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 Many questions need to be answered to fully understand the function of these transporters.

To determine the roles of the genes encoding the KUP/KT/HAK family of proteins in plant development, cell expansion, and in environmental stress responses, we will have embarked on a project to characterize:

• temporal and spatial patterns of KUP gene expression;

• developmental and physiological functions of KUPs in whole plants;

• biophysical functions such as selectivity, kinetics and mode of transport of KUP transporters;

• contribution to cytoplasmic or vacuolar K⁺ homeostasis through membrane localization.

KUP transporters are found in plant, bacterial and fungal genomes. They were first identified in bacteria and fungi. They encode both high affinity and low affinity potassium transporters.

Plants (click for published references)
In Arabidopsis the KUP proteins form the largest family of potassium transporters. The relationship between these transporters is shown in the unrooted tree below (taken from Maser et al. 2001, Plant Physiology)

While the role of many of these transporter is unknown, in some cases, these transporters appear to be involved in the K⁺ uptake that drives cell expansion and plant development. If these proteins function in the plasma and vacuolar membranes, they may be involved in uptake into the cell and remobilization from the vacuole. Recent evidence suggests that at least one rice KUP is localized to the vacuolar membrane.

In plants very little is know about how KUP transporters function precisely. For some of the transporters in Arabidopsis, barley and cyanobacteria the kinetics and selectivity have been studied. Some of the plant KUPs have been shown to have a dual affinity for potassium and others had some permeability to Na⁺ as well as K⁺.

Characterization of two Arabidopsis mutants in KUP genes has revealed that they play a role in cell expansion. The tiny root hairs (trh1) mutant has a defect in root hair elongation and is a null allele of KUP4. The shy3-1 mutant has a dwarf phenotype due to decreased cell expansion in several tissues as a result of a missense mutation in the KUP2 gene. These results highlight the importance of these two KUP transporters in plant development. Another report also suggests that KUP transporter expression is correlated with the development and elongation of one of the largest single cells in the plant kingdom: the cotton fiber.

Bacteria (click for published references)
The KUP (TrkD) protein has been identified in E. coli and homologues exist in both Gram-positive and Gram-negative bacteria. In E. coli KUP mediates low affinity K⁺ uptake. It has been suggested that the E. coli KUP protein is a secondary transporter. Uptake is blocked by protonophores such as CCCP and there is evidence for a proton symport mechanism.

Yeast (click for published references)
The KUP transporter system in Schwanniomyces occidentalis has been shown to function as a high affinity (20 µM) K⁺ uptake transporter. In Neurospora crassa the genes encoding KUPs have been cloned and the K⁺ uptake system encoded by KUP was shown to be a high affinity K⁺:H⁺ symporter.