Fig. 4.

The IDF on Ca²⁺-dependent activation of SK channels. (A) NS309 increases the open probability of SK2 at a given Ca²⁺ concentration. Dose–response curves of Ca²⁺-dependent activation of SK2-a were obtained with or without 3 μM NS309 in the bath solution. (B) Application of the cross-linker MTS-1-MTS (1 mM) doubles the current size of the same patch in the presence of 300 nM Ca²⁺. After the bath solution was switched to 0 Ca²⁺, subsequent application of 10 μM Ca²⁺ no longer could activate the channels modified by MTS-1-MTS, indicating that cross-linking has changed the structure of the IDF of the SK channels irreversibly. (C) Cross-linking A477C of the CaMBD and F410C of the IDF mimics the effect of NS309. The membrane patches expressing A477C/F410C were perfused sequentially with a solution of 10 μM Ca²⁺, 300 nM Ca²⁺, and 300 nM Ca²⁺ plus 1 mM MTS-1-MTS. The current amplitudes were normalized to that at 10 μM Ca²⁺. Values in parentheses indicate the number of experiments. (D) Effects of mutations on the salt bridge between K75 of CaM and E404 of the IDF. The mutant pair K75R:E404D was able to rescue activation of the mutant channel by Ca²⁺. NS309 is able to potentiate the Ca²⁺-dependent activation of the mutant pair.