Interaction and ligand gating within Arabidopsis guard cell hormone-receptor anion-channel complex
Stomata optimize plant CO2 uptake for photosynthesis and the associated water loss via transpiration. As a key site, stomatal anion channels are addressed by a multitude of stimuli, including drought stress and pathogen-derived ligands. We uncovered the molecular nature of the major plant plasma membrane anion channel family (SLAC/SLAHs) and the signaling pathways through which they are regulated. The SLAC/SLAH channels are characterized by slow voltage-dependent (S-type) activation. Under drought S-type anion channels in guard cells are stimulated by the stress hormone abscisic acid (ABA), which triggers a decrease in cell volume and turgor pressure and thereby causing stomatal closure.
Within the scope of our proposed project, we will focus on the guard cell expressed anion channel isoforms SLAC1 and SLAH3 that represent master switches within the ABA-stimulated signaling complex. We reconstituted the core ABA-signalosome, consisting of an ABA-receptor/phosphatase complex and an anion channel-activating kinase, in vitro as well as in the Xenopus oocyte expression system. In the absence of ABA, the protein phosphatase inhibits SLAC/SLAH-activating kinases and keeps the anion channels dephosphorylated and thus inactive. Upon perception of ABA, the phosphatase becomes inactivated by interaction with the cytosolic ABA receptors. Thereby the inhibition of protein kinases is relieved and anion channels are activated through phosphorylation. However, the knowledge about the spatio/temporal dynamics of interactions within the ABA-signalosome remains elusive.
Using super-resolution microscopy in combination with electrophysiological techniques we intent to quantify and to decipher the membrane-delimited ABA signaling complex with spatio/temporal resolution. Similar to rafts in animals, membrane lipid nanodomains constitute signaling platforms in the plasma membrane of plants. Upon ABA stimulation the anion channel SLAH3 physically interacts with a protein kinase/phosphatase pair in these sterol-rich nanodomains. To better resolve the spatio/temporal and dynamic distribution of SLAH3 and its interacting partners we will utilize super-resolution microscopy (PALM and dSTORM), FRET/FLIM and single molecule tracking techniques. Very recently, we could show that the two members SLAH1 and 4 of the SLAC/SLAH anion channel family are electrically silent as homomers. When co-expressed with SLAH3, however, SLAH1 modulates the activation mechanism and the selectivity of SLAH3. In contrast, the formation of SLAH4/SLAH3 heteromers suggests an electrically silent state. We will follow the interaction of SLAH3 with its modulating beta-subunits using super-resolution microscopy and electrophysiology. Our approach will gain the first insight into the dynamics of i) drought hormone ABA recognition by the guard cell receptor complex, ii) the complex addressing the S-type anion channel(s), and iii) functional heteromeric assembly of SLAC/SLAH-type subunits.
Geringer, Andreas Michael
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