Activation of the SK potassium channel-calmodulin complex by nanomolar concentrations of terbium

Small conductance [Ca.sup.2+]-activated [K.sup.+] (SK) channels sense intracellular [Ca.sup.2+] concentrations via the associated [Ca.sup.2+]-binding protein calmodulin. Structural and functional studies have revealed essential properties of the interaction between calmodulin and SK channels. However, it is not fully understood how the binding of [Ca.sup.2+] to calmodulin leads to channel opening. Drawing on previous biochemical studies of free calmodulin using lanthanide ions as [Ca.sup.2+] substitutes, we have used the lanthanide ion, [Tb.sup.3+], as an alternative ligand to study the activation properties of SK channels. We found that SK channels can be fully activated by nanomolar concentrations of [Tb.sup.3+], indicating an apparent affinity >100-fold higher than [Ca.sup.2+]. Competition experiments show that [Tb.sup.3+] binds to the same sites as [Ca.sup.2+] to activate the channels. Additionally, SK channels activated by [Tb.sup.3+] demonstrate a remarkably slow deactivation process. Comparison of our results with previous biochemical studies suggests that in the intact SK channel complex, the N-lobe of calmodulin provides ligand-binding sites for channel gating, and that its ligand-binding properties are comparable to those of the N-lobe in isolated calmodulin. lanthanide | EF hand | gating | calcium-activated potassium channel