Major selective outward-rectifying potassium channel of the guard cell membrane. Involved in regulation of stomatal movements according to the water status. Assuming opened or closed conformations in response to the voltage difference across the membrane, the channel is activated by depolarization. Conductance of the channel is modulated in a potassium-dependent fashion. May interact with the cytoskeleton or with regulatory proteins. The potassium channel is probably composed of a homo- or heterotetrameric complex of pore-forming subunits. Expressed in guard cell-containing tissues, in root epidermal cells and in root hairs. Detected in vascular cells of the root and shoot. Up-regulated under drought, salt stress and cold conditions. Induced by abscisic acid (ABA) treatment in roots and shoots but not in guard cells. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids. The pore-forming region H5 is enclosed by the transmembrane segments S5 and S6 in the Shaker-type (1P/6TM) and contains the GYGD signature motif which seems to be involved in potassium selectivity. The KHA domain (rich in hydrophobic and acidic residues) present in the C-terminal part is likely to be important for tetramerization. Belongs to the potassium channel family. Plant (TC 1.A.1.4) subfamily.