Plant HAK/KUP/KT K⁺ transporters: Function and regulation

The HAK/KUP/KT family of potassium (K⁺) transporters belongs to the amino acid-polyamine-organocation (APC) superfamily of carriers for secondary active transport and has been widely associated with K⁺ transport across membranes in bacteria, fungi, and plants. The plant genome contains large number of HAK/KUP/KT transporters, and they show the diverse roles in K⁺ uptake and translocation, salt tolerance and osmotic potential regulation, as well as in controlling root morphology and shoot phenotyping. Recently, significant progress has been achieved towards uncovering the regulatory mechanisms of HAK/KUP/KT transporters at both transcriptional and post-translational levels. Most of the HAK/KUP/KT genes were regulated at transcriptional level, and such regulation may contribute to the alteration of root cell membrane potential by different growth conditions. At least six transcription factors have been identified as positive or negative regulators of HAK/KUP/KT gene expression in responding to external K⁺ supply. The HAK/KUP/KT transporter proteins can be phosphorylated by CIPK-CBL complexes for activating their function in K⁺ uptake and probably signaling. Nevertheless, it is still not known if HAK/KUP/KT transporters are involved in K⁺-sensing and K⁺-compartmentation in plant cells. Some orthologues of the HAK/KUP/KT transporters from different species show varied physiological functions and some plant species lack an entire sub-clade of HAK/KUT/KT transporters. We are still a long way from unraveling the molecular mechanism of HAK/KUP/KT involved in K⁺-sensing and signaling pathways in plants.