The KcsA prokaryotic potassium ion channel from the soil bacteria Streptomyces lividans that has been studied extensively in ion channel research. It acts as a pH-gated potassium ion channel and changes the cytosolic pH from 7 to 4 opens the channel, although it is not clear if this is the physiological stimulus for channel opening. Monovalent cation preference is K+> Rb+> NH4+>> Na+> Li+.