Small conductance Ca2+-activated K+ channels and calmodulin: cell surface expression and gating.

Small conductance Ca2+-activated K+ channels (SK channels) are heteromeric complexes of pore-forming alpha subunits and constitutively bound calmodulin (CaM). The binding of CaM is mediated in part by the electrostatic interaction between residues Arg-464 and Lys-467 of SK2 and Glu-84 and Glu-87 of CaM. Heterologous expression of the double charge reversal in SK2, SK2 R464E/K467E (SK2:64/67), did not yield detectable surface expression or channel activity in whole cell or inside-out patch recordings. Coexpression of SK2:64/67 with wild type CaM or CaM1,2,3,4, a mutant lacking the ability to bind Ca2+, rescued surface expression. In patches from cells coexpressing SK2:64/67 and wild type CaM, currents were recorded immediately following excision into Ca2+-containing solution but disappeared within minutes after excision or immediately upon exposure to Ca2+-free solution and were not reactivated upon reapplication of Ca2+-containing solution. Channel activity was restored by application of purified recombinant Ca2+-CaM or exposure to Ca2+-free CaM followed by application of Ca2+-containing solution. Coexpression of the double charge reversal E84R/E87K in CaM (CaM:84/87) with SK2:64/67 reconstituted stable Ca2+-dependent channel activity that was not lost with exposure to Ca2+-free solution. Therefore, Ca2+-independent interactions with CaM are required for surface expression of SK channels, whereas the constitutive association between the two channel subunits is not an essential requirement for gating.