Redox-dependent Cd²⁺ inhibition of BK-type Ca²⁺-activated K⁺ channels

Large-conductance Ca²⁺-activated K⁺ channels (BK channels) are formed by Slo1 subunits as a homotetramer. Besides Ca²⁺, other divalent cations, such as Cd²⁺, also activate BK channels when applied intracellularly by shifting the conductance-voltage relation to more negative voltages. However, we found that if the inside-out patch containing BK channels was treated with solution containing reducing agents such as dithiothreitol (DTT), then subsequent Cd²⁺ application completely inhibited BK currents. The DTT-dependent Cd²⁺ inhibition could be reversed by treating the patch with solutions containing H₂O₂, suggesting that a redox reaction regulates the Cd²⁺ inhibition of BK channels. Similar DTT-dependent Cd²⁺ inhibition was also observed in a mutant BK channel, Core-MT, in which the cytosolic domain of the channel is deleted, and in the proton-activated Slo3 channels but not observed in the voltage-gated Shaker K⁺ channels. A possible mechanism for the DTT-dependent Cd²⁺ inhibition is that DTT treatment breaks one or more disulfide bonds between cysteine pairs in the BK channel protein and the freed thiol groups coordinate with Cd²⁺ to form an ion bridge that blocks the channel or locks the channel at the closed state. However, surprisingly, none of the mutations of all cysteine residues in Slo1 affect the DTT-dependent Cd²⁺ inhibition. These results are puzzling, with an apparent contradiction: on one hand, a redox reaction seems to regulate Cd²⁺ inhibition of the channel, but on the other hand, no cysteine residue in the Slo1 subunit seems to be involved in such inhibition.