AtKUP1: A Dual-Affinity K + Transporter from Arabidopsis

Abstract

Plant roots contain both highand low-affinity transport systems for uptake of K 1 from the soil. In this study, we characterize a K 1 transporter that functions in both highand low-affinity uptake. Using yeast complementation analysis, we isolated a cDNA for a functional K 1 transporter from Arabidopsis (referred to as AtKUP1 for Arabidopsis thaliana K 1 uptake). When expressed in a yeast mutant, AtKUP1 dramatically increased K 1 uptake capacity at both a low and high [K 1 ] range. Kinetic analyses showed that AtKUP1-mediated K 1 uptake displays a “biphasic” pattern similar to that observed in plant roots. The transition from the high-affinity phase ( K m of 44 m M) to the low-affinity phase ( K m of 11 mM) occurred at 100 to 200 m M external K 1 . Both lowand high-affinity K 1 uptake via AtKUP1 were inhibited by 5 mM or higher concentrations of NaCl. In addition, AtKUP1-mediated K 1 uptake was inhibited by K 1 channel blockers, including tetraethylammonium, Cs 1 , and Ba 2 1 . Consistent with a possible function in K 1 uptake from the soil, the AtKUP1 gene is primarily expressed in roots. We conclude that the AtKUP1 gene product may function as a K 1 transporter in Arabidopsis roots over a broad range of [K 1 ] in the soil.