Characterization of SKT1, an Inwardly Rectifying Potassium Channel from Potato, by Heterologous Expression in Insect Cells

A cDNA encoding a novel, inwardly rectifying K(+) (K(+)(in)) channel protein, SKT1, was cloned from potato (Solanum tuberosum L.). SKT1 is related to members of the AKT family of K(+)(in) channels previously identified in Arabidopsis thaliana and potato. Skt1 mRNA is most strongly expressed in leaf epidermal fragments and in roots. In electrophysiological, whole-cell, patch-clamp measurements performed on baculovirus-infected insect (Spodoptera frugiperda) cells, SKT1 was identified as a K(+)(in) channel that activates with slow kinetics by hyperpolarizing voltage pulses to more negative potentials than −60 mV. The pharmacological inhibitor Cs(+), when applied externally, inhibited SKT1-mediated K(+)(in) currents half-maximally with an inhibitor concentration (IC(50)) of 105 μm. An almost identical high Cs(+) sensitivity (IC(50) = 90 μm) was found for the potato guard-cell K(+)(in) channel KST1 after expression in insect cells. SKT1 currents were reversibly activated by a shift in external pH from 6.6 to 5.5, which indicates a physiological role for pH-dependent regulation of AKT-type K(+)(in) channels. Comparative studies revealed generally higher current amplitudes for KST1-expressing cells than for SKT1-expressing insect cells, which correlated with a higher targeting efficiency of the KST1 protein to the insect cell's plasma membrane, as demonstrated by fusions to green fluorescence protein.