KCa2.2 is a member of the Ca2+-activated K+ channels family with small conductance that includes KCa2.1 (SK1) and KCa2.3 (SK3). The channel is voltage insensitive and is activated by intracellular Ca2+ in the submicromolar range. It has, though, a similar topology to that of voltage-dependent K+ channels (KV channels), that is six transmembrane domains and intracellular N- and C-termini. The functional channel of all the KCa2 family members is a multimeric protein composed of four pore-forming subunits.KCa2 channels are extremely sensitive to the levels of intracellular Ca2+ and concentrations as low as 300-700 nM can open the channels very rapidly (5-15 ms). Hence, the KCa2 channels are highly sensitive and fast Ca2+ sensors resembling other known Ca2+-binding proteins. This type of Ca2+-dependent activation is achieved by the constitutive binding of the KCa2 channels to calmodulin, a highly expressed Ca2+-binding protein via a calmodulin-binding domain situated at the cytoplasmic C-terminus.
