Na+ Sensitivity of the KAT2-Like Channel Is a Common Feature of Cucurbits and Depends on the S5-P-S6 Segment.

Inhibition of Shaker K⁺ channel activity by external Na⁺ was previously reported in the melon (Cucumis melo L.) inwardly rectifying K⁺ channel MIRK and was hypothesized to contribute to salt tolerance. In this study, two inward Shaker K⁺ channels, CsKAT2 from cucumber (Cucumis sativus) and ClKAT2 from watermelon (Citrullus lanatus), were identified and characterized in Xenopus oocytes. Both channels were inwardly rectifying K⁺ channels with higher permeability to potassium than other monovalent cations and more active when external pH was acidic. Similarly to MIRK, their activity displayed an inhibition by external Na⁺, thus suggesting a common feature in Cucurbitaceae (Cucumis spp. Citrullus spp.). CsKAT2 and ClKAT2 are highly expressed in guard cells. After 24 h of plant treatment with 100 mM NaCl, the three KAT2 -like genes were significantly downregulated in leaves and guard cells. Reciprocal chimeras were obtained between MIRK and Na⁺-insensitive AtKAT2 cDNAs. The chimera where the MIRK S5-P-S6 segment was replaced by that from AtKAT2 no longer showed Na⁺ sensitivity, while the inverse chimera gained Na⁺ sensitivity. These results provide evidence that the molecular basis of the channel blockage by Na⁺ is located in the S5-P-S6 region. Comparison of the electrostatic property in the S5-P-S6 region in AtKAT2 and MIRK revealed four key amino acid residues potentially governing Na⁺ sensitivity. [ABSTRACT FROM AUTHOR]