Roles of higher plant K+ channels.
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DNA, Complementary↗
Biomedical subjects
A M Ichida
Publications and source records attributed to A M Ichida.
Expression of a Cs(+)-resistant guard cell K+ channel confers Cs(+)-resistant, light-induced stomatal opening in transgenic arabidopsis.
Inward-rectifying K+ (K+in) channels in the guard cell plasma membrane have been suggested to function as a major pathway for K+ influx into guard cells during stomatal opening. When K+in channels were blocked with external Cs+ in wild-type Arabidopsis guard cells, light-induced stomatal opening was reduced. Transgenic Arabidopsis plants were generated that expressed a mutant of the guard cell K+in channel, KAT1, which shows enhanced resistance to the Cs+ block. Stomata in these transgenic lines opened in the presence of external Cs+. Patch-clamp experiments with transgenic guard cells showed that inward K+(in) currents were blocked less by Cs+ than were K+ currents in controls. These data provide direct evidence that KAT1 functions as a plasma membrane K+ channel in vivo and that K+in channels constitute an important mechanism for light-induced stomatal opening. In addition, biophysical properties of K+in channels in guard cells indicate that components in addition to KAT1 may contribute to the formation of K+in channels in vivo.
Arabidopsis↗
Increased resistance to extracellular cation block by mutation of the pore domain of the Arabidopsis inward-rectifying K+ channel KAT1.
Inward-rectifying potassium channels in plant cells provide important mechanisms for low-affinity K+ uptake and membrane potential control in specific cell types, including guard cells, pulvinus cells, aleurone cells and root hair cells. K+ channel blockers are potent tools for studying the physiological functions and structural properties of K+ channels. In the present study the structural and biophysical mechanisms of Cs+ and TEA+ block of a cloned Arabidopsis inward-rectifying K+ channel (KAT1) were analyzed. Effects of the channel blockers Cs+ and TEA+ were characterized both extracellularly and intracellularly. Both external Cs+ and TEA+ block KAT1 currents. A mutant of KAT1 ("m2KAT1"; H267T, E269V) was produced by site-directed mutagenesis of two amino acid residues in the C-terminal portion of the putative pore (P) domain. This mutant channel was blocked less by external Cs+ and TEA+ than the wild-type K+ channel. Internal TEA+ and Cs+ did not significantly block either m2KAT1 or KAT1 channels. Other properties, such as cation selectivity, voltage-dependence and proton activation did not show large changes between m2KAT1 and KAT1, demonstrating the specificity of the introduced mutations. These data suggest that the amino acid positions mutated in the inward-rectifying K+ channel, KAT1, are accessible to external blockers and may be located on the external side of the membrane, as has been suggested for outward-rectifying K+ channels.
Amino Acid Sequence↗
Multiple genes, tissue specificity, and expression-dependent modulationcontribute to the functional diversity of potassium channels in Arabidopsis thaliana.
K+ channels play diverse roles in mediating K+ transport and in modulating the membrane potential in higher plant cells during growth and development. Some of the diversity in K+ channel functions may arise from the regulated expression of multiple genes encoding different K+ channel polypeptides. Here we report the isolation of a novel Arabidopsis thaliana cDNA (AKT2) that is highly homologous to the two previously identified K+ channel genes, KAT1 and AKT1. This cDNA mapped to the center of chromosome 4 by restriction fragment length polymorphism analysis and was highly expressed in leaves, whereas AKT1 was mainly expressed in roots. In addition, we show that diversity in K+ channel function may be attributable to differences in expression levels. Increasing KAT1 expression in Xenopus oocytes by polyadenylation of the KAT1 mRNA increased the current amplitude and led to higher levels of KAT1 protein, as assayed in western blots. The increase in KAT1 expression in oocytes produced shifts in the threshold potential for activation to more positive membrane potentials and decreased half-activation times. These results suggest that different levels of expression and tissue-specific expression of different K+ channel isoforms can contribute to the functional diversity of plant K+ channels. The identification of a highly expressed, leaf-specific K+ channel homolog in plants should allow further molecular characterization of K+ channel functions for physiological K+ transport processes in leaves.
Amino Acid Sequence↗