High-affinity potassium transporter. Can also transport rubidium and cesium (Ref.1). Is essential with AKT1 for high-affinity potassium uptake in roots during seedling establishment and postgermination growth under low potassium conditions (PubMed:20413648). Mediates potassium uptake by plant roots in response to low potassium conditions, by a calcium-, CBL-, and CIPK-dependent pathway. Positively regulated by the calcium sensors calcineurin B-like genes CBL1, CBL8, CBL9 and CBL10, and by phosphorylation by CIPK23 (PubMed:26474642). Interacts with ILK1. Predominantly expressed in the roots. Induced in both roots and shoots by potassium starvation (Ref.1). Down-regulated by salt stress in roots (PubMed:20028724). Phosphorylated at the N-terminus (amino acids 1-95) by CIPK23. Belongs to the HAK/KUP transporter (TC 2.A.72.3) family.