The Potassium Transporter Hak1 in Regulation and Physiological Effects at Limiting Potassium and under Acidic Conditions.

The three families of yeast plasma membrane potassium influx transporters are represented in : Trk, Acu, and Hak proteins. Hak transporters work as K-H symporters, and the genes coding for Hak proteins are transcriptionally activated under potassium limitation. This work shows that mutant cells lacking display a severe growth impairment at limiting potassium concentrations under acidic conditions. This is the consequence of a defective capacity to transport K, as indicated by potassium absorption experiments and by the kinetics parameters of Rb (K) transport. Moreover, cells are more sensitive to the toxic cation lithium. All these phenotypes became much less robust or even disappeared at alkaline growth conditions. Finally, transcriptional studies demonstrate that the mutant, in comparison with cells, activates the expression of the K/Na ATPase coded by in the presence of Na or in the absence of K.