Threonine at position 306 of the KAT1 potassium channel is essential for channel activity and is a target site for ABA-activated SnRK2/OST1/SnRK2.6 protein kinase. Biochem J

Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Aobayama 6-6-07, Sendai 980-8579, Japan.
Biochemical Journal (Impact Factor: 4.4). 09/2009; 424(3):439-48. DOI: 10.1042/BJ20091221
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The Arabidopsis thaliana K+ channel KAT1 has been suggested to have a key role in mediating the aperture of stomata pores on the surface of plant leaves. Although the activity of KAT1 is thought to be regulated by phosphorylation, the endogenous pathway and the primary target site for this modification remained unknown. In the present study, we have demonstrated that the C-terminal region of KAT1 acts as a phosphorylation target for the Arabidopsis calcium-independent ABA (abscisic acid)-activated protein kinase SnRK2.6 (Snf1-related protein kinase 2.6). This was confirmed by LC-MS/MS (liquid chromatography tandem MS) analysis, which showed that Thr306 and Thr308 of KAT1 were modified by phosphorylation. The role of these specific residues was examined by single point mutations and measurement of KAT1 channel activities in Xenopus oocyte and yeast systems. Modification of Thr308 had minimal effect on KAT1 activity. On the other hand, modification of Thr306 reduced the K+ transport uptake activity of KAT1 in both systems, indicating that Thr306 is responsible for the functional regulation of KAT1. These results suggest that negative regulation of KAT1 activity, required for stomatal closure, probably occurs by phosphorylation of KAT1 Thr306 by the stress-activated endogenous SnRK2.6 protein kinase.

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    • "Subsequently, the phosphatase activity of PP2Cs is inhibited, which leads to autophosphorylation of SNF1-Related protein kinases (SnRK2s; Ma et al., 2009; Park et al., 2009; Santiago et al., 2009; Umezawa et al., 2009; Vlad et al., 2009). Stomatal closure is initiated by the depolarisation of guard cells, which is triggered by anion release through guard cell anion channel slow anion channel-associated 1 (SLAC1) (Geiger et al., 2009; Lee et al., 2009; Negi et al., 2008) and the inhibition of potassium channel 1 (KAT1) in Arabidopsis via phosphorylation mediated by OST1, a SnRK2 homologue (Joshi-Saha et al., 2011; Mustilli et al., 2002; Sato et al., 2009). Both channels facilitate the efflux of ions and are reciprocally regulated by the ABA signaling pathway and by Ca 2+ (Siegel et al., 2009). "
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    • "activates the anion channel SLOW ANION CHANNEL- ASSOCIATED 1 (SLAC1; Lee et al., 2009; Vahisalu et al., 2010), which has a central role in guard cells. Another ion channel targeted by SnRK2/OST1 is the KAT1 potassium channel, which loses its activity upon phosphorylation (Sato et al., 2009). The rise in cytoplasmic Ca 2+ concentration activates multiple calcium-dependent protein kinases CPK3, CPK6 (Mori et al., 2006), CPK4, CPK11 (Zhu et al., 2007), CPK5 (Dubiella et al., 2013), CPK21, and CPK23 (Geiger et al., 2010). "
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    • "This interaction can inhibit the phosphorylation activity of PP2Cs and release SnRKs from PP2C inhibition (Ma et al., 2009; Park et al., 2009; Santiago et al., 2009). In turn, SnRKs phosphorylate downstream ion channels (Geiger et al., 2009; Lee et al., 2009; Sato et al., 2009) and transcription factors (Fujii et al., 2009) to trigger the ABA response. The ubiquitin/26S proteasome pathway plays a very important role in hormone signaling, including the perception of auxin (Dharmasiri et al., 2005; Kepinski and Leyser, 2005), jasmonates (Sheard et al., 2010), and gibberellins (Murase et al., 2008), and in signal transduction in the ABA and ethylene pathways (Santner and Estelle, 2009). "
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