Activation of the SK potassium channel-calmodulin complex by nanomolar concentrations of terbium.
Small conductance Ca(2+)-activated K(+) (SK) channels sense intracellular Ca(2+) concentrations via the associated Ca(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(2+) to calmodulin leads to channel opening. Drawing on previous biochemical studies of free calmodulin using lanthanide ions as Ca(2+) substitutes, we have used the lanthanide ion, Tb(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(3+), indicating an apparent affinity >100-fold higher than Ca(2+). Competition experiments show that Tb(3+) binds to the same sites as Ca(2+) to activate the channels. Additionally, SK channels activated by Tb(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.
Li
Weiyan
W
Section of Neurobiology, Center for Learning and Memory, University of Texas, Austin, TX 78712, USA.
Aldrich
Richard W
RW
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002147:Calmodulin / Q000378:metabolism*; D015640:Ion Channel Gating / Q000187:drug effects*; D007700:Kinetics; D051381:Rats; D051640:Small-Conductance Calcium-Activated Potassium Channels / Q000378:metabolism*; D013725:Terbium / Q000494:pharmacology*; D014981:Xenopus
D002147:Calmodulin; D051640:Small-Conductance Calcium-Activated Potassium Channels; D013725:Terbium; D002118:Calcium