomes of 32 plant species. Phylogenetic analyses provided new insights into the molecular evolution of these channel types. We also combined sequence alignment and clustering with predictions of protein features, leading to the identification of known conserved phosphorylation sites in SLAC1-like channels along with potential sites that have not been yet experimentally confirmed. Using a similar strategy to analyze the hydropathicity of ALMT/QUAC-like channels, we propose a modified topology with additional transmembrane regions that integrates structure and function of these membrane proteins. Our results suggest that cross-referencing phylogenetic analyses with position-specific protein properties and functional data could be a very powerful tool for genome research approaches in general.

Altered Expression of a Malate-Permeable Anion Channel, OsALMT4, Disrupts Mineral Nutrition

“…For example, three members in Arabidopsis (AtALMT6, AtALMT9, and AtALMT12) and one in barley (Hordeum vulgare; HvALMT1) contribute to guard cell function by transporting organic and inorganic anions across the plasma membrane or tonoplast. This is consistent with the involvement of anion transport in signal transduction and osmotic adjustment in modulating stomatal aperture (Kovermann et al, 2007;Gruber et al, 2010;Meyer et al, 2010;Sasaki et al, 2010;Dreyer et al, 2012;Xu et al, 2015). Electrophysiological studies in Xenopus laevis oocytes revealed similarities between the currents generated by AtALMT12 and the previously characterized rapid or R-type/QUAC anion currents (Hedrich, 2012).…”

“…The aluminum-activated malate transporter (ALMT) family of genes encode anion channels in plants, with 14 members in the Arabidopsis (Arabidopsis thaliana) genome and nine in rice (Oryza sativa; Delhaize et al, 2007b;Barbier-Brygoo et al, 2011;Dreyer et al, 2012;De Angeli et al, 2013). The family is named after the first member to be identified, TaALMT1, which controls the major mechanism for aluminum (Al 3+ ) resistance in wheat (Triticum aestivum; Sasaki et al, 2004).…”

A Single-Pore Residue Renders the Arabidopsis Root Anion Channel SLAH2 Highly Nitrate Selective

“…When expressed under the control of the SLAC1 promoter, SLAH1 and SLAH3 are able to rescue the slac1-2 stomatal phenotype (Negi et al, 2008). By contrast, SLAH2, which is the closest homolog of SLAH3 (Dreyer et al, 2012), did not complement the phenotype caused by the loss of the chloride-and nitrate-permeable SLAC1 (Negi et al, 2008). This raises the question of whether SLAH2 exhibits different biophysical properties or interacting regulators than those identified in SLAH3 and SLAC1.…”

“…The crystal structure of Hi-TehA, a bacterial homolog of SLAC1, suggests that an inner five-helix transmembrane ring forms the pore with a central phenylalanine residue, which is invariant in this superfamily and seemingly blocks the pore (Chen et al, 2010;Dreyer et al, 2012). Whereas wild-type Hi-TehA showed no ion transport when expressed in oocytes, mutation of the blocking phenylalanine residue (Phe-262) to alanine rendered this channel constitutively active (Chen et al, 2010).…”

A molecular pathway for CO2 response in Arabidopsis guard cells

“…In addition to SLAC1, Arabidopsis guard cells also express SLAH3, a second S-type anion channel 25 , which is especially important for the extrusion of NO 3 À . In contrast to S-type channels, R-/QUAC-type anion channels may be essential for the efflux of organic acids such as malate, which can accumulate during stomatal opening and be released during closure [26][27][28] .…”