Probable accessory potassium channel protein which modulates the activity of the pore-forming alpha subunit. Forms heteromultimeric complexes with potassium channel alpha subunits. Expressed in late-developed leaves with the highest expression in the flag leaf (at protein level). Down-regulated in late-developed leaves when grown on potassium-deficient soil (at protein level). In vitro, interacts with the N-terminal part of A.thaliana KAT1, a voltage-gated potassium channel alpha subunit. Belongs to the shaker potassium channel beta subunit family.