13 research outputs found
The pH sensor of the plant K+-uptake channel KAT1 is built from a sensory cloud rather than from single key amino acids
Gonzalez, W (reprint author), Univ Talca, Ctr Bioinformat & Simulac Mol, 2 Norte 685, Talca 3465548, Chile.The uptake of potassium ions (K+) accompanied by an acidification of the apoplasm is a prerequisite for stomatal opening. The acidification (approximately 2-2.5 pH units) is perceived by voltage-gated inward potassium channels (K-in) that then can open their pores with lower energy cost. The sensory units for extracellular pH in stomatal K-in channels are proposed to be histidines exposed to the apoplasm. However, in the Arabidopsis thaliana stomatal K-in channel KAT1, mutations in the unique histidine exposed to the solvent (His(267)) do not affect the pH dependency. We demonstrate in the present study that His(267) of the KAT1 channel cannot sense pH changes since the neighbouring residue Phe(266) shifts its pK(a) to undetectable values through a cation-pi interaction. Instead, we show that Glu(240) placed in the extracellular loop between transmembrane segments S5 and S6 is involved in the extracellular acid activation mechanism. Based on structural models we propose that this region may serve as a molecular link between the pH- and the voltage-sensor. Like Glu(240), several other titratable residues could contribute to the pH-sensor of KAT1, interact with each other and even connect such residues far away from the voltage-sensor with the gating machinery of the channel
The receptor-like pseudokinase MRH1 interacts with the voltage-gated potassium channel AKT2
The potassium channel AKT2 plays important roles in phloem loading and unloading. It can operate as inward-rectifying channel that allows H+-ATPase-energized K+ uptake. Moreover, through reversible post-translational modifications it can also function as an open, K+-selective channel, which taps a 'potassium battery', providing additional energy for transmembrane transport processes. Knowledge about proteins involved in the regulation of the operational mode of AKT2 is very limited. Here, we employed a large-scale yeast two-hybrid screen in combination with fluorescence tagging and null-allele mutant phenotype analysis and identified the plasma membrane localized receptor-like kinase MRH1/MDIS2 (AT4G18640) as interaction partner of AKT2. The phenotype of the mrh1-1 knockout plant mirrors that of akt2 knockout plants in energy limiting conditions. Electrophysiological analyses showed that MRH1/MDIS2 failed to exert any functional regulation on AKT2. Using structural protein modeling approaches, we instead gathered evidence that the putative kinase domain of MRH1/MDIS2 lacks essential sites that are indispensable for a functional kinase suggesting that MRH1/MDIS2 is a pseudokinase. We propose that MRH1/MDIS2 and AKT2 are likely parts of a bigger protein complex. MRH1 might help to recruit other, so far unknown partners, which post-translationally regulate AKT2. Additionally, MRH1 might be involved in the recognition of chemical signals
The pH sensor of the plant K plus uptake channel KAT1 is built from a sensory cloud rather than from single key amino acids
The uptake of potassium ions (K+) accompanied by an acidification
of the apoplasm is a prerequisite for stomatal opening.
The acidification (approximately 2â2.5 pH units) is perceived by
voltage-gated inward potassium channels (Kin) that then can
open their pores with lower energy cost. The sensory units for
extracellular pH in stomatal Kin channels are proposed to be histidines
exposed to the apoplasm. However, in the Arabidopsis
thaliana stomatal Kin channel KAT1, mutations in the unique
histidine exposed to the solvent (His267) do not affect the pH
dependency. We demonstrate in the present study that His267 of
the KAT1 channel cannot sense pH changes since the neighbouring
residue Phe266 shifts its pKa to undetectable values through
a cationâp interaction. Instead, we show that Glu240 placed in
the extracellular loop between transmembrane segments S5 and
S6 is involved in the extracellular acid activation mechanism.
Based on structural models we propose that this region may
serve as a molecular link between the pH- and the voltage-sensor.
Like Glu240, several other titratable residues could contribute to
the pH-sensor of KAT1, interact with each other and even connect
such residues far away from the voltage-sensor with the gating
machinery of the channel