Small-conductance Ca2+-activated K+ channels (SK channels) are independent of voltage and gated solely by intracellular Ca2+. These membrane channels are heteromeric complexes that comprise pore-forming alpha-subunits and the Ca2+-binding protein calmodulin (CaM) [1]. CaM binds to the SK channel through this the CaM-binding domain (CaMBD), which is located in an intracellular region of the alpha-subunit immediately carboxy-terminal to the pore. Channel opening is triggered when Ca2+ binds the EF hands in the N-lobe of CaM. The structure of this domain complexed with CaM is known [1]. This domain forms an elongated dimer with a CaM molecule bound at each end; each CaM wraps around three alpha-helices, two from one CaMBD subunit and one from the other.

1: Structure of the gating domain of a Ca2+-activated K+ channel complexed with Ca2+/calmodulin. Schumacher MA, Rivard AF, Bachinger HP, Adelman JP; Nature 2001;410:1120-1124. PMID:11323678