Comparative functional features of plant potassium HvHAK1 and HvHAK2 transporters

Plant K+ transporters of the HAK family belong to four rather divergent phy logenetic clusters, although most of the transporters belong to clusters I or II. A simple phylogenetic analysis of fungal and plant HAK transporters suggests that an original HAK gene duplicated even before fungi and plants diverged, generating transporters that at present fulfill different functio ns in the plant. The HvHAK1 transporter belongs to cluster I and mediates h igh-affinity K+ uptake in barley roots, but no function is known for the cl uster II transporter, HvHAK2, which is not functional in yeast. The functio n of HvHAK2 was investigated by constructing HvHAK1-HAK2 chimeric transport ers, which were not functional even when they included only short fragments of HvHAK2. Then, amino acids characteristic of cluster II in the N terminu s and in the first transmembrane domain were introduced into HvHAK1. All of these changes increased the Rb+ K-m, introducing minimal changes in the Na + K-m, which suggested that HvHAK2 is a low-affinity, Na+-sensitive K+ tran sporter. Using a K+-defective Escherichia coli mutant, we functionally expr essed HvHAK2 and found that the predicted characteristics were correct, as well as discovering that the bacterial expression of HvHAK2 is functional a t pH 5.5 but not at 7.5. We discuss whether HvHAK2 may be a tonoplast trans porter effective for vacuolar K+ depletion in K+ starved plants.