AKT3, a phloem-localized K+ channel, is blocked by protons

The potassium-channel gene, AKT3, has recently been isolated from an Arabid opsis thaliana cDNA library, By using the whole-mount and in situ hybridiza tion techniques, we found AKT3 predominantly expressed in the phloem, To st udy the physiological role of this channel type, AKTS was heterologously ex pressed in Xenopus oocytes, and the electrical properties were examined wit h voltage-clamp techniques. Unlike the plant inward-rectifying guard cell K t channels KAT1 and KST1, the AKT3 channels were only weakly; regulated by the membrane potential. Furthermore, AKT3 was blocked by physiological conc entrations of external Ca2+ and showed an inverted pH regulation. Extracell ular acidification decreased the macroscopic AKT3 currents by reducing the single-channel conductance. Because assimilate transport in the vascular ti ssue coincides with both Hf and Bf fluxes, AKT3 K+ channels may be involved in K+ transport accompanying phloem loading and unloading processes.