State-dependent FRET reports calcium- and voltagedependent gating-ring motions in BK channels

Large-conductance voltage- and calcium-dependent potassium channels (BK, "Big K+") are important controllers of cell excitability. In the BK channel, a large C-terminal intracellular region containing a "gating-ring" structure has been proposed to transduce Ca²⁺ binding into channel opening. Using patch-clamp fluorometry, we have investigated the calcium and voltage dependence of conformational changes of the gating-ring region of BK channels, while simultaneously monitoring channel conductance. Fluorescence resonance energy transfer (FRET) between fluorescent protein inserts indicates that Ca²⁺ binding produces structural changes of the gating ring that are much larger than those predicted by current X-ray crystal structures of isolated gating rings.