AtKUP1: A Dual-Affinity K⁺ Transporter from Arabidopsis

Correspondence to: Sheng Luan, sluan{at}nature.berkeley.edu (E-mail), 510-642-4995 (fax).

Plant roots contain both high- and low-affinity transport systems for uptake of K⁺ from the soil. In this study, we characterize a K⁺ transporter that functions in both high- and low-affinity uptake. Using yeast complementation analysis, we isolated a cDNA for a functional K⁺ transporter from Arabidopsis (referred to as AtKUP1 for Arabidopsis thaliana K⁺ uptake). When expressed in a yeast mutant, AtKUP1 dramatically increased K⁺ uptake capacity at both a low and high [K⁺] range. Kinetic analyses showed that AtKUP1-mediated K⁺ uptake displays a "biphasic" pattern similar to that observed in plant roots. The transition from the high-affinity phase (K_m of 44 µM) to the low-affinity phase (K_m of 11 mM) occurred at 100 to 200 µM external K⁺. Both low- and high-affinity K⁺ uptake via AtKUP1 were inhibited by 5 mM or higher concentrations of NaCl. In addition, AtKUP1-mediated K⁺ uptake was inhibited by K⁺ channel blockers, including tetraethylammonium, Cs⁺, and Ba²⁺. Consistent with a possible function in K⁺ uptake from the soil, the AtKUP1 gene is primarily expressed in roots. We conclude that the AtKUP1 gene product may function as a K⁺ transporter in Arabidopsis roots over a broad range of [K⁺] in the soil.

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