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 tuberosom 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 (IC50) of 105 mu M. An almost identical high Cs+ sensitivity (IC50 = 90 mu M) 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.