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

Abstract

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.

Molecular Mechanism for Inhibition of a Critical Component in the Arabidopsis thaliana Abscisic Acid Signal Transduction Pathways, SnRK2.6, by Protein Phosphatase ABI1

“…SnRK2-dei, the triple knock-out of subclass III SnRK2s gave rise to an ABA-insensitive phenotype (10 -12). The substrates for SnRK2.6/3/2 include transcription factors responsible for the expression of the ABA-responsive genes (13)(14)(15), as well as ion channels that control the osmotic homeostasis, such as the anion channel SLAC1 (16) and potassium channel KAT1 (17). Activation of SnRK2s requires autophosphorylation (18), a process that is inhibited by group A PP2Cs, whose members include ABI1, ABI2, HAB1, HAB2, etc.…”

Nitric oxide negatively regulates abscisic acid signaling in guard cells by S-nitrosylation of OST1

“…Among the three SnRK2s, SnRK2.2, -2.3, and -2.6, which are most important for ABA signaling, SnRK2.6 is preferentially expressed in guard cells and plays a critical role in stomatal regulation, whereas SnRK2.2 and -2.3 are mainly expressed in seeds and young seedlings and are thus more important for seed germination and seedling growth (4,11). SnRK2.6 phosphorylates the slow (S-type) anion channel associated 1 and inward potassium channel KAT1 (K + channel in Arabidopsis thaliana 1) to cause stomatal closure (12,13).…”

Abscisic Acid- and Stress-Induced Highly Proline-Rich Glycoproteins Regulate Root Growth in Rice

“…The active SnRK2 then phosphorylates certain transcription factors to promote the transcription of downstream ABA-responsive genes (Cutler et al, 2010;Kline et al, 2010;Raghavendra et al, 2010). In guard cells, SnRK2E (also known as OPEN STOMATA1 [OST1]) activated by ABA signaling phosphorylates POTASSIUM CHANNEL IN ARABIDOPSIS THALIANA1 (KAT1) to allow the entry of K + (Sato et al, 2009). In addition, SLOW ANION CHANNEL-ASSOCIATED1 (SLAC1), which is also phosphorylated by the activated SnRK2E/OST1, interacts with calcium-dependent protein kinases 21 and 23 to stimulate the efflux of Cl 2 and malate 22 from guard cells (Negi et al, 2008;Geiger et al, 2009Geiger et al, , 2010.…”