Histidine(118) in the S2-S3 linker specifically controls activation of the KAT1 channel expressed in Xenopus oocytes.

The guard cell K(+) channel KAT1, cloned from Arabidopsis thaliana, is activated by hyperpolarization and regulated by a variety of physiological factors. Low internal pH accelerated the activation kinetics of the KAT1 channel expressed in Xenopus oocytes with a pK of approximately 6, similar to guard cells in vivo. Mutations of histidine-118 located in the putative cytoplasmic linker between the S2 and S3 segments profoundly affected the gating behavior and pH dependence. At pH 7.2, substitution with a negatively charged amino acid (glutamate, aspartate) specifically slowed the activation time course, whereas that with a positively charged amino acid (lysine, arginine) accelerated. These mutations did not alter the channel's deactivation time course or the gating behavior after the first opening. Introducing an uncharged amino acid (alanine, asparagine) at position 118 did not have any obvious effect on the activation kinetics at pH 7.2. The charged substitutions markedly decreased the sensitivity of the KAT1 channel to internal pH in the physiological range. We propose a linear kinetic scheme to account for the KAT1 activation time course at the voltages where the opening transitions dominate. Changes in one forward rate constant in the model adequately account for the effects of the mutations at position 118 in the S2-S3 linker segment. These results provide a molecular and biophysical basis for the diversity in the activation kinetics of inward rectifiers among different plant species

KDC1 , a novel carrot root hair K+ channel. Cloning characterization and expression in mammalian cells

Potassium is an essential nutrient which plays an important role in many aspects of plant growth and development. Plants have developed a number of highly specific mechanisms to take up potassium from the soil; these include the expression of K(+) transporters and potassium channels in root cells. Despite the fact that root epidermal and hair cells are in direct contact with the soil, the role of these tissues in K(+) uptake is not well understood. Here we report the molecular cloning and functional characterization of a novel potassium channel KDC1 which forms part of a new subfamily of plant K(in) channels. Kdc1 was isolated from carrot root RNA and in situ hybridization experiments show Kdc1 to be highly expressed in root hair cells. Expressing the KDC1 protein in Chinese hamster ovary cells identified it as a voltage and pH-dependent inwardly rectifying potassium channel. An electrophysiological analysis of carrot root hair protoplasts confirmed the biophysical properties of the Kdc1 gene product (KDC1) in the heterologous expression system. KDC1 thus represents a major K(+) uptake channel in carrot root hair cells